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.
Contents
Phenomenon
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.
Criteria
Squall warnings, reduce visibility
Canada
In Canada, snow squall warnings are issued for conditions expected to be:
Heavy snow off an open body of water producing at least 15 cm accumulations per 12 hours and reducing visibility to less than 400 meters over a limited region.
Line of convective snow showers, associated with a cold front, reducing suddenly and for a short period the visibility in snow and blowing snow, causing difficult road conditions.
United States
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:
Visibility of less than one quarter of a mile
Sub-freezing temperatures on the ground
Expected to last in one area less than 60 minutes
Cause dangerous and life-threatening conditions
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.
US Example
The following is an example of a snow squall warning issued by the US National Weather Service in Binghamton, New York, on February 2, 2018.
148
WWUS51 KBGM 021452
SQWBGM
BULLETIN - IMMEDIATE BROADCAST REQUESTED
Snow Squall Warning
National Weather Service Binghamton NY
950 AM EST Fri Feb 2 2018
NYC007-017-023-053-077-107-109-021545-
/O.NEW.KBGM.SQ.W.0002.180202T1450Z-180202T1545Z/
Otsego NY-Broome NY-Madison NY-Cortland NY-Tioga NY-Tompkins NY-
Chenango NY-
950 AM EST Fri Feb 2 2018
The National Weather Service in Binghamton has issued a
* Snow Squall Warning for...
Central Otsego County in central New York...
Northwestern Broome County in central New York...
Southern Madison County in central New York...
Cortland County in central New York...
Tioga County in central New York...
Tompkins County in central New York...