Hurricane Schwartz: Severe storm alert for Philadelphia

Note: This is a previous forecast.  For the current outlook please go to Philly.com's weather page, powered by NBC10.

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A line of showers in Central Pennsylvania will move into the Philadelphia region this afternoon and strengthen into thunderstorms. NBC10 has issued our own "First Alert" for the chance some of the storms could become severe. That means downpours, locally damaging winds, and hail. Most of us won't experience those conditions, but some could.

Several factors go into making a storm threat severe. It's like "Goldilocks" - everything has to be just right: ground-to-atmosphere temperatures, winds and moisture. If the ground doesn't heat up enough, there will be no severe storms. If the wind is too strong or in the wrong direction at any level, there will be no severe storms. If it's too humid throughout the atmosphere, there will be no severe storms. This is why most thunderstorms are not severe.

What's the official definition of a severe thunderstorm? The storm has to either produce wind gusts of 58 mph or more, hail at least 1" in diameter, or a tornado. Heavy rain or lightning are not specific parts of the National Weather Service definition, but we do consider those threats in deciding on a "First Alert" weather day.

Thunderstorms will be moving through, generally after 3 p.m. Some computer models suggest the storms could stall over part of our area this evening. That would limit the severe storm threat but increase the localized flood threat. The evening rush will likely be affected by the storms, so a slow commute should become even slower.

Even though some thunderstorms are possible Thursday, the threat of them being severe is significantly lower than today.

Note: The National Weather Service has issued a severe thunderstorm watch for Philadelphia and the surrounding area through 7 tonight.

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You can watch Glenn "Hurricane" Schwartz every weekday on NBC10 at 11 a.m., 4 p.m. and 6 p.m. Follow him on Twitter @HurricaneNBC and stay informed when weather changes on the NBC10 app and with the NBC10 First Alert Weather Team on Facebook: NBC10Weather.