What’s Fueling Pennsylvania’s Recent Severe Weather Pattern?

April 20, 2019 GMT

The severe weather that has battered Pennsylvania and other parts of the Mid-Atlantic region this month is due to an active spring pattern bringing just the right mix of atmospheric ingredients together, weather analysts say. The pattern is no surprise to the experts, as April kicks off what is typically the most active three-month period of the year for severe weather outbreaks in the United States. What may bewilder more astute weather watchers is preliminary data from the National Oceanic and Atmospheric Administration depicting a significant year-over-year increase in severe weather events across the state. As of Tuesday, it showed 85 instances of tornadoes, large hail or wind damage so far this year. Only 10 instances were reported in the same time period in 2018. The comparison of year-to-year data needs perspective to be understood and isn’t as alarming at it may seem, says John Hart, lead forecaster, operations branch, for NOAA’s Storm Prediction Center. He said there is no known trend in the data or pattern that indicates severe storms are becoming more frequent in Pennsylvania. The state is simply stuck in an active period and typically has such patterns separated by a few quiet years. What makes this spring different Empire Weather meteorologist Ed Vallee, who provides forecasts for The Morning Call, said the setup for severe weather has been key thus far. That’s because we’re still inside the time of year where heat and humidity are limited. Vallee said a volatile weather pattern begins with a strong cold front — typically one that’s been moving across the country for several days. The cold, dry air of this front plows directly into an early season warm, moist airmass, forcing the warm air upward in the atmosphere, where it condenses into clouds that produce severe storms. “Beyond this forcing factor, it’s usually tough to get the pop-up storms that we can typically see in the summer months because we don’t have enough instability,” Vallee explained. How CAPE fuels severe weather There is one part of the severe weather pattern with a memorable acronym — CAPE. It sadly has nothing to do with superheroes, but stands for convective available potential energy. Quite simply, CAPE is the amount of “fuel” available to a developing thunderstorm, Vallee says. More specifically, it describes the instability of the atmosphere and is calculated based on temperatures changing with altitude, as well as the amount of moisture available. It’s used by weather officials to understand what the potential might be for thunderstorms to develop, as well as how powerful those storms might become if they do materialize. “I really like the ‘fuel’ analogy,” Vallee said. “When you bring a match (in this case, the approaching cold front) into an airmass with fuel (CAPE), typically things go boom.” In sustained pattern, forecasters focus on lead time As severe weather approaches, forecasters race to convert the shifting data into a public warning that could save lives. They’re getting faster at doing it. When the National Weather Service issues a warning, it means severe weather is occurring or imminent. But the lead time on these storms for the purpose of protecting life and property has become a focus for meteorologists in the private and public sectors. “We’ve gotten a lot better at the lead time of these systems because technology has advanced,” Vallee said. Hazardous weather outlooks are sometimes issued 5-to-7 days in advance of severe weather, while watches are issued when storms are in or near a specific area. Warnings come once severe weather has been reported by trained spotters or indicated by radar. Vallee pointed out the timing of watches and warnings ultimately depends on the setup of the storm and how it evolves. No matter what, residents of storm-prone states are encouraged to rely on weather radios specifically designed to monitor government alerts, as well as smartphones and other technology, to keep on top of potentially deadly storms. Hart said continued severe weather outbreaks in Pennsylvania will depend on whether this current pattern continues, especially into May and June, when severe weather is more typical in our area.