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Climate of Seattle
Climate of Seattle
Climate of Seattle
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Seattle
Climate chart (explanation)
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█ Average max. and min. temperatures in °F
█ Precipitation totals in inches
Source: [1]
Metric conversion
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█ Average max. and min. temperatures in °C
█ Precipitation totals in mm
Rainy day in Capitol Hill, Seattle. Seattle experiences around 150 days with at least 0.01 inches (0.25 mm) precipitation each year.

The climate of Seattle is temperate, classified in the warm-summer (in contrast to hot-summer) subtype of the Mediterranean zone by the most common climate classification (Köppen: Csb)[2][3][4] although some sources put the city in the oceanic zone (Trewartha: Do).[5][6] It has cool, wet winters and warm, dry summers, covering characteristics of both.[7][8] The climate is sometimes characterized as a "modified Mediterranean" climate because it is cooler and wetter than a "true" Mediterranean climate, but shares the characteristic dry summer and the associated reliance upon cooler-season precipitation (which has a strong influence on the region's vegetation).[9] The city is part of USDA hardiness zone 9a, with surrounding pockets falling under 8b.[10]

Records for the Seattle City area date back to 1894, with records at Seattle-Tacoma International Airport beginning in 1945, a location notably not within Seattle. Prior to 1945 the official temperatures were observed in locations in downtown Seattle, which tends in general to be somewhat warmer and drier than the airport location.[11] The hottest officially recorded temperature was 108 °F (42 °C) on June 28, 2021; the coldest recorded temperature was 0 °F (−18 °C) on January 31, 1950;[12] the record cold daily maximum is 16 °F (−9 °C) on January 14, 1950, while, conversely, the record warm daily minimum is 73 °F (23 °C) on June 27, 2021.[13]

Seattle generally does not experience many extremes of weather. However, the 21st century has seen a trend towards more extreme high-temperature and large-precipitation events. In July 2009 Seattle's all-time high temperature was broken by a margin of 4 degrees Fahrenheit (2.2 Celsius),[14] then broken again by a margin of 5 F (2.8 C) in June 2021. The single-day precipitation record set in October 2003 saw higher precipitation by nearly 2 inches (50mm) than any other day on record. However, thunderstorms are still rare,[15] as the city reports thunder on just seven days per year.[16] Similarly, the city typically receives at least light snowfall every year, though heavy snowfall is uncommon.

Temperature

[edit]
Weather report from Seattle-Tacoma Airport in June 2021. Seattle recorded its highest temperature ever on June 28, reaching 108 °F (42 °C).[17]

The city's regime of temperature features small seasonal swings, due to its proximity to the ocean. The Pacific Ocean, Puget Sound and Lake Washington serve as moderators of the temperature meaning the city is milder than areas inland during the winter and cooler during the summer. Extreme heatwaves are rare, as are cold temperatures. Hot temperature extremes are enhanced by dry, compressed wind from the west slopes of the Cascades,[18] while cold temperatures are generated mainly from the Fraser Valley in British Columbia.[19] Records are taken from the Seattle City area from 1894 to 1944 and at Sea-Tac Airport from 1945.

Averages

[edit]

In an average year, the temperature will usually be between 21 °F (−6 °C) and 94 °F (34 °C) with temperatures greatly exceeding these values being uncommon. Temperatures above 97 °F (36 °C) and below 15 °F (−9 °C) are very rare, with the last occurrences being June 28, 2021 and November 24, 2010, respectively.[13]

Climate data for Seattle (Seattle-Tacoma International Airport), 1991–2020 normals
Month Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Year
Mean maximum °F (°C) 57.0
(13.9) 		59.1
(15.1) 		66.4
(19.1) 		74.3
(23.5) 		81.9
(27.7) 		85.8
(29.9) 		91.2
(32.9) 		89.9
(32.2) 		84.1
(28.9) 		72.0
(22.2) 		61.6
(16.4) 		56.8
(13.8) 		94.1
(34.5)
Mean daily maximum °F (°C) 48.0
(8.9) 		50.3
(10.2) 		54.2
(12.3) 		59.3
(15.2) 		66.3
(19.1) 		71.1
(21.7) 		77.4
(25.2) 		77.6
(25.3) 		71.6
(22.0) 		60.5
(15.8) 		52.1
(11.2) 		47.0
(8.3) 		61.3
(16.3)
Daily mean °F (°C) 42.8
(6.0) 		44.0
(6.7) 		47.1
(8.4) 		51.3
(10.7) 		57.5
(14.2) 		62.0
(16.7) 		67.1
(19.5) 		67.4
(19.7) 		62.6
(17.0) 		53.8
(12.1) 		46.5
(8.1) 		42.0
(5.6) 		53.7
(12.1)
Mean daily minimum °F (°C) 37.7
(3.2) 		37.7
(3.2) 		39.9
(4.4) 		43.3
(6.3) 		48.7
(9.3) 		53.0
(11.7) 		56.8
(13.8) 		57.2
(14.0) 		53.6
(12.0) 		47.0
(8.3) 		40.9
(4.9) 		37.1
(2.8) 		46.1
(7.8)
Mean minimum °F (°C) 26.1
(−3.3) 		27.3
(−2.6) 		31.3
(−0.4) 		35.6
(2.0) 		40.6
(4.8) 		46.6
(8.1) 		51.5
(10.8) 		51.7
(10.9) 		45.8
(7.7) 		36.8
(2.7) 		29.2
(−1.6) 		25.4
(−3.7) 		21.5
(−5.8)
Source: NOAA[13]

Daily record warm minima

[edit]

Highest averages

[edit]

NOTE: in the tables below, all numbers before 1945 come from locations in downtown Seattle, which tends to be somewhat warmer than the current official location of Sea-Tac airport.[11] To distinguish these older numbers we mark them below in italics.

Precipitation

[edit]
Cherry Street after heavy snowfall on January 10, 1880

The city sees frequent, though light rainfall between October and May, with rainfall becoming lighter and sparser between June and September. With many more "rain days" than other major American cities, Seattle has a well-earned reputation for frequent rain.[20] In an average year, at least 0.01 inches (0.25 mm) of precipitation falls on 150 days, more than nearly all U.S. cities east of the Rocky Mountains. In November, Seattle averages more rainfall than any other U.S. city of more than 250,000 people; it also ranks highly in winter precipitation. Conversely, the city receives some of the lowest precipitation amounts of any large city from June to September. Seattle is one of the five rainiest major U.S. cities as measured by the number of days with precipitation.[21][16] However, because Seattle often has merely a light drizzle falling from the sky for many days, it actually receives significantly less rainfall (or other precipitation) overall than many other U.S. cities like New York City, Miami, or Houston. Seattle experiences its heaviest rainfall during November, December, and January, receiving roughly half of its annual rainfall (by volume) during this period. In late fall and early winter, atmospheric rivers (also known as "Pineapple Express" systems), strong frontal systems, and Pacific low-pressure systems are common. Light rain and drizzle are the predominant forms of precipitation during the remainder of the year. On average, less than 1.6 in (41 mm) of rain falls in July and August combined when rain is less common.

