Updated Wed 7:33 AM — Current weather analysis (RTMA) shows the front only made it as far westward as Chester county, not Lancaster county as predicted by last night’s models.

This boundary is expected to move back eastward today and the convergence zone of the winds (see graphic below) will be a focal point for showers and some thunderstorms. Lowered CAPE values and high CIN values show any activity to be garden variety thunderstorms and showers later this afternoon and this evening.

Updated Tue 11:06 PM — Tonight’s models show the backdoor cold front moving as far westward as Lancaster county. Most models have very little storm activity during the afternoon as CIN values remain high. Some models (but not all) have storms redeveloping in the late afternoon and evening and moving in from the northwest as the wind convergence zone moves back eastward. Some of these may be strong. I’ll update tomorrow.

Updated Tue 6:11 PM — This afternoon’s models, particularly the HRRR, has the backdoor front moving so slowly back eastward that conditions near Philadelphia are not as unstable as shown in the table below. Less instability means less activity is likely in the immediate PHL area on Wednesday. So forget about the green severity table below for now.

Indeed, the latest RAP shows elevated ”CIN” during the afternoon. (Think of CIN as opposing CAPE.)

Any activity is predicted to from in the far northwest (Reading – Allentown area) and far western suburbs. The NAM-NEST continues with some storms making it into our area, but they diminish in strength as they move through. Any storms will be scattered; this will not be a line of storms. Should the backdoor front change its forecast position, this forecast will change.

Heavier storms still expected Thursday evening.

Updated Tue 11:34 AM — The backdoor cold front boundary, expected to stall just west of Philadelphia on Wednesday, will gradually move back eastward during Wednesday afternoon. Areas of moisture convergence and high CAPE will result in some heavy thunderstorms in some areas later Wednesday afternoon.

Update Tue @ 8:35 AM — Temperatures are still expected to be in the range shown in last night’s post, perhaps 1 degree less overall.

A weak backdoor cold front is still expected to move in before daybreak Wednesday morning. Here’s the RAP model position of the front—

There’s a lack of agreement about how far the front will move back eastward and how much hot air will return Wednesday afternoon. A large gradient in temperatures across this boundary.

Originally Posted Mon @ 7:40 PM —

A strong upper air ridge will bring very hot temperatures to our area on Tuesday. The latest NBM model shows maximum temperatures to be higher than previous forecasts—

With dew point temperatures in the range of 65-67º, Tuesday will feel uncomfortably hot.

From Wednesday through Thursday, a back-door cold front will gradually sink closer to us from the northeast

With more clouds, Wednesday’s highs may not reach 90

Some showers/thunderstorms may break out Wednesday ahead of some upper air disturbances (areas of vorticity) rotating around the upper ridge. Additional activity far northwestern suburbs late Wednesday afternoon.

From Thursday into early Friday, a cold front will approach closer with low pressure developing along the front. Some thunderstorms and rain will break out, especially late Thursday afternoon and evening. Exact timing and details vary from model to model. Several models have some fairly heavy rain later Thursday.

Things clear later Friday and the weekend looks pretty good.

Stay tuned for updates.