Updated Thu 10:59 PM — Tonight’s models have failed to clarify the most likely location of the heaviest precipitation tomorrow. The NAM NEST continues with the heaviest rain falling to the west of the low center, west of Philadelphia, while the RAP and HRRR have the heaviest rain in NJ.  The HIRESW models have very heavy rain right over Philadelphia.  (The GFS won’t be available until 11:40.)  Amounts continue to be 1.25-2 inches with some areas getting 3-5 inches. Rain starts 7-8 AM.
Updated Thu 04:40 PM and Thur 5:46 PM— The coastal storm is named Tropical Storm Fay.

This afternoon’s NAM-NEST model as well at the Canadian High Resolution and German ICON model have moved the heaviest rain bullseye eastward into NJ. The afternoon GFS is not yet available. Still a lot of rain!

NAM-NEST (High resolution) accumulated rain Friday.  (Click on image for a larger view.)


The NAM model is further eastward with the heavy rain and somewhat captures the ICON and CMC models—

NAM model accumulated precipitation 

This is almost as challenging as trying to predict a snowstorm!

And the afternoon run of the GFS has just become available and maintains the the precip maximum over Philadelphia —

Afternoon GFS accumulated rain forecast. (Click on image for a larger view.)

from earlier today…

The models continue to bring the center of the semi-tropical low directly over our area.  As with most tropical systems, the amount of available moisture for rain will be high.   (Measured as the model parameter PWAT, or “precipitable water”.)

Here’s the current forecast amount of rain expected by 8 PM Friday—

GFS forecast accumulated rain by 8 PM Friday.   (Click on image for a larger view.)

Please keep in mind that model rain predictions can’t be taken literally, especially in regard to the exact placement of the heaviest rain.  But it does give a good idea of the general amount of rain expected and the most likely area for the heaviest rain.

(The NAM-NEST model has a similar forecast to the GFS for the heaviest rain in the western suburbs.  The NAM has the heaviest rain bullseye in NJ. )


Updated Wed 10:42 PM — Tonight’s NAM NEST model has generalized 4 inches of rain Friday, starting early morning and ending in the evening.

The coastal low expected to take on tropical characteristics and affect our area Friday has had an interesting history as far as the model predictions  are concerned.

It was just a day ago that the much touted European model (ECMWF) and the GFS model were predicting it to just brush the coastline, missing us.  Meanwhile the current track had been predicted by the NAM and SREF models, often wrong about such storms.

The new GFS, still under development and experimental, had picked up the current track sooner than the current operational GFS model. It suggests that weather forecast accuracy may improve when the new GFS is released in 2021.

It appears that the storm will affect us starting early Friday morning. About 1.5 -2 inches of rain will fall on Friday, with locally heavier amounts possible. Heaviest amounts just west of Philadelphia.

GFS forecast Friday 2 PM


Updated Mon 01:39 PM — The storms developed around 1 PM, but not where predicted by the models.  Also, the storm movement is different than predicted as well (or there’s something wrong with my data.)  We’re getting some small size hail around here.

Live Radar at 12:56PM- Starting Thunderstorm development.  (Click on image for a larger view.)

Radar image courtesy of

Considerable development 30 minutes later—

Live Radar Capture 1:27 PM  (Click on image for a larger view.)

Radar image courtesy of

From earlier this morning…

The models from last night and this morning show thunderstorms developing over central NJ near Bucks County about 1-3 PM today and moving from northeast to southwest!   Some of these thunderstorms will have very heavy rain!

HRRR storm motion direction at 3 PM

Most areas near Philadelphia  will have showers and thunderstorms, but some areas which have been dry over the past few weeks may again miss some of the heaviest rains.

(There’s a negative feedback loop where there are existing dry conditions. When soil moisture is low, it negatively affects the development of thunderstorms.  Dry begets dry!)

Storms will be with us through 7 to 8 PM


Updated Sun 07:29 AM — The latest NAM NEST keeps the showers south of the immediate PHL area. High temperature 93.6° sd 1.3º  (Blue Bell),     94.9º sd 1.3º (Phila Airport)

NAM NEST 1 hour precip forecast 3 PM Sunday.  (Click on image for a larger view.)

Updated Sat 10:42 PM — Tonight’s models keep the scattered showers south of Philadelphia. The position in the GFS graphic below remains unchanged. High temperatures 94°.

The focus of this weekend’s weather has been the very warm (hot) temperatures expected.  But there’s a forecast change regarding showers.

The frontal boundary that passed through Friday night had been expected to ‘wash out’.  Instead, it has stalled to our south and is now expected to become an area of weak moisture convergence for this evening and Sunday.

GFS Forecast vertical velocity (yellow/pink)  and lower wind flow. (Click on image for a larger view.)

With daytime heating and the vertical motion from the wind flow convergence, scattered showers and thunderstorms are possible in the area shown in pink above.   (South of the Philadelphia area.)

This afternoon’s NAM NEST has this area of convergence a little further north, and a ripple ( vorticity disturbance) in the upper air flow with showers developing closer to Philadelphia area tomorrow afternoon—

NAM NEST 2 PM Sunday precipitation forecast (Click on image for a larger view.)

So there’s some uncertainty about the exact location of these possible showers.    Also timing uncertainty, with the GFS having these showers as early as late morning and NAM NEST as early as 2PM Sunday.

Weather… and other things "up in the air"