Daily Archives: Wednesday, February 8, 2017 9:19 pm

Winter Weather

Philly Snow Update

As I’ve been saying for several days, the final forecast can’t be made until Wednesday night’s data comes in.  And latest NAM model data has just become available.

In conjunction with my previous post this evening regarding warm surface temperatures as an impediment to high snow accumulation, the latest NAM data suggests that much of the QPF will occur before surface temperatures in the immediate PHL area drop to freezing.

The NAM has 0.59 inches water falling before 7AM, but much of that will be rain,  snow mixed with rain,  and finally snow about 7AM.    The NAM has reduced the QPF to only 0.20 inches water after 7AM.

So I’m going to go against the TV forecasts and predict only 3-4 inches of heavy wet snow for the immediate PHL area, ending about 10 AM.

I might be wrong about this forecast, but you’re reading this blog to get the real scoop without the hype.  While everyone gets all excited about a big storm, I simply can’t ignore the warm surface temperatures with this one.

10:30 PM Update:  Latest GFS supports the forecast of only 3, maybe 4, inches of heavy snow accumulation in the immediate Philadelphia  area.  The GFS has only 0.18 inches QPF falling after 7AM.  It’s looking like most of the precipitation occurs before the cold air moves in.

I don’t think the snow will be very heavy after daybreak.

 

Philadelphia Area Weather Forecasts

Some concerns about the current snow forecast

The big focus of this impending snow has been on the QPF and the duration of the precipitation.

One thing that seems to have escaped higher scrutiny are the warm temperatures.  Looking at the current short range GFS LAMPS forecast, it has surface temperatures well above freezing past the peak precipitation time.

While the current models show the upper atmosphere cooling down to support snow by about 4-5 AM the big unanswered question, will it be cold enough for the snow to accumulate to the levels being forecast?

Reviewing the current short range MOS and LAMPS, I’m beginning to think that the temperatures may not drop soon enough for as much accumulation to occur as being forecast.

So what is being forecast as a major snowstorm of 6-8 inches may actually only accumulate 3-4 inches if the cold surface temperatures arrive too late in the morning.   (In that case, ice under the snow. will be a bigger issue with this storm.)

I wanted to share this concern that the current forecast may not be correct.  Indeed, that’s why many of you read this blog. 

The next NAM data will be available within the hour.  I’ll update then.