Philly Weekend Weather

The intense low that brought the stormy weather will expand as it moves a slowly away, keeping us in windy conditions through Saturday.  Saturday will have a mix of clouds and sun.   It appears that the cloudiness may be considerable.  Winds diminish to about 15 mph with higher gusts .  High 47.

Sunday will be mostly sunny and a bit milder.  High 50.

SO WHAT HAPPENED?

The snow totals exceeded everyone’s expectations, including my own.   So what happened?

I can only speak for myself— I was very reluctant to predict snow totals based only on the raw model numbers for a storm in March.  Indeed, I felt I over-estimated last night when I said 2-4 inches and reneged this morning.

But if I had gone by the raw model numbers, the forecast would have been more on target.   So I can’t say the models got it wrong, except for perhaps surface temperatures, which showed to be colder than forecast. They apparently didn’t get the ‘dynamic cooling’ factored in at the surface.

Indeed, as said in my blog post yesterday, if this had been  a storm in January, I would have predicted 7+ inches.   But, I felt that warm surface temperatures and radiant energy through clouds would severely reduce accumulations and over-ride the raw numbers.

But the raw model numbers (at least based on the NAM FOUS) had enough QPF and low enough temperatures to account for what we have received.   So the models did well, based on my usual criteria.  I just didn’t trust it explicitly for the forecast.

I use the NAM FOUS data, an odd, tabular set of numbers that often provides everything I need for a good snow forecast; I rarely use the preconfigured snow totals of some of the model outputs.  

BTW, the NAM from this afternoon shows snow ending shortly after 7PM, although current radar would suggest otherwise.   We’ll see.

 

PHILLY STORM UPDATE 9 AM

This morning’s NAM data just coming in. (Some data I depend upon won’t be available until about 9:40AM.)   Latest NAM has critical temperatures colder.  QPF still about 0.55 inches water during the daytime hours.  Precip ending by very early evening (5-7 PM)

NAM 1PM Fri Forecast

Based on my usual forecasting criteria, I am still looking to a changeover and mix with wet snow by later this morning.  With all the mitigating factors still in play regarding accumulations (March solar insolation, above freezing air surface temperatures, etc.), it’s still impossible to give a snow accumulation total.    Some accumulation on grassy surfaces expected, a slushy wet coating on roadways if we get a burst of heavy precip .

Either way, it will be very stormy looking, despite minimal snow totals.

Highly unusual intense storm with blocked movement and a track that proceeds southeastward instead of the usual northeast track.

High winds a real concern, as sustained winds 25-35 mph with gusts near 55, especially during the afternoon hours.

9:50AM- Latest NAM FOUS data confirms the changeover to snow by noon or 1 PM.  Total QPF is about 0.69 inches water.  Some accumulation on grassy surfaces expected.  High winds!

PHILLY STORM UPDATE – 7:30 AM

The 1AM model runs (06Z) show increased QPF for the hours now through about 6 PM- Over 1.0 inches water falling.

The secondary low pressure is developing south of Long Island, somewhat closer and westward, accounting for the increased QPF.   With the westward development, warmer air is predicted to intrude in the upper levels of the atmosphere at times during the day.  Critical temperatures remain warmer until about noon here in PHL, delaying the transition to snow.  Still expect snow to mix in but very uncertain about grassy accumulations.

I’ve been hanging my hat on the NAM model which has been a bit of an outlier, a somewhat colder forecast than some of the other models.   With these latest model run, the NAM is even a bit warmer, so I think the 2-4 inch grassy surface accumulation is wrong now.

Indeed, some of the short range models now show nothing accumulating around here.   I’m hard-pressed to give a number now.

(Indeed, the TV forecasters suddenly dropped any mention of snow accumulation in our area last night and went on the bandwagon with a high wind concern.)

Still expecting strong winds, especially in the afternoon.

The next model data becomes available about 9:45 AM this morning.   I’ll update then.