Annually, total precipitation averages 39.3 in (998 mm), with winter being the wettest season and July the driest month. At Sea-Tac, rain has fallen in every month since records began there in January 1945, previously in the Seattle City area, the Julys of 1896 and 1922 reported no precipitation. Long stretches of little precipitation can occur. No measurable precipitation, greater than 0.01 inches (0.25 mm), fell between June 18 and August 13, 2017.[22] The city also sees snow, primarily in winter, but sometimes in the late autumn and early spring. Snowfall averages 6.3 in (16.0 cm) per year but is highly variable between winter seasons. The most rainfall in 24 hours was 5.02 in (127.5 mm) on October 20, 2003, and the most snowfall was 21.5 in (55 cm) on February 2, 1916.[13] Seattle typically receives some snowfall on an annual basis but heavy snow is rare. Average annual snowfall, as measured at Sea-Tac Airport, is 6.3 inches (16.0 cm). From winter season to winter season, amounts can be extremely variable.

Due to local variations in microclimate, Seattle also receives significantly lower precipitation than some other locations west of the Cascades. Around 80 mi (130 km) to the west, the Hoh Rain Forest in Olympic National Park on the western flank of the Olympic Mountains receives an annual average precipitation of 142 in (3,600 mm). Sixty miles (97 km) to the south of Seattle, the state capital Olympia, which is out of the Olympic Mountains' rain shadow, receives an annual average precipitation of 50 in (1,270 mm).[13] The city of Bremerton, about 15 mi (24 km) west of downtown Seattle on the other side of the Puget Sound, receives 56.4 in (1,430 mm) of precipitation annually.[13]

One of many exceptions to Seattle's reputation as a damp location occurs in El Niño years, when marine weather systems track as far south as California and less than the usual precipitation falls in the Puget Sound area.[23] However, the El Nino of 2015-2016 caused an increase in rainfall. Since the region's water comes from mountain snow packs during the dry summer months, El Niño winters can not only produce substandard skiing but can result in water rationing and a shortage of hydroelectric power the following summer.[24]

Averages

[edit]
Climate data for Seattle (SeaTac International Airport), 1991–2020 normals
Month Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Year
Average precipitation inches (mm) 5.78
(147) 		3.76
(96) 		4.17
(106) 		3.18
(81) 		1.88
(48) 		1.45
(37) 		0.60
(15) 		0.97
(25) 		1.61
(41) 		3.91
(99) 		6.31
(160) 		5.72
(145) 		39.34
(999)
Average snowfall inches (cm) 2.5
(6.4) 		2.2
(5.6) 		0.9
(2.3) 		0
(0) 		0
(0) 		0
(0) 		0
(0) 		0
(0) 		0
(0) 		0
(0) 		0.5
(1.3) 		2.1
(5.3) 		8.2
(20.9)
Average precipitation days (≥ 0.01 in) 18.7 15.9 17.1 15.0 11.3 9.2 4.7 4.9 8.3 14.3 18.4 18.4 156.2
Average snowy days (≥ 0.1 in) 2.3 2.0 0.9 0 0 0 0 0 0 0 0.2 1.5 4.7
Source: NOAA[13]

Rainfall Extremes

[edit]

Snowfall

[edit]

Highest

[edit]
Period Record most snowfall Year
Year[a] 63.6 in (162 cm) 1916
Seasonal (July–June) 67.5 in (171 cm) 1968–1969
Spring (March–May) 18.2 in (46 cm) 1951
Autumn (September–November) 20.5 in (52 cm) 1896
Winter (December–February) 58.7 in (149 cm) 1915–1916
January 57.2 in (145 cm) 1950
February 35.4 in (90 cm) 1916
March 18.2 in (46 cm) 1951
April 2.4 in (6.1 cm) 1920
May Trace 1993, 1990, 1989,
1974, 1965, 1955,
1953, 1951, 1925
June–September 0 in (0 cm) –
October 2.0 in (5.1 cm) 1971
November 20.5 in (52 cm) 1896
December 22.1 in (56 cm) 1968

Other phenomena

[edit]
Vew of the downtown Seattle skyline, on the waterfront, with the Seattle Aquarium on the left and Seattle Great Wheel on the right.
Downtown Seattle averages 71 completely sunny days a year, with most of those days occurring between May and September[25]

Sunshine, UV and daylight

[edit]

The city generally experiences cloudy conditions, with clear days occurring infrequently. As a result of the city's latitude, it experiences a moderate difference in daylight hours between summer and winter, though is not subject to the extremes of cities further north.[26] The Seattle area is the cloudiest region of the United States, due in part to frequent storms and lows moving in from the adjacent Pacific Ocean. Seattle is cloudy 201 days out of the year and partly cloudy 93 days.[25] (Official weather and climatic data is collected at Seattle–Tacoma International Airport, located about 19 km (12 mi) south of downtown in the city of SeaTac, which is at a higher elevation, and records more cloudy days and fewer partly cloudy days per year.[25])

Climate data for Seattle
Month Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Year
Mean monthly sunshine hours 69.8 108.8 178.4 207.3 253.7 268.4 312.0 281.4 221.7 142.6 72.7 52.9 2,169.7
Mean daily daylight hours 9.0 10.3 12.0 13.7 15.2 15.9 15.5 14.2 12.5 10.8 9.3 8.5 12.2
Percentage possible sunshine 25 38 48 51 54 56 65 64 59 42 26 20 49
Average ultraviolet index 1 2 3 5 6 7 7 6 5 3 1 1 4
Source 1: NOAA (relative humidity and sun 1961–1990)[27]
Source 2: Weather Atlas[26]

Wind

[edit]

The Puget Sound Convergence Zone is an important feature of Seattle's weather. In the convergence zone, air arriving from the north meets air flowing in from the south. Both streams of air originate over the Pacific Ocean; airflow is split by the Olympic Mountains to Seattle's west, then reunited to the east. When the air currents meet, they are forced upward, resulting in convection.[28] Thunderstorms caused by this activity are usually weak and can occur north and south of town, but Seattle itself rarely receives more than occasional thunder and small hail showers. The Hanukkah Eve Wind Storm in December 2006 is an exception that brought heavy rain and winds gusting up to 69 mph (111 km/h), an event that was not caused by the Puget Sound Convergence Zone and was widespread across the Pacific Northwest. In December 2007, a strong windstorm brought hurricane force winds and heavy rain, leading to 5 deaths.[29][30]

Climate change

[edit]

