Hurricane Stan was the deadliest tropical cyclone of the 2005 Atlantic hurricane season. A relatively weak system that affected areas of Central America and Mexico in early October 2005, Stan was the eighteenth named storm and eleventh hurricane of the 2005 season, having formed from a tropical wave on October 1 after it had moved into the western Caribbean. The depression slowly intensified, and reached tropical storm intensity the following day, before subsequently making its first landfall on the Yucatán Peninsula. While traversing the peninsula, the tropical storm weakened, but was able to re-intensify once it entered the Bay of Campeche. Under favorable conditions for tropical cyclogenesis, Stan attained hurricane strength on October 4, and later reached peak intensity with winds of 80 mph (130 km/h) and a minimum barometric pressure of 977 mbar (28.9 inHg). The hurricane maintained this intensity until landfall in the Mexican state of Veracruz later the same day. Once over the mountainous terrain of Mexico, however, Stan quickly weakened, and dissipated on October 5.

Due to Stan's position within a large area of convective activity and thunderstorms, the hurricane's effects were far-reaching and widespread across Central America. Flash floods generated by the hurricane caused severe crop losses, particularly to coffee crops. Overall, Stan caused at least 1,673 deaths across six countries, with many others unaccounted for. Most of these fatalities occurred in Guatemala, and were mostly caused by mudslides triggered by torrential rainfall. The floods in Guatemala destroyed entire towns and disrupted exportation of petroleum. In Mexico, the heavy rains triggered additional mudslides and caused rivers to overflow, flooding nearby villages. Despite being relatively far from Stan as opposed to other countries, El Salvador was also severely affected by the hurricane. The Santa Ana Volcano erupted while Stan was producing heavy rains in the country, which contributed to the damage already wrought by mudslides. Transportation in the country was disrupted. Across the region, Stan caused $3.9 billion in damages, primarily due to torrential rainfall.

A tropical wave left the west coast of Africa on 17 September 2005, which the National Hurricane Center considered the likely origin of Hurricane Stan. For several days, the wave moved westward across the tropical Atlantic Ocean without any signs of development. The associated thunderstorms, or convection, increased on September 22, although the presence of wind shear prevented further organization. The wave entered the eastern Caribbean Sea on September 25. Around that time, the NHC identified the wave in their tropical weather outlook, noting that "any development [would] be slow to occur." The thunderstorms became more consolidated by September 27, as upper-level conditions became more favorable. The system's organization fluctuated, developing a weak circulation and broad low-pressure area on September 28 over the western Caribbean Sea. The Hurricane Hunters flew into the system on September 29, observing a broad area of thunderstorms with insufficient organization to be classified a tropical cyclone. Over several days, the system failed to consolidate and was "slowly festering", as described by NHC forecaster Stacy Stewart. On October 1, the circulation became more defined, developing into Tropical Depression Twenty about 215 km (130 mi) southeast of Cozumel.

Upon its development, the depression's circulation was broad, with several small vorticities. The associated convection developed outflow as the overall system moved generally westward, steered by a ridge over the northern coast of the Gulf of Mexico. The NHC anticipated that the system's passage over the Yucatán Peninsula would tighten the wind field and allow for further development in the Bay of Campeche. Early on October 2, the depression intensified into Tropical Storm Stan, and at 10:00 UTC that day it made landfall in eastern Mexico near Punta Hualaxtoc, Mexico, roughly 35 mi (55 km) south of Tulum. For much of its duration, Stan was associated with a Central American gyre, which was a much larger circulation covering eastern Mexico and Central America. After moving ashore the Yucatán, Stan traversed the peninsula in about 18 hours, emerging into the Bay of Campeche as a tropical depression on October 3.

After emerging into the Bay of Campeche, Stan had a well-defined circulation with an anticyclone aloft, and was moving over an area of nearly 30 °C (86 °F) waters. Thunderstorms soon redeveloped, and the depression quickly reattained tropical storm status. A strong area of high pressure over the western Gulf of Mexico forced the storm to turn southwestward, back to the Mexican coastline. Initially, the convection was mostly limited to the eastern periphery. Early on October 4, the convection increased significantly over the center as well-defined rainbands organized into an eyewall. At 06:00 UTC that day, Stan intensified into a hurricane, and around that time it turned more to the southwest. Six hours later, the hurricane made landfall in the Mexican state of Veracruz near Punta Roca Partida, about 150 km (90 mi) east-southeast of the city of Veracruz. It was the first hurricane landfall in the state since Hurricane Gert in 1993. Stan moved ashore with sustained winds of 80 mph (130 km/h), and a minimum pressure of 28.85 inHg. The storm rapidly weakened over the mountainous terrain, and Stan dissipated early on October 5 over the state of Oaxaca.

After Stan's development, the Government of Mexico issued tropical storm warnings from Chetumal to Cabo Catoche along the eastern Yucatán peninsula, with a tropical storm watch issued westward to the city of Campeche. These were dropped after Stan moved over the Yucatán peninsula. After the storm moved over the Bay of Campeche, Mexico issued hurricane warnings between Cabo Rojo to Punta El Lagarto on October 3, about 27 hours before landfall. Additional warnings were expanded to include more of the coast. The NHC had anticipated that landfall would not occur for 48 hours; it later wrote that the agency had "not predicted very well", describing the southwest path as "unexpected".

Ahead of the storm, state oil company Pemex evacuated 270 employees from five platforms in the Gulf of Mexico, while also closing three oil exporting ports.

In Veracruz state where Stan moved ashore, about 38,000 people evacuated their homes, utilizing thousands of shelters. Schools were canceled in the state. Elsewhere, about 600 families evacuated in Chiapas in areas near the Guatemalan border.