When you think of winter weather hazards, what comes to mind? Do you think of big snow storms that drop a significant amount of snow? Do you think of ice storms that cover trees, powerlines, and roadways in a glaze of ice? Or maybe you think of flooding that can occur when a storm drops heavy rain on top of melting snow, overwhelming creeks, streams, and rivers with water.
Pennsylvania sees many forms of hazardous winter weather, including snow squalls. A snow squall is a brief but intense period of heavy snow (up to 2 inches in 30 minutes), strong winds (30+ mph), and whiteout conditions (visibility less than ¼ mile). Snow squalls often occur on days with otherwise partly cloudy skies. Coming on so suddenly, snow squalls can catch drivers off guard and lead to major transportation impacts, including deadly multi-vehicle accidents.
One of the things that makes snow squalls so dangerous is their tendency to produce icy roadways. Since snow squalls usually occur on days with partly cloudy skies, road surface temperatures can start off well above the freezing point before snow squalls arrive. When the snowflakes start to fall, they initially melt on the warm road surface, making the road wet. However, as the snow becomes heavier and air temperatures fall, the road surface cools and liquid water turns to ice. Ice can also form on road surfaces that start out below freezing, especially when moving traffic drives over fallen snow, packing it down and warming it just enough to become icy.
The National Weather Service uses several tools to predict and detect snow squalls. First, forecasters analyze weather model simulations and apply the latest forecast methods to diagnose setups that are favorable for the development of snow squalls. When snow squalls are anticipated, the NWS gets the word out through forecast discussions, social media, and special weather statements. The National Weather Service briefs partners, including PennDOT, PEMA, and many others, including public safety and emergency management officials.
Broadcast meteorologists also do a great job in getting the forecast out to the public.
As the snow squalls begin to develop, the NWS uses weather surveillance radar and high-resolution satellite imagery to monitor their intensity and track their movement. The NWS also monitors ground-based weather observations from the Pennsylvania State Mesonet and the PennDOT Road Weather network. All of these instruments measure temperature, wind speed, and precipitation rate, and many of them also measure visibility and road surface temperature. SKYWARN observers also notify the NWS when they observe snow squalls.
In 2018, the National Weather Service began issuing Snow Squall Warnings to alert for the sudden onset of life-threatening conditions encountered by highway travelers during snow squalls. The warnings are "short-fused," only issued for up to about 1 hour, and are designed to be acted upon immediately. To make sure that the public receives them, these warnings activate the Wireless Emergency Alert system and will push audible alert notifications to smartphones.
If you are driving on an interstate when a Snow Squall Warning is issued, the best thing to do is to exit the roadway at the next opportunity. If you do get caught driving in a snow squall, avoid slamming on your brakes, turn on your headlights and hazard lights, stay in your lane, and increase your following distance.
Before you get on the road this winter, make sure you are weather-aware. Check the NWS forecasts at weather.gov and on social media. If a snow squall warning is issued by the National Weather Service, delay travel until after the snow squall moves through. Use the 511 PA App or visit 511pa.com to check the latest traffic conditions, traffic speeds, and live camera images. For more, visit www.weather.gov/ctp/snowsquall