Being a coastal city, Seattle may experience significant effects from rising sea levels. The sea has risen by 6 inches (15 cm) in the past century, and is expected to rise 28 inches (71 cm) by 2100 and 47 inches (120 cm) by 2150. It is expected that by this time, frequent flooding will become a problem, with now-annual extreme king tide's becoming monthly or even daily events.[31]

Temperature

[edit]

The temperature in Seattle has generally increased steadily and this trend is expected to continue due to anthropogenic warming. For the last century, the average has increased roughly 0.3 degrees Fahrenheit each decade. By the end of the century, it is predicted that there will be on average around two weeks of 90 °F (32 °C) days each year.[31]

Homes in the area have historically not used air conditioning due to the temperate summer climate; the United States Census Bureau found that 31 percent of households in the Seattle metropolitan area used air conditioning. Several major heat waves in the 2010s and 2020s, including the June 2021 heat dome that set record temperatures in the state, led to a large increase in air conditioning use.[32] By the end of 2021, the Census Bureau's biannual housing survey found that 53 percent of households in the metropolitan area had air conditioning.[33]

Precipitation

[edit]

Precipitation in the city has increased slightly, but this trend is expected to continue. Extreme rainfall events have become more frequent over the previous years and this trend is also expected to continue.[31]

Station data

[edit]
Climate data for Seattle (SeaTac Airport), 1991–2020 normals,[b] extremes 1894–present[c]
Month Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Year
Record high °F (°C) 67
(19) 		70
(21) 		79
(26) 		89
(32) 		93
(34) 		108
(42) 		103
(39) 		99
(37) 		98
(37) 		89
(32) 		74
(23) 		66
(19) 		108
(42)
Mean maximum °F (°C) 57.0
(13.9) 		59.1
(15.1) 		66.4
(19.1) 		74.3
(23.5) 		81.9
(27.7) 		85.8
(29.9) 		91.2
(32.9) 		89.9
(32.2) 		84.1
(28.9) 		72.0
(22.2) 		61.6
(16.4) 		56.8
(13.8) 		94.1
(34.5)
Mean daily maximum °F (°C) 48.0
(8.9) 		50.3
(10.2) 		54.2
(12.3) 		59.3
(15.2) 		66.3
(19.1) 		71.1
(21.7) 		77.4
(25.2) 		77.6
(25.3) 		71.6
(22.0) 		60.5
(15.8) 		52.1
(11.2) 		47.0
(8.3) 		61.3
(16.3)
Daily mean °F (°C) 42.8
(6.0) 		44.0
(6.7) 		47.1
(8.4) 		51.3
(10.7) 		57.5
(14.2) 		62.0
(16.7) 		67.1
(19.5) 		67.4
(19.7) 		62.6
(17.0) 		53.8
(12.1) 		46.5
(8.1) 		42.0
(5.6) 		53.7
(12.1)
Mean daily minimum °F (°C) 37.7
(3.2) 		37.7
(3.2) 		39.9
(4.4) 		43.3
(6.3) 		48.7
(9.3) 		53.0
(11.7) 		56.8
(13.8) 		57.2
(14.0) 		53.6
(12.0) 		47.0
(8.3) 		40.9
(4.9) 		37.1
(2.8) 		46.1
(7.8)
Mean minimum °F (°C) 26.1
(−3.3) 		27.3
(−2.6) 		31.3
(−0.4) 		35.6
(2.0) 		40.6
(4.8) 		46.6
(8.1) 		51.5
(10.8) 		51.7
(10.9) 		45.8
(7.7) 		36.8
(2.7) 		29.2
(−1.6) 		25.4
(−3.7) 		21.5
(−5.8)
Record low °F (°C) 0
(−18) 		1
(−17) 		11
(−12) 		29
(−2) 		28
(−2) 		38
(3) 		43
(6) 		44
(7) 		35
(2) 		28
(−2) 		6
(−14) 		6
(−14) 		0
(−18)
Average precipitation inches (mm) 5.78
(147) 		3.76
(96) 		4.17
(106) 		3.18
(81) 		1.88
(48) 		1.45
(37) 		0.60
(15) 		0.97
(25) 		1.61
(41) 		3.91
(99) 		6.31
(160) 		5.72
(145) 		39.34
(999)
Average snowfall inches (cm) 1.8
(4.6) 		2.2
(5.6) 		0.4
(1.0) 		0.0
(0.0) 		0.0
(0.0) 		0.0
(0.0) 		0.0
(0.0) 		0.0
(0.0) 		0.0
(0.0) 		0.0
(0.0) 		0.2
(0.51) 		1.7
(4.3) 		6.3
(16)
Average precipitation days (≥ 0.01 in) 18.7 15.9 17.1 15.0 11.3 9.2 4.7 4.9 8.3 14.3 18.4 18.4 156.2
Average snowy days (≥ 0.1 in) 1.4 1.2 0.4 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.2 1.5 4.7
Average relative humidity (%) 78.0 75.2 73.6 71.4 68.9 67.1 65.4 68.2 73.2 78.6 79.8 80.1 73.3
Average dew point °F (°C) 33.1
(0.6) 		35.1
(1.7) 		36.3
(2.4) 		38.8
(3.8) 		43.5
(6.4) 		48.2
(9.0) 		51.4
(10.8) 		52.7
(11.5) 		50.2
(10.1) 		45.1
(7.3) 		38.8
(3.8) 		34.3
(1.3) 		42.3
(5.7)
Mean monthly sunshine hours 69.8 108.8 178.4 207.3 253.7 268.4 312.0 281.4 221.7 142.6 72.7 52.9 2,169.7
Percentage possible sunshine 25 38 48 51 54 56 65 64 59 42 26 20 49
Average ultraviolet index 1 2 3 5 6 7 7 6 5 3 1 1 4
Source 1: NOAA (relative humidity, dew point and sun 1961–1990)[34][35][36]
Source 2: Weather Atlas (UV)[37]

See or edit raw graph data.

Explanatory notes

[edit]

References

[edit]
Revisions and contributorsEdit on WikipediaRead on Wikipedia

Climate of Seattle

View on Grokipedia
from Grokipedia
The climate of Seattle, Washington, is a mild temperate maritime regime classified as warm-summer Mediterranean (Csb) under the Köppen-Geiger system, marked by cool, rainy winters, mild and relatively dry summers, and persistent throughout much of the year. This pattern arises from the city's position in the lowlands, where prevailing westerly winds from the carry moisture inland, but the create a pronounced effect that substantially reduces precipitation compared to windward coastal areas, while and the Puget Sound Convergence Zone enhance local rainfall through atmospheric convergence and uplift. Annual precipitation at Seattle-Tacoma International Airport, the primary observing station, averages approximately 38 inches, with over 70% falling between and , often as light drizzle rather than heavy downpours, contributing to more than 150 days per year with at least 0.01 inches of measurable —far exceeding the volume of many eastern U.S. cities despite the city's "rainy" reputation stemming more from gloom than volume. Mean annual hovers around 53°F, with averages of 47°F highs and 36°F lows, and July peaks at 76°F highs and 56°F lows; extremes are rare, with freezing temperatures occurring on only about 30 days annually and temperatures above 90°F on fewer than five, moderated by marine influences that prevent sharp continental swings. Sunshine is limited to roughly 2,200 hours per year, or about 57% possible, due to frequent stratus clouds, while snowfall averages under 6 inches annually in the city core, though occasional events like the 1880 blizzard or 2021 of 108°F highlight vulnerabilities to atypical synoptic patterns amid an otherwise equable regime. Recent U.S. normals (1991–2020) indicate a slight warming and wetting trend relative to prior periods, consistent with broader observations, though empirical station data emphasize the enduring dominance of seasonal marine air flows over long-term shifts in defining daily variability.

