By Kare Bilo
The Memorial Day holiday has come and gone, and it didn’t quite feel like May – with a high of 91 and afternoon Heat Index values as high as 95 in the city, it felt more like the 4th of July. And today is even hotter!
The mechanics are these: a summerlike pattern with a large Bermuda high pumping moisture-rich air in from the South (moisture that has been amplified by the tropical system Beryl over the southeast), and a cold front slowly pushing in from the west. The advance of the cold front is serving to push in even more heat and humidity ahead of it’s passage, meaning temperatures as of mid-afternoon are already in the 90′s and climbing.
The National Weather Service in Mount Holly added the following to their discussion today: THE HEAT INDICES TODAY WILL PROBABLY RUN 2 TO 4 DEGREES WARMER THAN YESTERDAYS MAXIMUMS WHICH WILL PROBABLY OCCUR AGAIN TODAY AROUND 4 PM OR 5 PM. Yesterday’s maximum heat index values were up around 95-96, which means today’s values could reach close to 100 degrees. Because of this, Philadelphia Public Schools have been dismissed as of 1:30 this afternoon. In this kind of heat, you want to stay in air-conditioned areas as much as possible (malls, libraries, churches are all good ideas in a pinch), drink lots of water and don’t over-exert yourself. Also check on anyone you think may need some help.
Now on to the thunderstorm threat. Behind the cold front, there is much cooler and drier air and the clash of these two drastically different air-masses has already turned violent this afternoon – and the storms are headed in our direction. The heat and humidity have fostered a very unstable environment and the storms will intensify quickly as they progress eastward. While the greatest risk for tornadoes exists off to our north over NY state and New England thanks to increased directional shear, the threat does exist for super cell thunderstorms with severe impacts in our region – damaging straight-line wind gusts in excess of 60mph, large hail and, with all the moisture and precipitable water in the atmosphere, drenching downpours. In fact, a Flash Flood Watch has already been issued for our area under the supposition that any storm could drop a quick 1-2″+ of rainfall in the course of an hour or less, leading to the risk for quick ponding and flooding on the roadways.
Storms should really begin moving through our area by about 5 o’clock, with the greatest risk in the city of Philadelphia around 7 p.m. (a few rogue cells have been developing in advance of the main line of storms, meaning we could potentially get hit a little earlier than that).
After all that, I have to give you some good news! Behind the front we’ll usher in a less humid, more comfortable air-mass later Wednesday into Thursday. Stay safe out there this evening!