2021 Severe Weather Outlook: How do we forecast severe weather?


Springtime in Pennsylvania is the season of change, and that change is what often leads to our most violent displays of mother nature.

Last year’s severe weather season was fairly quiet, with only 10 tornadoes confirmed state-wide during the entire year, none in our viewing area. This year, we’ve seen an early start to severe weather across the southern states. What about here in Pennsylvania?

Dr. Greg Forbes was the severe weather expert on the Weather Channel for 20 years. He grew up in Pennsylvania and spent his graduate life studying tornadoes alongside Ted Fujita, the man responsible for developing the F scale. Dr. Forbes was a professor at Penn State during the infamous 1985 tornado outbreak.

“I was initially monitoring the outbreak using the radar that we had at the Penn State meteorology department and taking pictures of the radar screen, and afterwards I did damage surveys for myself as well as to help Dr. Fujita out,” said Dr. Forbes.

Outbreaks like 1985 are extremely rare, but we do get tornadoes every year in Pennsylvania.

The ingredients we look for when forecasting severe weather include a moist, unstable air mass and strong vertical wind shear, or the change of wind with height. The typical setup for tornadoes in our area is southerly or southeasterly winds at the ground, and southwesterly or westerly winds aloft. Historically, our southern counties have been hit by tornadoes the most.

“For the Lancaster and York area, they’re in the Lee of the Appalachian mountains, so there’s a little bit of a funneling upward or a lee trough that helps pull in that moisture there,” said Dr. Forbes

He says something else to watch out for are areas of drought to our west, which can aid in capping storms initially, thereby making them stronger when do develop. 

“It can inhibit ordinary thunderstorms and allow the storms to hold off until colder air and a bit of jet stream energy comes in aloft,” said Dr. Forbes. “The cap breaks and you get an outbreak.”

Forecasting long range severe weather is difficult, but the global weather pattern can give us clues as to what will happen.

Dr. Forbes says a La Nina typically brings more intense severe weather outbreaks nationally, but they tend to be earlier in the season. This month, high pressure along the North Pole will bring down cooler and more stable air into the northeast, making severe weather less likely during that optimal time.

As warm air surges back north, look for an uptick in severe weather season as we head into late April and May. The most frequent severe weather will happen in June and July, when Pennsylvania sees the most Gulf of Mexico moisture. However…

“You can get severe weather year-round and sometimes if there are strong cold fronts, even in November,” Dr. Forbes added. “There can be a bit of a second season and sometimes those are little tornado circulations and especially high wind events. 

A good reminder is to be prepared no matter your location – and no matter the time of year. For ABC27 News I’m meteorologist Adis Juklo.

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