Snow squall warning

A snow squall warning (SAME code: SQW) is a bulletin issued by Environment Canada's Meteorological Service of Canada and the National Weather Service in the United States to warn population of two types of snow events reducing visibility in blowing snow: lake effect snow squalls and frontal snow squalls.

Lake effect snow squalls are generated by cold arctic air moving over unfrozen water of lake or sea. These will reduce visibility to less than 1 km and produce large accumulations of snow on the ground along narrow corridors in lee of the waters. Duration of these events can extend for days.

Frontal snow squalls are associated with a fast moving intense cold front in winter. Visibility must be reduce to less than 500 m, wind over 40 km/h with a wind shift. Quantities of snow is not important with this type but intensity is heavy for a short period, typically 10 to 20 minutes.

Squall warnings, reduce visibility

In Canada, snow squall warnings are issued for conditions expected to be:

Beginning November 1, 2018, the snow squall warning was declared operational in the United States, and the capability to issue these warnings was rolled out to all NWS offices in the US. These issuances are intended to warn drivers of potentially life-threatening road conditions. The warnings are issued in a polygon area, to make the warned area as localized as possible, similar to a tornado, severe thunderstorm, or flash flood warning.

Criteria for the National Weather Service to issue a snow squall warning include:

In the winter season of 2023-2024, the National Weather Service started using Impact-Based Warnings for snow squall warnings. Previously, all snow squall warnings would prompt a Wireless Emergency Alert. However, only warnings tagged 'Significant' will trigger an alert, while 'General' warnings, those without an impact tag, will not.