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Saturday Forecast Update

Update Sat 05/13 @ 10:58 AM — Here’s what we have at 11 AM

Current 11AM MRMS radar and RAP data. (Click on image for a larger view.)

This is last night’s NBM (not previously posted)—

Last night’s 00z NBM (available at 9:30 PM EDT) showing the extent of the light showers. (Click on image for a larger view.)

Update Sat 5/13 8:12 AM — I knew my ‘dry’ forecast was a low confidence forecast. A few sprinkles have fallen this morning. Last night’s HRRR and Canadian maintain a dry forecast as shown in last night’s graphic.

Last night’s 00z NAM-NEST also showed a dry forecast, but its 06z run shows light sprinkles from the city southward, especially in the early afternoon.

It should be noted that NOAA- NWS is working on a next generation model, called the RRFS (Rapid Refresh Forecast System), scheduled for release sometime this fall. It will actually replace several models: the NAM-NEST, HRRR, HIRESW-ARW (all three versions) .

While not ready yet for prime time, here’s its current forecast compared to the HRRR:

Pre-release beta version of the RRFS. It shows more more shower activity north of the city (blue) than the current HRRR (red) at 2 PM. (Click on image for larger view)

Anyhow, I’m going to backtrack on my ‘update’ from last night and removed the strike-out from the forecast posted below to its original form.

Saturday Forecast Update

Update Fri 05/12 @ 9:58 PM — It hasn’t been even an hour since I posted this Weekend Weather Forecast, but I’m going to remove the mention of showers in the immediate PHL area for Saturday, based on the latest HRRR just becoming available and the consistent forecast of the Canadian models.

So I’m going to call for no rain here, despite the GFS, NBM and ensemble forecasts to the contrary.

Tonight’s 00z HRRR shows most of the immediate PHL area dry on Saturday. (Click on image for a larger view.)

Previously Posted Fri 9:23 PM —

A disturbance is expected to move to our south on Saturday with the rain shield staying in Maryland and southern Delaware. As has been the case over the past few days, some models keep the immediate PHL area dry.

However, at this point there are enough models, particularly the statical ensemble models, forecasting very light scattered showers moving through Montgomery county and into New Jersey, that I need to include that possibility in Saturday’s forecast. I’m not all that sure about this aspect of the Saturday forecast because the Canadian models have consistently kept us dry.


Cloudy with very light and scattered showers as early as 8 AM. Heaviest showers (if they occur here) early to mid afternoon.

High temperature 78.4º with a high uncertainty as expressed as a higher than normal standard deviation of 4.4° ( NBM model location Blue Bell, PA)


Some cloudiness early morning, becoming sunny by late morning.

High temperature 72.5º sd 1.5º ( NBM model location Blue Bell, PA)