Category Archives: Philadelphia Area Weather Forecasts

Philly Winter Weather Update

With this mornings models both having a QPF of 0.46 inches water, it’s time to put my own bias of a fizzler to rest and predict the snow based on the model QPF.

The GFS model has some heavier snow occurring early, with about two inches or so at daybreak Tuesday,  then light snow for the rest of the day. The NAM has been consistent with very light accumulation at daybreak Tuesday and  then 4-5 inches occurring afternoon and and evening.

Impossible to tell which, if any,  of these models is correct.  As I’ve said, these secondary lows can’t be accurately modeled in advance regarding moisture distribution on a three dimensional level.

If we go by QPF, 4-6 inches is possible by late Tuesday evening.  Impossible to say where the heaviest amounts may setup.    There’s still my suspicion, that the snow accumulation could be significantly less.  A low confidence forecast.

It’s 1pm and there’s breaks of sun. Not captured by any model forecast. Doesn’t bode well for the rest of the forecast.

Philly Winter Weather Update 7 AM

So the issues with last evening’s model runs continue with the 1 AM reruns of the GFS and NAM.

Here are  the contradictions and forecast dilemma:  Both models have high QPF values (GFS about 0.50) and  NAM (increased to over 0.60 inches water) but they differ in the handling of the low pressure development.  The GFS has us waking up to 2 inches of snow Tuesday morning.   The NAM has us waking up to a light coating at best.  They differ by almost 18 hours for the heavy QPF.   So something is wrong, possibly with both models handling of this developing system.

If this were a different scenario, I would change my current forecast from the current 1-3 inches to the QPF-based 5-7 inches when all is said and done Tuesday night.

Essentially, I don’t think the models are right and the 1-3 inches is a hedge on what could be a fizzler of a storm.   HOWEVER,  a small voice is beginning to say, “don’t ignore the models QPF”

(I heard KYW’s forecast this morning and they’re going with the same “1-3 ….with 6 inches in some locations”.   Hey, what locations??   Wow, talk about a hedged forecast!  Yet another forecast from them ‘sanitized for your protection’. )

So here’s where we are at:  If this morning’s models continue to crank out high QPF values, we have to start taking this seriously and expect higher snow amounts on Tuesday.  I’ll update sometime mid day today….(this is not my full-time job).

 

 

 

Philly Weekend Weather Forecast

High pressure builds into our area for the weekend, but an upper air low pressure system to our west will affect our weather.

On Saturday, the upper air low with some upper air vorticities will bring mid-level cloudiness (altocumulus).  It will start out sunny, but expect the mid-level cloudiness for much of the day on Saturday.  High 42.

There wasn’t as much cloudiness as I predicted.

On Sunday, it will be mostly sunny and a bit milder. Some cloudiness moves in during the late afternoon.  High 44.

All models are showing a coastal storm to form off the southeast coast and move east of our area, missing us early Monday.  Not much variation with the various models, but it we must keep in mind that the same models didn’t capture the westward track of this morning’s snow.   So right now, no snow is expected from this storm.

Later Monday, the upper low to our west is expected to spawn a weak secondary surface low around Delaware bay, giving us light to moderate snow late Monday through Tuesday.   This is a low confidence forecast.

Here are the concerns: 1. The initial southeast low tracks closer to the coast than the models currently forecast.  2, The upper low phases with the coastal low that tracks closer instead of spawning a separate weaker low in Delaware on Tuesday.  While this is not the current expectation, this possibility needs to be watched, especially since the models did so poorly in advance of this morning’s snow.  

I’ll keep an eye on this, but right now, the models are going with a diffusely organized secondary low with snow on Tuesday.