A warm front moving northward tonight into Sunday should be the initial focus for scattered showers and isolated thunderstorms. Some of the showers and thunderstorms will be slow moving and could yield some heavy rainfall. Later on Sunday, a weak trough will move into the increasingly humid air. This should increase the shower and thunderstorm activity. Some thunderstorms could become severe with the main threat being damaging wind gusts or wet microbursts. These storms could be slow moving and there is the potential for some flash flooding. No flash flood watches have been posted at the moment, but this does not mean a short-fused warning cannot be issued. At present, the Day Two Outlook from the Storm Prediction Center has us in a see text or 5% probability of severe weather within 25 miles of a point. But this may be upgraded to a slight risk in later outlooks, especially if some sun can manage to peak though the clouds in a large portion of the area before the trough quickly bubbles up new development after the warm front passage.
Temperatures on Sunday will be very tricky. Quite a bit of cloudiness may prevent us from reaching into the nineties. Should the sun come out for a substantial period of time, then 90 degrees or better is not out of the question. The most sunshine should be south of Philadelphia. We saw that the cloudiness we had today held temperatures down into the seventies and lower eighties. I am not very confident at all about the temperature forecast for Sunday. There is going to be likely a huge difference between Mount Pocono and the Delmarva with the airport falling somewhere in the middle.
On Monday, there does appear to be some decent breaks developing in the clouds to push our temperatures past 90 degrees. On Tuesday, temperatures will be pushing into the upper nineties with some isolated 100 degree readings. A very isolated thunderstorm may develop on these two days in the afternoon or evening, unless we see a MCS develop. We are going to be watching for any potential MCS that develop to our west during this period, but it is difficult to forecast if they will make it into our region this far out. An MCS or remnant cloud debris, depending on the timing, would play a role in our temperatures.
For Wednesday, it will be oppressive with a surge of hot air ahead of a cold front swinging down from the northwest. Temperatures should at least rise into the mid and upper nineties, if not close to 100 degrees again. Heat Index Values of at least 100 to 108 degrees are possible. As the cold front interacts with the extreme heat, another round of organized severe thunderstorms is possible. Excessive Heat Watches are posted for Monday into Wednesday for the urbanized counties surrounding and including Philadelphia and this will likely be upgraded to a warning on Sunday. Heat Advisories or Excessive Heat Warnings may be issued for rural counties when the time comes for most likely Tuesday and Wednesday when the heat peaks.