Another quiet weekend coming up weather wise.  A well-advertised dip in the jet stream will bring us unseasonably cool weather this weekend. This is easily seen on the Short Range Ensemble Forecast (SREF) model—

SREF Forecast Saturday 3PM  (Note the 540 thickness line [red] and a tropical disturbance in the Guif.  (Click on image for a larger view.)
Notice the red “540 thickness line” which is often used to forecast the approximate rain-snow dividing line in winter.  It’s not common that we see that drop into the northeastern US in September.

Average high temps are around 74º this week, but we’ll have high temps 63.8º Saturday and 64.1º (Model Blend version 4 mean high Temp.)

Sunny both Saturday and Sunday and low dew points!

NBM Temp/Dewpoints for Blue Bell PA  (Click on image for a larger view.)

Winds will be a bit gusty in the early afternoon both Saturday and Sunday.

The tropical disturbance in the Gulf of Mexico appears to be blocked from moving north, but there may be more development.

On the weather front, new models are becoming available in the next two weeks.  A final version 4.0 of the National Blend of Models  (NBM) is expected to be released as well as a new version of the GEFS, the Global Ensemble Forecast System.

A new version of the HRRR (High Resolution Rapid Update) model is hoped to be released next month, as well as a new RAP model (Rapid Update). (Previously planned for release in June, these new models were crashing when modeling severe storms. It was back to the drawing boards for these models)

A new version of the GFS model is gradually moving towards release by the end of the year or in January.

These new models should enhance weather forecasts, especially snow forecasts for the winter.  Say what you want about weather forecasts, these NOAA scientists are amazing.


A front slowly moving through Thursday will cause heavy rain to fall in the Philadelphia area.  The latest NBM cranks out several inches of rain for our area, about 2.5 -4+ inches, with even some higher localized amounts!  The heaviest rain north of the city.

Model Blend 5 PM accumulated Rain—

NBM accumulated rain 5 PM Thursday  (Click on image for a larger view.)

I’m going to be without weather model access for a few days, so there won’t be the usual sort of updates for this weather event tomorrow, nor will there be my regular weekend weather forecast.


It’s been a long time since we had an uneventful three day forecast, and it seems even longer that this occurred on a major holiday weekend.

The  general pattern is one of high pressure building in from Canada, bringing clear skies and comfortable temperatures with low humidity.

The model blend (NBM) should be more than adequate for this forecast —

Sunny skies Saturday, Sunday and Monday.  Highs 78°, 83° and 84°



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.)