Category Archives: Thunderstorm Outlook


Updated on Wed 7:49 AM highlighted below:

The latest NBM continues to show the highest probability of thunderstorms from Philadelphia eastward at about 4 PM.

NBM Location of highest probability at 4 PM Wednesday. (Click on image for a larger view.)
NBM 1 hour Thunderstorm probability for Blue Bell PA

Updated Tues 10:35 PM with HIRESW highlighted below

A cold front will move through Wednesday afternoon and evening. The actual upper trough is not very sharp and the energy of the front will come through not as a single punch.

Prior to today, the models were showing very little in the way of thunderstorms. Today’s models have changed somewhat with some activity being forecast by most models.

The HREF shows scattered areas of strong vertical motion between 12 PM and 4 PM. Peak time as early as 2 PM Main activity currently appears to be north and east of the city.

HREF Vertical Velocity, jet stream level, at 3 PM Wednesday. The areas with the darkest coloration have the best chance of thunderstorms. (Click on image for a larger view.)
NBM Thunderstorm 1 hour probability Blue Bell. Anything more than 18% is a good probability.

Precipitable water (1.4″ to 1.7″) is not as high as recent past thunderstorms and CAPE levels are in the 1100 J/Kg range. Enough for some strong storms but right now, it doesn’t look like a major storm event for most areas.


Fri 08:18 PM Forecast Review — The storms developed and moved just south of Philadelphia as picked up by the afternoon HRRR.
NEXRAD radar at 6:55 PM

Updated Fri 4:25 PM, highlighted below, based on new HRRR
Updated Fri12:30 PM, highlighted below, based on new HIRESW

Strong thunderstorms expected early this evening. The latest HRRR shows all the ingredients coming together about 4:30 -8:30 PM 5:30 – 9 PM (peak 7-8 PM) in Philadelphia.

Update: The latest HRRR has the heaviest storms just to the south of Philadelphia

CAPE values in the 1600-2200 J/Kg range. Lifted Index moderately negative at -5. Very strong upward vertical motion is predicted to develop (blue shading/white arrow below) along with cyclonic upper air contour (blue arrows).

Heavy rain expected, especially from Philadelphia south and east.

HRRR 12z forecast for 8 PM showing very strong vertical velocity (blue shading/white arrow) right over Philadelphia and cyclonic upper air curvature. (blue arrows) (Click on image for a larger view.)

Thunderstorms WEDNESDAY

Updated Wed 8:40AM, highlighted below:

Updated Wed 8:25 PM: This afternoon’s GFS has heavy rain blossoming around Philadephia between 8-10 PM, while the HRRR and the RAP do not. Current radar seems to support the GFS. We’ll see soon enough.

Wednesday’s thunderstorm outlook is looking a bit like Tuesday’s, but with some differences. These storms will also be slow moving and will develop like today’s but will be moving southward instead of northeastward.

CAPE values only about 2000 instead of Tuesday’s 2800 J/Kg. Precipitable water still high at 1.9-2.0″ water, but also a bit lower than Tuesday. Instability values also lower.

Storms are possible in as early as 2-4 PM but eventually blossoming in the immediate Philadelphia area by 4 -7 PM and continuing into the late evening. There is some suggestion that areas south of the city and in NJ may be more affected, especially in the evening. Last nights’s models continue with a timeframe of 4-7 PM for the onset in the immediate PHL area, then continuing southward. Highest probability is 6 – 8 PM

Here’s their afternoon’s HREF precip forecast

High Resolution Ensemble 1 hour precip forecast for Wednesday. (Click on image for a larger view.)