Climatic Classification and Influences

Köppen-Geiger Classification and Comparisons

Seattle's climate is classified as Csb (warm-summer Mediterranean) under the Köppen-Geiger system, featuring mild winters with the coldest month averaging above 0°C (32°F), cool summers where the warmest month averages below 22°C (72°F), and a pronounced dry summer season where in the driest summer month falls below 30 mm and less than one-third of the wettest winter month's total. This designation captures the temperate oceanic moderation from the Pacific, tempered by seasonal aridity driven by the subtropical high-pressure ridge in summer, distinguishing it from purely oceanic regimes without such dryness. In comparison to , where continental influences dominate with Dfb (cold, humid continental) classifications yielding sharper temperature swings—winters often dipping below -10°C (14°F) and summers exceeding 30°C (86°F)— maintains narrower diurnal and annual ranges, with historical extremes rarely below 0°F (-18°C) or above 100°F (38°C) due to marine air persistence. The exacerbates this east-west divide by creating a that desiccates the interior, resulting in semi-arid to continental dryness east of the mountains, while benefits from Puget Sound's moderated flows. Proximity to the further nuances Seattle's profile, as their windward slopes intercept Pacific moisture, fostering a partial in the Puget Lowland that yields annual around 37 inches (940 mm)—wetter than eastern leeward zones but far drier than the Olympics' western flanks exceeding 100 inches (2,540 mm). This topographic buffering underscores Seattle's mildness as a geographic anomaly, prioritizing coastal marine effects over broader continental or arid norms.

Topographic and Oceanic Factors

The to the west of create a pronounced effect, whereby moist Pacific air masses are orographically lifted and precipitate heavily on the windward western slopes, resulting in substantially drier conditions on the leeward eastern side encompassing the lowlands. Annual in averages approximately 37 inches, in contrast to over 100 inches on the southwestern slopes of the Olympics and up to 150 inches in adjacent coastal areas. This topographic barrier not only limits total rainfall but also influences storm tracks, channeling much of the into focused bands rather than uniform distribution across the region, thereby contributing to Seattle's relative climate stability by mitigating the intensity of direct Pacific frontal assaults. The , a large semi-enclosed , exerts a moderating influence on local temperatures through its thermal , absorbing in summer and releasing it in winter to dampen seasonal extremes. This marine proximity ensures that winter minimums rarely drop below freezing, with December averages remaining above 32°F, and prevents summer highs from exceeding 80°F on most days, fostering a narrow diurnal and annual range compared to inland locales. Oceanic factors amplify this effect; the , a southward-flowing cold water mass along the North American coast, maintains cooler sea surface temperatures off Washington, which in turn cools onshore air flows and reinforces the temperate marine air dominating Seattle's climate. Prevailing westerly winds transport moisture-laden air from the eastward, particularly during the cool season when storm tracks align favorably, driving the majority of Seattle's rainfall through successive frontal passages rather than convective thunderstorms. These winds, combined with the topographic sheltering of the Olympic and Cascade ranges, historically limit the incursion of severe continental weather systems, such as intense arctic outbreaks or tropical cyclones, resulting in fewer tornadoes and no recorded hurricane landfalls in the area over the past century, unlike more exposed U.S. coastal cities.

Natural Climate Oscillations

The (PDO), a long-term pattern of variability in the North Pacific, modulates decadal-scale climate fluctuations in the , including Seattle. In its negative (cool) phase, characterized by cooler waters in the eastern North Pacific, the region tends toward wetter winters, as enhanced tracks bring increased ; conversely, positive (warm) phases correlate with drier conditions due to suppressed activity. This phase-dependent influence has contributed to observed 20th-century precipitation shifts, such as relatively drier periods during the positive PDO regime from approximately 1977 to 1998. The PDO entered a sustained negative phase around 2020, with indices reaching record lows in 2022 and 2023, and remaining negative through August 2025 at -3.18. The El Niño-Southern Oscillation (ENSO), operating on interannual timescales, further drives variability in Seattle's winter weather through teleconnections that alter the position and storm paths. During El Niño phases, such as the 2023-24 event, suppressed and above-average temperatures prevail in the , with reduced winter rainfall due to a southward-shifted storm track; La Niña phases, including the weak 2024-25 occurrence, typically enhance northerly flow, yielding wetter and cooler winters with greater snowfall potential at low elevations. These opposing effects amplify or dampen seasonal extremes, as seen in historical La Niña winters featuring increased snow accumulation in Washington. PDO and ENSO interact to explain substantial variability in , with ENSO forcing contributing to the PDO's expression and together modulating up to half of interannual fluctuations through reinforced or canceled teleconnection patterns. NOAA analyses of reconstructed indices highlight their role in capturing 30-50% of variance on these timescales, underscoring natural Pacific modes as primary non-anthropogenic drivers of Seattle's fluctuating winter patterns.

Temperature Patterns

Averages and Seasonal Cycles

The annual mean temperature in , based on observations at Seattle-Tacoma International Airport (Sea-Tac), is approximately 53.5°F for the 1991-2020 normal period, reflecting a mild maritime climate with limited seasonal extremes. Winters are mild from to , featuring average daily highs of 47–50°F and lows of 37–38°F, while summers from to see highs of 71–78°F and lows of 53–57°F, with a gradual spring warming and autumn cooling. This progression maintains relative consistency across decades, as evidenced by the 30-year normals derived from long-term station data, underscoring the stabilizing influence of the and . Diurnal temperature ranges remain modest year-round, typically 10–15°F, due to the moderating that suppresses sharp daily fluctuations; for instance, winter months average about 10°F ranges, expanding slightly to 19–21°F in peak summer. The following table summarizes monthly high and low temperatures (°F) from Sea-Tac for the 1991-2020 period:
MonthHigh (°F)Low (°F)
4838
5038
March5440
April5943
May6649
June7153
July7757
August7857
September7254
October6147
November5241
December4737
These averages highlight Seattle's transition from cool, overcast winters to drier, milder summers, with the marine proximity ensuring highs rarely exceed 80°F on average and lows seldom drop below freezing.

