Updated Wed 8:39 AM — Minor changes from last night. Chance of thunderstorms today is low in the PHL area. Several areas of rain will move through, occurring earlier than previously expected. What best captures the rain is this HREF (High Resolution Ensemble Forecast) meteogram for Blue Bell PA which shows mean hourly rain amounts with the “spread” (which is considerable)—

Updated Tue 9:52 PM highlighted below

For tonight (Tuesday), the latest models are showing a chance of thunderstorms somewhat later, again mostly north and west of the city.

NBM thunderstorm probability 9 PM Thursday (Click on image for a larger view.)

Not much happening tonight in the immediate PHL area. The only thunderstorms are occurring in the orange area as depicted in the NBM probability map above. All models from this morning (HIRESW, RAP, HRRR) and even this afternoon did not get this right.


The models are showing the cold front slowly moving through on Wednesday with two centers of low pressure moving up the front.

There’s a chance of rain throughout the day, with two maxima.

The first batch of rain will start as early as 9-11 AM Wednesday. While embedded thunderstorms are possible, the morning hours generally do not support much in the way of heavy thunderstorms activity.

A second wave of low pressure moves through during the afternoon hours. More rain is likely about 3-5 PM. Thunderstorms are most likely from Philadelphia south and east. (although tonight’s NBM is unimpressive with thunderstorm probability)

Here’s an NBM model hourly rain probability meteogram for Blue Bell PA for Tuesday night and Wednesday—

I’ll be looking at the new models later tonight.

Tonight’s NBM and HRRR is very unimpressive with the chance for thunderstorms on Wednesday, although it continues to show several slugs of rain, very similar to the peaks shown in the meteogram above from an earlier model run this afternoon.

Some embedded thunderstorms still possible from PHL south and east.


Forecast Updated Tue 5:40 PM — This afternoon’s HRRR and HREF models have the heaviest activity to our northwest—

This afternoon’s HRRR shows the storms mostly to the northwest at 8 PM. with storm motion vectors and shear vectors most aligned in these areas. We’ll see if the earlier HIRESW-ARW-MEM2 forecast from this morning does a better job with the thunderstorm placement than the more recent HRRR. (Click on image for a larger view.)

Tue 5:35 PM —Updated with storm motion/shear vector graphic from this morning‘s HIRESW

An upper air disturbance will approach Tuesday evening ahead of a cold front Wednesday.

Several parameters that are used to predict severity of thunderstorms are not all that impressive in the immediate PHL, in the low-moderate range. (CAPE values in the 600s)

Nonetheless, there are several factors including a strong jet streak to our northwest and considerable daytime heating to 82º that can create some interesting weather. Additionally, the storm shear vectors align with the predicted storm motion vectors, especially north west of our area, resulting in some stronger storms north and west.

Look for thunderstorms developing between 6 and 8 PM tonight.

This morning’s HIRESW simulated Radar with shear direction and motion direction vectors. at 7 PM and 8 PM Highest support is north and west of the city. (Click on image for a larger view.)

More on the thunderstorm threat Wednesday with a later post.