Forecasting Severe Weather

Although not as common as in other parts of the country, the Mid-Atlantic Region does have severe thunderstorms on occasion.  Severe thunderstorms require a unique set of elements to develop such as a warm and humid air mass combined with a lifting mechanism such as a cold front.  When cooler and drier air displaces warm and humid air, strong to severe thunderstorms often ensue.  That’s different than the typically short-lived showers and thunderstorms that develop on a hot and humid afternoon or near the beach during the summer.  Specific criteria that makes a thunderstorm severe include wind gusts of at least 58 mph, hail at least 1” in diameter or a tornado. 

Since so many elements go into forecasting thunderstorms and severe weather – such as atmospheric stability, the presence of a lifting mechanism (like a front), as well as local geography – the forecast needs to be refined multiple times to ensure accuracy.  To give members of the public an idea of when and where thunderstorms may occur, NOAA’s Storm Prediction Center issues “Convective Outlooks” for the United States.  Their outlooks are great at providing a benchmark a few days in advance of any potential severe weather.  However, it’s critical that people pay attention for updates to their local weather forecast since so many, ever-changing variables are involved in forecasting showers and thunderstorms.

This past week, the Mid-Atlantic Region had a significant severe weather outbreak with multiple reports of severe wind gusts and hail.  In fact, unusually large baseball-sized hail occurred in Loudoun County, Virginia.  NOAA’s Convective Outlook for June 16 showed an “enhanced risk” for severe weather from southwest Pennsylvania to west-central Virginia.  The map of where severe weather occurred corresponded remarkably well to the outlook meaning this severe weather event was well-forecast.

Earlier this month, there was another instance where a large portion of the Mid-Atlantic Region was under an “enhanced risk” for severe weather.  The area at “enhanced risk” was larger on June 5 than it was on June 16.  It included the major metropolitan areas of Washington, D.C., Baltimore, MD and Philadelphia, PA.  Unlike June 16, different weather conditions on June 5 caused most of the severe weather to occur outside the DC Metro Region.  While it’s important to notify the public to the potential for thunderstorms on a given day, it’s impossible to say a thunderstorm will develop or track over a particular town more than a few hours in advance.

A common misconception is that the entire area under an “enhanced risk” for severe thunderstorms will see it.  What it actually illustrates is that a particular part of the country has the right combination of elements for a greater than average or an “enhanced” chance for severe thunderstorms.  People can understandably get upset if they change their plans when there is an elevated potential for a given type of weather that doesn’t ultimately occur in their town.  However, people also need to realize that there is an inherent amount of uncertainty in weather forecasting.  That said, the accuracy of weather forecasts have improved greatly – even in the last ten to twenty years – as the forecasting tools such as the computer forecasting models and weather radar have improved.