Record Extremes

The all-time record high temperature in Seattle, measured at Seattle-Tacoma International Airport (Sea-Tac), is 108°F (42°C), recorded on June 28, 2021, during a regional heat dome event that shattered numerous Pacific Northwest records. This surpassed the previous record of 104°F set on July 20, 1994. Prior to 2021, Seattle had only three verified instances of reaching 100°F or higher since official observations began in the late 19th century: 100°F on July 16, 1941 (downtown station), 100°F on July 20, 1994 (Sea-Tac), and 103°F on July 29, 2009 (Sea-Tac). The 2021 event added three more: 102°F on June 26, 104°F on June 27, and the 108°F peak, bringing the total to six days of 100°F or higher in recorded history. The all-time record low is 0°F (-18°C), observed on January 31, 1950, at the Sea-Tac station during an outbreak. Other notable cold extremes include a low of 1°F on , 1950, and 4°F on , 1963.
CategoryValueDateNotes/Source
Highest temperature108°FJune 28, 2021Sea-Tac; event
Second highest104°FJune 27, 2021Sea-Tac
Third highest103°FJuly 29, 2009Sea-Tac
Lowest temperature0°FJanuary 31, 1950Sea-Tac; air mass
Warmest minimum (night low)70°FJune 28, 2021During 2021 ; previous record 68°F in 2021 earlier
Coldest maximum (day high)18°FJanuary 31, 1950Coincided with record low
Days exceeding 90°F have occurred approximately 3 times per year on average since , with 246 such instances through 2022, though many cluster during multi-day heat events like 2021 (five consecutive days ≥95°F). Recent records for warm minima include 69°F on 11, 2025, setting a new August low-high mark.

Recent Temperature Anomalies

Seattle experienced its all-time record high temperature of 108°F (42°C) at Seattle-Tacoma International Airport (Sea-Tac) on June 28, 2021, during a multi-day event that produced three consecutive days above 100°F from June 26 to 28. This outlier shattered previous records by 5°F and highlighted episodic extreme heat in the post-2000 period, with the event driven by a persistent high-pressure ridge over the . In 2024, Sea-Tac recorded an temperature of 69.8°F (21.0°C), ranking as the third-warmest July on record since 1945, with an high of 81.5°F (27.5°C) exceeding the normal of 77.4°F (25.2°C). 16, 2025, marked the hottest day of that year to date at Sea-Tac with a high of 94°F (34°C), accompanied by a advisory for mid-90s temperatures across the region. Later in 2025, September 16 saw a near-record high of 91°F (33°C) at Sea-Tac, tying the daily record and underscoring late-season warmth amid an overall dry summer that amplified perception through reduced evaporative cooling. Analyses of Sea-Tac data indicate extended periods of summer-like warmth in recent years, with Seattle experiencing approximately five additional days of above-normal summer temperatures compared to three decades prior, contributing to longer streaks of elevated highs without implying uniform trends. Despite a cool phase of the (PDO) in 2024-2025, which typically moderates regional temperatures toward cooler anomalies, episodic heat events persisted, as evidenced by 2024's above-average annual warmth at Sea-Tac counterbalanced by oscillatory influences.

Precipitation Characteristics

Rainfall Distribution and Intensity

Seattle's annual precipitation averages 37.1 inches, with roughly 70 percent concentrated from through due to the influence of Pacific storm tracks. stands as the wettest month, averaging 6.3 inches of rainfall, which contributes to having among the highest November precipitation totals for U.S. cities exceeding 250,000 residents. The city records approximately 152 days per year with measurable of at least 0.01 inches, reflecting frequent light rain events rather than intense deluges. Rainfall intensity remains generally moderate, with daily accumulations exceeding 2 inches occurring infrequently outside of rare events. The highest single-day total measured 5.02 inches on October 20, 2003, driven by a stalled frontal system. Annual extremes highlight variability, as the wettest year on record, 1950, delivered 55.1 inches, contrasting with drier periods like the top-10 driest water years. Recent patterns underscore ongoing fluctuations, including a pronounced dry streak through early 2025 with an 8.5-inch deficit since October 2024. The summer of 2025 ranked as Seattle's 10th driest on record, exacerbating seasonal amid below-normal totals from June through August. These deviations align with natural oscillations but have prompted localized water management concerns without altering the core seasonal distribution.

Snowfall and Winter Precipitation

Snowfall in Seattle is infrequent and typically light at low elevations, averaging around 5 to 7 inches annually at Seattle-Tacoma International Airport, the official climate station situated near sea level. Recent decades show an average of about 6 inches per season at the airport, with variability driven by occasional arctic outflows that allow cold air to pool in the Puget Sound lowlands. These events require specific synoptic conditions, such as easterly winds channeling frigid continental air over the Cascades, often resulting in brief snow episodes rather than prolonged accumulation. Suburban areas to the east, including higher elevations around , experience greater snowfall totals due to orographic enhancement and the Convergence Zone, which can amplify during winter storms. In contrast, and coastal lowlands see less, as the marine influence moderates temperatures, frequently causing phase changes from to or mixed . This results in winter precipitation featuring primarily rain with virtually no accumulating snow in the city—perhaps a rare dusting every few years that melts quickly. Historical records indicate exceptional seasons, with the highest winter total of 67.5 inches occurring in 1968–1969, though such extremes are outliers tied to rare persistent cold snaps. Earlier measurements from downtown stations captured up to 63.6 inches in the 1916 calendar year, including a single-day record of 21.5 inches on February 2, 1916. At Sea-Tac, post-1948 data show more modest peaks, such as 21 inches in 2018–2019, underscoring the rarity of deep snow amid the region's mild maritime climate. Recent years have trended toward lower accumulations, with many seasons below 5 inches, though variability persists without a clear long-term decline in event frequency. Winter precipitation often manifests as rain-on-snow events rather than sustained whiteouts, as near-freezing temperatures lead to rapid melting and increased risk from slushy runoff rather than burial under deep powder. Approximately 80% of lowland snowstorms stem from easterly cold air incursions, but the ' rain and Pacific moderation limit duration and depth, making Seattle's winters wetter than snowy compared to continental climates at similar latitudes.

