Monthly Archives: March 2020

Philadelphia Area Weather Forecasts

WEATHER UPDATE FOR FRIDAY

Last night’s models continue with the premise that the lowest layer of the atmosphere (from 1500-2000 feet above ground level)  will be above freezing during the day tomorrow, preventing us from getting snow or having any snow that forms above melt before we see it.

All models have us getting rain rather than snow from Allentown south and east due to this shallow warm layer. 

However,  temperatures above this lower shallow layer drop off rapidly, to well below freezing at 3000 feet and above.  This may be important.

Latest NAM model Simulated Radar 1 PM Friday. Critical temperature at 1500 feet is the blue line with the red arrows.

A notable model change is a forecast increase in amount of total accumulated precipitation (0.4 -0.6 inches water) and the rate of precipitation (PRATE).  .

With the higher PRATE, dynamic cooling may bring the cold air aloft down to the lower layer.  The models don’t seem to account well for dynamic cooling.

We’ve all seen forecasts where rain in predicted, but we get rain mixed with snow or even non-accumulating snow. This is a possibility tomorrow, especially later in the afternoon and evening.

I’m always reluctant to go against the models.   But you people read this blog to get another perspective.   So expect rain, but if we get rain mixed with snow or even non-accumulating snow, it won’t be a surprise.

Weather Updates

WEATHER UPDATE FOR FRIDAY

[su_note note_color=”#defcdc”]The mystery of why the models are showing rain instead of snow has been solved.  The models are forecasting a layer of warm air in the lowest 1500 feet of the atmosphere.[/su_note]

The Friday storm discussed earlier this week still remains a forecast challenge.  Let me start by saying that ALL models currently forecast rain, not snow, for our area because near surface temperatures are expected to be well above freezing.

That said, there are many model parameters that historically predict snow for us and many features that still need to watched.

Basically a low pressure system was expected to come together and phase with an upper low resulting in an intense coastal storm.   That scenario has faded, as phasing is expected to occur later, out in the Atlantic.

Here’s the current GEFS (Statistical Ensemble) mean forecast for Friday at 12 PM:

GEFS 12 PM Friday forecast  Blue Arrow points to upper low and the black arrow to the surface low.

Below is the current SREF (Short Range Ensemble) forecast, showing a somewhat similar position of the surface low, but with this forecast, I have drawn in the snow-critical temperature lines.  Notice that we are on the north side of these critical temperatures, usually meaning snow for us.

SREF 12 PM with critical snow thickness.  We are north of these critical snow temperatures, usually meaning snow for us.

I’m sure the models ‘know what they’re doing’ regarding snow forecasting, but with this complex upper-low/surface low combination, it will be interesting to see what actually happens.

What I expect is for some snow to mix in with the rain especially later in the day, but no accumulation.  Let’s see what happens.