Category Archives: Weather Forecasts


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°



Fri 11:00 PM Update — Tonight’s models suggest the rain will be more showery in nature on Saturday— Breaks in the rain interrupted by some heavy rain periods. Winds look a bit more gusty as well. Some clearing late afternoon will be interrupted by scattered thunderstorms late afternoon and during the evening. Some areas will receive much more rain than others. Not a uniform continuous rain event. The HIRESW shows some of that extreme tropical moisture makes it into Philadephia, so localized heavy rain possible. The forecast below remains mostly intact.
The moisture and energy remnants of Hurricane Laura will move into our area on Saturday.  The low pressure system that was Laura will merge with a large non-tropical low pressure system and associated cold front on Saturday into Saturday night.

The moisture maximum, as reflected in the precipitable water parameter, or “PWAT” maximum,  will move just to our south, as previously forecast —

GFS PWAT forecast for 2 PM Saturday afternoon (Click on image for a larger view.)

The energy associated with both Laura, the non-tropical low, and the cold front will move through in separate batches.  Showers and rain will move through during the overnight hours.  There may be a break in the action Saturday morning, but rain returns around noon and continues though the mid afternoon.  Total rain about 1 inch, locally higher.

GFS (pre-release version) 6 hour rain accumulation at 2 PM Saturday. (Click on image for a larger view.)

The GFS (new pre-release version) has rain ending around 5PM with even some sun possible later Saturday afternoon.  The HIRESW has similar forecast, but has additional showers and clouds after 8 PM with the actual cold front passage.


  • Rain and cloud details above
  • High Temp 81º sd 2.1º  Dew Point 73-74º
  • Winds SW 5-10 mph   Gusty mid afternoon.  Becoming NW around 10 PM after the cold front passage

High pressure builds in for Sunday. A few upper air disturbances may produce periods of “fair weather clouds” and instability stratocumulus.


  • Sunny early, then  a mix of sun and clouds in the afternoon.
  • High temp 78.6º sd 1.3º
  • Low Humidity. Dew points upper 50s.
  • Winds WNW  10 mph becoming gusty to 20mph in the afternoon.