Drought and Flood Events

Seattle has experienced periodic multi-year droughts characterized by below-average precipitation and reduced streamflows, with historical records indicating cyclical occurrences rather than unprecedented events. One notable prolonged dry period occurred in the 1890s, when Washington State faced extended low precipitation and low flows on rivers like the Columbia, contributing to agricultural stress before widespread irrigation development. Similarly, the 1928–1938 Dust Bowl era and the 1977 single-year event marked severe droughts in the Pacific Northwest, as evidenced by standardized anomaly drought indices that highlight multi-decadal variability in precipitation deficits. More recently, partial drought conditions from 2021 onward affected the region, with Seattle recording extended dry spells, such as nearly four weeks without measurable precipitation in July 2021, exacerbating low streamflows in local basins. The Palmer Drought Severity Index (PDSI) for the underscores these patterns, showing values below -2 (moderate drought) during the 1890s, 1930s, and 2000–2013 periods, with tree-ring reconstructions confirming subdecadal droughts over millennia that align with precipitation shortfalls rather than novel anomalies. data from USGS gauges near Seattle, such as those in the Cedar and Green River basins, reflect vulnerability during these events, often falling into the 0–10th percentile of historical norms, as seen in summer 2024 deficits persisting into early 2025 despite weak La Niña conditions typically favoring wetter winters. These metrics indicate that while urban Seattle's water supply is buffered by reservoirs, regional hydrological stress from cyclical dry spells has prompted emergency declarations, echoing precedents without evidence of irreversible shifts. Flood events in Seattle are often driven by atmospheric rivers, narrow corridors of intense moisture that deliver rapid heavy rainfall, leading to localized flooding and elevated river stages. A prominent example struck in November 2021, when a series of atmospheric rivers dumped over 10 inches of rain in parts of , causing record-breaking flooding on the north of Seattle and widespread landslides, with Seattle itself logging its third-wettest November on record. This event saturated soils and overwhelmed drainage, mirroring historical deluges but amplified by antecedent wet conditions in autumn. Streamflow surges during such episodes, as monitored by USGS, can exceed the 90th , straining infrastructure like closures. Cyclical alternation between and is evident in Seattle's record, with atmospheric river-driven peaks following dry phases, such as potential localized from October 2025 systems amid earlier deficits, demonstrating the region's variability tied to Pacific oscillations rather than linear intensification. Historical PDSI rebounds from lows to wet extremes further illustrate this balance, with no data supporting claims of novel magnitudes decoupled from natural variability.

Additional Weather Elements

Sunshine, Cloudiness, and Visibility

Seattle averages approximately 2,170 hours of sunshine per year, equivalent to about 50% of possible sunshine duration, which is lower than the U.S. average due to frequent cloud cover. Daily sunshine peaks at around 10 hours in July, the sunniest month with 312 total hours, while winter months average 2–3 hours per day, with December recording only 54 hours total. This seasonal disparity empirically underpins Seattle's overcast reputation, as persistent stratus decks from the cool marine layer advecting onshore suppress solar exposure, particularly from fall through spring. Cloud cover is heaviest in winter, with the sky or mostly cloudy (more than two-thirds obscured) about 70% of the time in , compared to roughly 40% in . experiences 226 days annually with heavy (at least 80% obscured during daylight hours), contributing to its ranking among the cloudiest U.S. cities. The marine stratus layer, influenced by the Pacific's cool waters and prevailing , forms low-level clouds that often persist for days, limiting breaks in cover even on non-precipitation days. Astronomical daylight varies from approximately 15 hours 59 minutes on the to 8 hours 28 minutes on the , amplifying the impact of clouds on perceived light levels. Combined with frequent overcast conditions, this results in lower effective (UV) exposure; the annual average UV index is about 4, with summer peaks of 6–7 moderated by cloudiness despite the 47°N . Visibility is routinely impaired by from the marine layer, especially in mornings when cool, moist air interacts with warmer land surfaces; reductions to less than 1 mile occur frequently from through . Dense events (visibility ≤ 0.25 miles) average 47 days per year at Seattle stations, peaking in early fall when temperature inversions trap the layer near the surface. These conditions, while rarely persistent beyond midday in summer, enhance the gloomy character of the climate during cooler seasons.

Wind Regimes and Storms

Seattle's are generally light, with average speeds at Seattle-Tacoma International Airport (Sea-Tac) ranging from 6 to 7 (mph) year-round, though slightly higher at 7-9 mph during the windier period from to . Winds exceeding 12 mph occur only 15-24% of the time, reflecting the region's sheltered position within the lowlands, which moderates velocities compared to exposed coastal areas. Seasonally, southerly or southwesterly winds dominate during the wet winter months (October-March), driven by frequent low-pressure systems approaching from the , while northerly or northwesterly flows prevail in summer, influenced by high-pressure ridges over the interior continent. These patterns result in calm conditions overall, with northerlies more frequent in the drier season due to under anticyclonic influence. Gusts commonly reach 20-50 mph ahead of cold fronts or during the passage of lines, but sustained gale-force winds (34-47 knots) are confined to major winter storms. Winter wind regimes are punctuated by extratropical cyclones that produce the region's strongest events, with gusts occasionally exceeding 60 mph inland; for instance, the Inauguration Day Storm of January 20, 1993, recorded gusts up to 70 mph at Boeing Field and 69 mph near Sea-Tac, ranking among the most intense on record for the area. Such storms, originating from atmospheric rivers or bomb cyclones, rarely sustain hurricane-force winds due to Seattle's latitude (47°N), which precludes tropical cyclone formation or persistence. Tornadoes and severe hail are exceptionally rare, with Washington State averaging only 2-3 confirmed tornadoes annually, most weak (EF0-EF1) and confined to eastern regions or isolated western outbreaks; hail exceeding 1 inch in diameter occurs sporadically in unstable summer thunderstorms but lacks frequency for routine concern.

Local and Urban Modifications

Urban Heat Island Effects

The (UHI) effect in manifests as elevated temperatures in densely developed areas compared to surrounding rural or less urbanized zones, primarily due to the absorption and re-emission of solar radiation by impervious surfaces such as and asphalt. Measurements indicate that daytime temperatures in Seattle's urban core can exceed rural baselines by 1-7°F on average, with peaks up to 17°F during extreme events, while nighttime minima remain 2-5°F warmer owing to slower heat dissipation in built environments. This local warming is distinct from regional trends, as evidenced by comparisons between downtown stations and rural references like Sea-Tac , which show urban-specific biases inflating city-center records by several degrees without corresponding rises in peripheral data. Contributing factors include reduced vegetative cover and expansive heat-retaining materials, which limit evaporative cooling and trap , effects exacerbated during dry summer periods like the 2021 heat dome when vegetation stress further diminished natural mitigation. In , areas with higher coverage exhibit amplified UHI intensity, with concrete-dominated neighborhoods retaining heat longer into evenings compared to vegetated rural outskirts. A 2024 analysis found that 58% of residents inhabit zones at least 8°F hotter than nearby non-urban areas during peak heating, with 20% facing differences exceeding 10°F; historically redlined districts average 2.1°F above the citywide UHI baseline, reflecting legacy patterns of denser development and lower tree canopy. These UHI-driven elevations in nighttime minima—often 3.5-9°F higher in terms—reduce diurnal cooling, prolonging heat stress in urban populations and biasing long-term station data toward apparent warming not mirrored in pre-urban or rural proxies. Such effects underscore the need to adjust for local anthropogenic influences when assessing broader climatic signals, as unadjusted downtown measurements overestimate regional temperature anomalies by incorporating growth-related artifacts rather than atmospheric drivers alone.

