Updated Tue 07:12 PM — Some showers and thunderstorms are possible after midnight tonight, before daybreak Wednesday morning.  Most likely north of the PHL area.

Most of today’s models have been more consistent with a forecast of severe thunderstorms between 3 PM and 8PM Wednesday in the PHL area.  The instability parameter values are very impressive and severe storms are likely. However, the GFS model still seems to keep the heavy activity north of the PHL area. .

Here’s a new HIREF (Experimental) model displaying “Maximum Updraft Helicity“;  high values are associated with potential tornadic development for 6 PM Wednesday—

We’ll have to wait for tomorrow morning’s models to pin down the exact placement of the most severe weather. 

There’s much uncertainty with the specifics of the weather forecast over the next few days.  While the general setup I discussed here on Sunday’s outlook post remains generally intact, the difficulty is in the details.

First, for today, there are wide differences in the model forecasts for the amount of cloudiness today.  Some models have it very cloudy; some like the NAM-NEST have considerable breaks of sunshine.

For the showers tonight, some models have them falling apart as they approach Philadelphia, others have the bulk of activity moving just to our north or just to our south. 

The same is true for Wednesday, where the timing and placement of the largest storms varies between models; the gist is that the strongest activity may miss Philadelphia.

I’ll update this evening if things clarify.

An active week, weather-wise, is on tap for Philadelphia.

An upper air high pressure system has set up in the center of the country and disturbances are forecast to rotate around the the edge of high pressure system, moving through our area from the northwest almost every 24 hours.

Following another cool and dry day on Monday, clouds move in Tuesday.  The first disturbance approaches from the northwest Tuesday night with showers and thunderstorms.  A second and third disturbance is forecast for Wednesday night and  again Thursday night.

Here’s the current GFS forecast, with the three disturbances shown—

Expect some changes in the exact track (for instance, the current NAMNEST has the first disturbance moving  more to our south) and timing of each disturbance.