Tuesday, May 3, 2022

Last-Night's Blow

A strong outflow boundary initiated by a precipitation system near the Utah–Nevada border produced strong winds across northern Utah last night.  Peak gusts at lowland locations included 70 mph in Dugway Proving Ground, 67 mph at the Salt Lake City International Airport,  and gusts above 60 mph at several sites near and around the Great Salt Lake.  

Outflow boundaries, also referred to as gust fronts, form from cooling within dry air beneath a thunderstorm or precipitation system.  This results in a downdraft that spreads when it hits the ground, with a sharp contrast in temperature, wind speed, and wind direction commonly found at the leading edge, which can travel in some instances more than 100 km from the initiating storm.  

Observations from the Salt Lake City International Airport show the structure of the outflow boundary as it entered the Salt Lake Valley.  Ahead fo the outflow boundary, temperatures were in the low 70s and the wind was from the south to southeast, gusting to as high as 40 mph.  With the passage of the outflow boundary, temperatures rapidly dropped about 15˚F, the wind shifted to westerly with a gusts of over 65 mph for a 5–10 minute period.  Then, the winds shifted more gradually to northwest-north and tapered off to gusts in the 20 to 35 mph range for about four hours.

The Severe Thunderstorm Warning issued by the National Weather Service for the storm caused some confusion since there only seemed to be strong winds and no thunder.  

I had a couple of friends text me asking where the storms were and why a hail threat was indicated because they saw nothing on radar. 

The National Weather Service can issue a variety of warnings, watches, advisories, and statements depending on the situation.  A severe thunderstorm warning is issued for a thunderstorm when radar, weather observations, or spotter reports indicate winds of 58 mph or greater, and/or hail of at least 1".  Lightning frequency is not a criteria.  Typically such warnings are for short periods of an hour or less.  A high-wind warning is issued when conditions are expected to produce sustained winds of at least 40 mph or gusts of 58 mph or greater.  

It could be that a decision was made to issue a severe thunderstorm warning because of the rapid onset, intensity, and origin of the winds.  A look at lightning data from yesterday shows some activity in the precipitation system that initiated the outflow boundary and winds well to the northwest of Salt Lake City.  In Utah, high wind warnings are often issued with significant lead time, as occurs for downslope windstorms or strong pre-frontal wind events, which wasn't possible last night.  These issues may have factored into the decision to issue a severe thunderstorm warning.  


  1. Severe thunderstorm warnings would be better called (but more poorly understood as) convective wind warnings. Lightning and precipitation aren't requirements, but usually it has to at least be a "trackable" feature with 50 kt gusts. High Wind Warnings, like other "long fuse" products are usually issued for events lasting 2 hours or more that are usually gradient related.

  2. It was definitely a combination of things. Looking at the data I would classify it as primarily a strong cold frontal passage, although precipitation behind the front strengthened the surface gradient.