Microclimatic Variations

Seattle's microclimatic variations stem from its topography, proximity to , and interactions with regional wind patterns, leading to measurable differences in temperature and precipitation across the metro area. Areas along the waterfront, such as , tend to be cooler and more prone to due to the persistent from , which moderates daytime highs by 1-3°F compared to inland neighborhoods during summer afternoons. This cooling is enhanced by local sea breezes that develop as land heats faster than the sound's waters, drawing cooler air onshore and creating relative lows near the shore. Elevation gradients in Seattle's hills, including and Queen Anne at 300-500 feet above , produce cooler conditions following the environmental of 3-5°F per 1,000 feet ascent in the region's mountains. These elevated areas experience lower temperatures, particularly at night, and slightly higher precipitation from orographic enhancement as upslope flows interact with terrain. In contrast, lowland and inland sites show warmer profiles, with empirical vehicle-based heat mapping in King County documenting intra-metro temperature spreads of 2-5°F on clear summer evenings, driven by these topographic and exposure differences. Precipitation exhibits spatial gradients influenced by local and partial rain shadows; for instance, northern and waterfront microclimates receive less rainfall than southern or inland hills like Beacon Hill, with drier conditions near water bodies attributed to reduced and evaporation in marine-influenced air. Eastside suburbs, such as Bellevue, average 2-4 inches less annual precipitation than central (approximately 35 inches versus 37-38 inches), owing to diminished moisture from westerly storms after crossing and subtle shielding from the Cascades. temperatures often run 2-3°F cooler than readings at SeaTac Airport during peak summer conditions, reflecting downtown's greater exposure to sound breezes and less open compared to the airport's southern, more continental-influenced site.

Historical and Long-Term Data

Climate Records and Station History

The for Seattle's modern climate records is the Seattle-Tacoma International Airport (Sea-Tac) station (GHCND:USW00024233), which has recorded continuous daily summaries of , , and other variables since January 1, 1948. This airport location, at an elevation of 427 feet and approximately 15 miles south-southwest of , provides a more rural and open siting compared to earlier urban stations, potentially mitigating some influences but introducing differences in local exposure. NOAA's National Centers for Environmental Information (NCEI) maintains these records, applying pairwise homogenization algorithms to adjust for non-climatic factors such as instrument changes and minor site alterations within the airport grounds. Prior to 1948, official observations occurred at multiple downtown Seattle sites under the U.S. Weather Bureau, beginning sporadically in 1870 and becoming more systematic from 1893. Key relocations included the New York Block (1893–1905), Alaska Building (1905–1911), Hoge Building (1911–1933), and Federal Office Building (1933–1948), all situated in densely urbanizing areas near , which likely amplified recorded temperatures due to proximity to buildings and pavement. These moves necessitated retrospective adjustments in NOAA's integrated datasets to account for discontinuities, such as a noted 1–1.5°F urban warming bias in later downtown records relative to earlier sites. Auxiliary airport-area stations, like (from 1928) and initial Sea-Tac operations (from November 1944), facilitated a smoother transition to the primary site. Seattle's climate normals—30-year averages for , , and derived metrics—are computed using the NOAA-standard 1991–2020 period from Sea-Tac data, reflecting the latest decadal baseline for comparability across U.S. stations. The overall record spans over 170 years when combining adjusted downtown and airport series, though gaps persist, notably from 1871–1877 and intermittent early lapses, often addressed via nearby cooperative observers or regional interpolations for historical reconstructions. High-quality, consistent data suitable for rigorous analysis extend reliably for 80+ years from the late 1940s onward, post-relocation to Sea-Tac. Over the course of the twentieth century, annual mean temperatures in the Seattle region, reflective of broader Pacific Northwest patterns, rose by 0.7–0.9°C (1.3–1.6°F), based on analysis of cooperative observer network data from multiple stations. This gradual increase aligns with a linear regression slope of approximately 0.08°C per decade across the region, derived from homogenized historical records spanning 1900–2000. Seasonal disparities characterize the trend, with winter seasons exhibiting stronger warming—particularly in daily minimum temperatures—than summers, where maximum temperatures showed more modest gains. For instance, minimum temperatures across sites increased by up to 1.5°C in winter halves of the year, compared to less than 0.5°C for summer maxima, as documented in twentieth-century station data. The Seattle-Tacoma International Airport (Sea-Tac) record, commencing in 1948, corroborates a comparable upward trajectory in annual means, with a total rise of roughly 0.8°C (1.5°F) through 2020, though decadal-scale oscillations tied to the (PDO) introduce variability exceeding 1°C, rendering the trend subordinate to natural fluctuations in many sub-periods. The 2015–2024 decade registered as Seattle's warmest, surpassing prior periods by 0.5–1°F in decadal averages, with 2015 achieving the highest annual mean at 55.7°F (highs averaging 63.4°F, lows 47.9°F). 2024 followed as one of the hottest years on record, aligning with a positive PDO phase that amplified coastal air temperatures by 0.5–1°F above neutral conditions. influences from Seattle's expansion contribute an estimated 0.3°C (0.5°F) to station-based trends, from altered and proximity to developed areas in long-term observing sites. Over the past century, annual precipitation in has shown no statistically significant linear trend, maintaining an average of approximately 37 inches (940 mm) based on records from downtown stations dating back to the late and Sea-Tac Airport since 1945. Variability driven by the (PDO) has dominated patterns, with positive PDO phases correlating to drier conditions and negative phases to wetter winters in the , overshadowing any subtle long-term shifts. Seasonally, observations indicate minor increases in winter and spring precipitation since the early 1900s, potentially linked to enhanced storm tracks, but these are offset by stagnant or slightly declining summer totals, resulting in neutral annual balances. For instance, November remains the wettest month with averages around 6-7 inches, while July typically sees under 1 inch, a pattern consistent across decades without pronounced shifts. Historical data reveal wet decades like the 1950s, when 1950 recorded the all-time high of 55.14 inches, contrasted by drier periods such as the early 1950s and 1920s. In the 2020s, has exhibited high variability, with persistent negative PDO phases contributing to episodic dryness; for example, through 2025, lagged 8.5 inches behind normal since 2024, including a 6-inch deficit since January, amid statewide rankings placing July 2025 as the 22nd driest on record. Regarding extremes, while individual events like atmospheric rivers have produced intense downpours (e.g., record daily totals in recent winters), analyses of hourly and daily records from 1949-2007 show no significant increase in frequency of heavy days in , with trends attributable more to natural oscillations than monotonic change.

