Category Archives: Thunderstorm Outlook

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.)


Forecast Review — Last night’s models, which had been more conservative with the forecast severe parameters, were more on target with their thunderstorm forecast than this morning’s models.  We had several hours of clouds this afternoon which reduced the available energy when the storms arrived. It’s not the first time I’ve seen the 24 hour forecast be more accurate than the “latest models”.  
With thunderstorms, sometimes you’ve got wait for the latest models.

This morning’s models just becoming available have a significant increase in the severity parameters- helicity, CAPE and vertical shear.  Precipitable water values (PWAT) are very high.

So my previous post is off the mark.

Expect thunderstorms, 5-8 PM in the immediate Philadelphia area.   Some could be severe.

HRRR Helicity forecast for 7 PM Thursday. Anything over 300 J/Kg is high. (Click on image for a larger view.)


Thu 07:12 AM Update —  Regarding the thunderstorms being forecast for Thursday evening, the storms are forecast to move through about 5-7 PM.  The severity parameters in the immediate PHL area are at low to moderate levels.  The latest models have most of the stronger activity closer to and south of the PA-Delaware border.  Localized heavy rain still possible.  I’ll keep an eye on this.

  • CMC HRDPS 6 PM 1 hour Precip
  • CMC HRDPS 7 PM 1 hour Precip


From Wednesday evening…

The models have been on-board with moderate to heavy thunderstorms moving through our area about 10-11PM tonight.

Severe parameters are in the moderate range, but precipitable water values (PWAT) are very high, so some heavy downpours are expected this evening.  The storms are fast moving— expect some very gusty winds with these storms, as they quickly move through.

A front moves through Thursday early evening between 5 and 8 PM.  I’ll be updating tomorrow morning on those storms.

Addendum 9:07 PM:  Tonight’s storms are expected to diminish in intensity as they move into NJ.  Model blend thunderstorm probability at 10 PM—

NBM 1 hour Thunderstorm Probability 10 PM