Most of yesterday’s storms only affected the northern parts of our area, from central Montgomery county,  Bucks county and northward.

A cold front moves through this evening. The models are again forecasting scattered thunderstorms, this time affecting more of the immediate Philadelphia area including areas not affected yesterday.  The storms are expected to be scattered; again all areas won’t receive rain.

The weather parameters that are indicative of severe weather (Helicity, Shear, CAPE) are not as elevated as yesterday.

  • Sunny in the morning.  Clouds increase about 3 PM
  • High temp 89.4º sd 1.8º  Dew points 68-70º
  • Scattered thunderstorms 3-7 PM
  • Winds W 5-8 mph, increasing to 15 mph late afternoon.


[su_note note_color=”#ffffff”]Updated Sat 11:14 AM — After looking at this morning’s models, I’m trying to get out for a bike ride before the next batch of showers moves in.  To my surprise, here’s the current radar—

NEXRAD Radar 11:06 AM courtesy of

I’m looking back at all the models.  The Canadian High Resolution  (HRDPS) model which isn’t bad but its timing is often not correct, has these rounds of showers moving through during the late morning and early afternoon. [/su_note]

What I meant by uncertainty in the forecast from last night is when we get showers moving through this morning not predicted by last night’s models.

(This morning’s showers WERE predicted by some of the late night models (2AM EDT runs) that became available around 4 AM.)

So what can we expect of the rest of the day?  I’m going to approach as a probability forecast, not a deterministic forecast.

Here’s the latest model blend (NBM) 1 hour rain probability for 5 PM Saturday afternoon—

NBM Model blend 5 PM – 1 hour precip probability (Click on image for a larger view.)

Note that these probabilities are 1 hour probabilities and generally anything greater than 20% is meaningful.

The NAM model just available this morning has showers and thunderstorms developing 2:30 PM northwestern suburbs and moving through between 3 and 6 PM.  The NAM’s showers are more widespread.

Southern areas will be least affected today, according the the model blend, but the NAM has a wider area affected.


[su_note note_color=”#ffffff”]Updated Sat 07:20 AM — A fast moving mesoscale complex (thunderstorm group) will be moving through shortly.   The complex this morning was not predicted by the models last night!

Radar image 7:28 AM Saturday. Courtesy

It appears to be weakening as it moves eastward.[/su_note]

There’s been uncertainty about the forecast for this weekend with most of the models from Friday continuing to show most of the energy and precipitation shearing off to our north.

An upper air disturbance will move through on Saturday, setting off thunderstorms.

Tonight’s models (HRRR,RAP) just becoming available suggest that the dynamics are there for thunderstorms to develop and move through anytime between 2 PM and 6 PM on Saturday. Parameters that forecast severe weather are sufficiently elevated that a cluster of moderate to severe thunderstorms may move through with an elevated likelihood about 5 PM.

Tonight’s HIRESW and the NAM NEST models still have the storms moving just north of Philadelphia, from west to east.  So uncertainty still exists.

NAM NEST Saturday 6pm 1hr precipitation forecast

Saturday —

  • A mix of sun and clouds in the morning, considerable cloudiness developing by mid afternoon.
  • High temperature 90.7° sd 1.7°  dew points near 68°
  • Fast moving thunderstorms developing between 2 PM and 6 PM
  • Winds 8 to 10 mph becoming gusty to 30 mph in thunderstorms

Sunday –

  • Considerable cloudiness especially in the afternoon
  • High temperature 89.3° sd 1.8°
  • An elevated risk of thunderstorms and showers during the afternoon.
  • Winds light



[su_note note_color=”#defcdc”] Updated Fri 05:15 PM —An interesting announcement from NOAA about the strangely large standard deviation on the NBM model today—

“Navy FNMOC updated their data distribution software on Wednesday, and there are some issues that have resulted in values of NAVGEMD being about 40F too cold.
Thus, the large spike in standard deviation fields for NBM that have been seen are caused by that model.
NCO is in the process of removing the NAVGEMD as an input to NBM today so that it can run without this issue over the weekend.”
So, the amount of uncertainty in the forecast  I spoke of today was not real; it was just caused by a large error in one of the updated model inputs.


The upcoming weekend forecast has a higher than usual level of uncertainty.

First, most of the scattered storms missed the immediate PHL area yesterday, the subject of my previous post about a persistent weather pattern.

So we’re having a conjectured persistent weather “pattern” of having storms repeatedly shear off to the north and west coming against a number of ingredients that add up to potentially moderate to severe thunderstorms for our area Saturday afternoon.

For several days, the statistical models (“Ensemble” models) – GEFS and the SREF – continue to forecast the thunderstorms moving off to our north and west.   The deterministic models, the GFS, NAM CMC, suggest the possibility of moderate to severe thunderstorms making it into the PHL area.

SREF Forecast Saturday afternoon 6 PM
NAM NEST accumulated precip Saturday 8 PM

The uncertainty in the forecast is captured by the extremely large standard deviation (sd) in the model blend forecast high temperature for Saturday— 89º sd 10.5º  (this standard deviation of 10.5º is 1 – 2 times the usual spread of 1º or 2º). In fact, I haven’t see the sd as high as this in long while.

So this means there’s a lot of spread in the various model forecasts that comprise the national model blend (NBM).    The two graphics above show the differences in the placement of the storms.

Too early to tell.   I’ll update later this evening.