Climate Change Analysis

Observed Changes and Attribution

Records from Seattle-Tacoma International Airport (Sea-Tac), the primary long-term station since 1945, indicate an increase of approximately 4.2°F in annual mean temperature from 1948 (49.7°F) to 2024 (53.9°F), equivalent to a warming rate of about 0.55°F per decade. This trend exceeds the global average of roughly 0.2°F per decade over the but reflects regional patterns in the , where observed changes include more frequent hot days and fewer cold extremes. totals have remained stable at around 30-40 inches annually over the same period, with no statistically significant long-term increase or decrease, though seasonality has shifted toward reduced snowfall—historical peaks like 53 inches in January 1950 contrast with rarer major events in recent decades—and more winter falling as rather than due to milder temperatures. Extreme heat events have become more notable in Seattle's record; prior to 2009, no daily maximum reached 100°F at Sea-Tac, but the July 2009 event hit 103°F, followed by multiple triple-digit readings during the June 2021 heat dome, which peaked at 108°F on June 28. Summers show no evidence of extension or drying trends in precipitation data, with average June-August totals stable and wettest summers like 1968 (8.43 inches) comparable to recent variability. The (IPCC) attributes virtually all global observed warming since the mid-20th century to anthropogenic , estimating human influence responsible for approximately 1.1°C of the total change. For the , however, analyses highlight substantial contributions from natural variability, including (PDO) phases and wind pattern shifts, which explain over 80% of coastal temperature trends in some 20th-century reconstructions. (UHI) effects from Seattle's metropolitan growth have amplified local readings, with urban areas experiencing boosts of 6-8°F above rural baselines, potentially accounting for a significant fraction of station-specific warming at the developed Sea-Tac site. Event-specific studies, such as for the 2021 , link anthropogenic warming to increased likelihood (at least 150-fold) and intensity of extremes, though baseline natural drivers remain influential regionally.

Natural vs. Anthropogenic Drivers

The (PDO) and El Niño-Southern Oscillation (ENSO) represent primary natural modes of variability influencing Seattle's climate, driving multiyear to decadal fluctuations in and through alterations in Pacific sea surface temperatures and . During positive PDO phases, such as the mid-20th century (roughly 1925–1946 and 1977–1998), the experienced warmer, drier conditions with reduced storminess, while negative phases, including the current one since approximately 2008, correlate with cooler s and enhanced , potentially masking any underlying long-term warming signal in the 2010s and 2020s. ENSO events amplify these effects: El Niño winters typically bring milder, drier conditions to Seattle with below-average rainfall, as seen in the strong 2015–2016 event that contributed to a 20% deficit, whereas La Niña phases enhance wet, stormy patterns. These oscillations introduce substantial internal variability, with PDO shifts explaining up to 50% of decadal variance in the region, complicating isolation of forced trends. Local factors, including the (UHI) effect, further amplify natural variability in Seattle's observed temperatures without invoking anthropogenic dominance. Urban development has elevated maximum daytime temperatures by 4–6°F above rural baselines through heat retention in and asphalt, with some analyses indicating up to 8°F enhancements in densely built areas like . This UHI signature persists year-round but intensifies during calm, clear conditions associated with natural modes like La Niña, contributing disproportionately to urban warming records independent of regional atmospheric forcings. Seattle's baseline mildness, characterized by oceanic moderation from and prevailing , underscores geography as the dominant control, rendering small-scale radiative perturbations from greenhouse gases secondary to these persistent natural and local dynamics. Anthropogenic influences, primarily through CO2 estimated at +1.0–1.5 W/m² since pre-industrial times, have contributed to the modest century-scale rise of about 1.5°F in , yet attribution remains contested due to intertwined natural signals and model limitations. Fingerprint analyses, which seek unique spatial-temporal patterns of warming (e.g., tropospheric amplification), reveal mixed evidence in the , with no unambiguous anthropogenic signal disentangled from PDO/ENSO modulation. trends exemplify this ambiguity: 's annual totals have shown no statistically significant century-scale increase, fluctuating around 37 inches with stasis or slight declines in extremes attributable to internal variability rather than forced changes, as confirmed by station data lacking clear humidification fingerprints expected under forcing. Critiques highlight that climate models, including early projections, often overestimated PNW warming by failing to fully resolve natural decadal oscillations, with CMIP ensembles exhibiting biases toward excessive tropical and mid-latitude amplification relative to observations. Such discrepancies, noted in evaluations of historical simulations, suggest over-attribution to anthropogenic drivers when empirical records prioritize variability-inclusive realism over smoothed forcing assumptions.

Projections, Uncertainties, and Empirical Critiques

Climate models project an increase in Seattle's average annual temperature of approximately 2–4°F by mid-century (around 2050) under moderate emissions scenarios, with summertime maximum temperatures potentially rising up to 6°F by the same period under higher-emissions pathways like RCP8.5. Annual precipitation is anticipated to rise modestly, by about 2–4 inches on average, though with greater uncertainty in seasonal distribution. Sea-level rise at Seattle's waterfront is forecasted at roughly 1 foot by 2050 and 2–5 feet by 2100, relative to late-20th-century baselines, driven primarily by thermal expansion and glacier melt. These projected shifts remain modest compared to Seattle's natural interannual variability, where temperature fluctuations can exceed 2°F and precipitation can vary by 10+ inches in any given year due to modes like the Pacific Decadal Oscillation (PDO). Major uncertainties in these projections stem from incomplete representation of cloud feedbacks in global climate models, which amplify or dampen warming and responses, particularly in mid-latitude regions like the (PNW). PDO phase transitions, which modulate PNW temperature and dryness over decades, introduce additional variability that models struggle to predict with high skill beyond a few years, as evidenced by initialized forecast limitations in coupled models. Regional models for the PNW, while improving resolution, have historically underperformed in capturing fine-scale orographic and blocking patterns, leading to biases in extreme event simulation. Empirical critiques highlight discrepancies between model projections and observations, such as overstated attribution of extremes to anthropogenic forcing without accounting for natural analogs. The 2021 PNW , which pushed -area temperatures above 108°F, has been linked by some studies to a 150-fold increase in likelihood due to , yet local analyses argue it aligns with rare but precedented blocking events akin to 1930s Bowl-era patterns, amplified by deficits rather than solely greenhouse gases. Projections of a shift toward wetter winters or monsoon-like patterns in the PNW have not materialized consistently, with historical model runs from the – overpredicting increases that failed to exceed natural variability. Recent underscore these gaps: despite expectations of rising overall wetness, recorded below-normal in 2024 (about 90% of average statewide) and into 2025, with a persistent dry streak since October 2024 leaving the city 8.5 inches in deficit by September 2025, and summer 2025 ranking as the 10th driest on record. Such mismatches suggest that internal variability and unresolved processes may dominate short-term regional outcomes over projected trends.

References

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