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.

Reviewing last night’s models,  here are the trends– Cold air in the upper atmosphere moves in sooner and supports snow sooner, however the models still have the lower atmosphere at 40 degrees around midnight, so accumulating snow likely won’t occur earlier.

Precipitation (as rain)  starts earlier, now  in the late evening Wednesday.  With the  earlier chilling of the upper atmosphere, the transition from rain to snow may occur earlier (about 2 AM), BUT it will be a wet snow and warmer temps at the surface will greatly reduce early accumulations, especially around PHL.

Accumulating snow may not get going until 4AM or later and the accumulating snow will be ending about 8 AM.  

With  colder air moving in earlier in the upper atmosphere, the NAM’s “precipitable water” is less and  QPF values have reduced  to about 0.50 inches water.

What this all means is that total snow accumulations are looking lower at this time, especially in the immediate PHL area.  I’m thinking 3-4 inches around here, greater accumulations in the north and west.  It will be an icy and heavy accumulation.

Competing aspects of temperature and precipitable water values will make this storm accumulation forecast lower than usual confidence, but I’m leaning on the low side.

We won’t really know until tonight’s models.   Stay tuned.