The storm today, which had always looked primarily like a rain maker here in Philly, appears to be even warmer than expected. Heavy rain most of the day….changeover to snow in the immediate PHL area late afternoon or after dark now. About 1 wet inch accumulation most likely.
Don’t let those TV weathercasters fool you. There weren’t any last minute changes they “tracked”. This wasn’t ever going to be a “snow shhtorm” They’ve been hyping the snow potential of this storm since Sunday, even when all indications were that it would be a mostly rain storm all along right in PHL.
The latest NAM and GFS models are wetter and warmer than previous model runs. Very heavy rain expected most of the day, changing to wet snow about 2-4 pm before ending early evening. Much of the snow won’t accumulate.
Temperatures don’t support much accumulation, both at the surface and a few thousand feet up. Still 1-2 inches most likely on streets and sidewalks, a bit more on grassy surfaces.
The NAM and GFS continue to predict different scenarios for the coastal storm tomorrow, Wednesday.
The NAM predicts amost twice as much precipitation with more falling as snow. The GFS has warmer temperatures at the surface and delays changeover to snow until late in the afternoon, as the amount of precipitation (QPF) winds down.
The NAM has often overstated QPF in past storms. So I’m going with the GFS model, which has a changeover to snow between 2-4 PM and conditions allowing accumulation after 4 PM. Still going with 1-2 inches of snow for Philadelphia and the immediate suburbs. Further NW, greater amounts possible.
The new models come out about 10:30 PM. I’ll update if things look different.
There are the usual differences between the GFS and NAM models – the NAM is colder and wetter- the GFS has much less precipitation and is a bit warmer in the upper atmosphere.
In recent years, the NAM has often over-predicted the amounts of snow with model runs more than a day in advance. So while the NAM is predicting several inches of snow in PHL, I think we can ignore it and go with the GFS.
The GFS model has temperatures falling in the upper atmosphere to support snow just as much of the precipitation winds down. It maintains surface temperatures that are above freezing until the precipitation shuts down. So conditions for snow and snow accumulation are very marginal with the GFS model. While a changeover to snow looks to occur about 2-3 PM, conditions for accumulation will not be optimal because the precip will be winding down. With warm and wet surface conditions, expect snow accumulations to be about 1 inch or so in the immediate Philadelphia area.
Traveling conditions will not be good late afternoon, but the precipitation ends about 7 PM.
Again, much of this will be RAIN, changing to snow 2-3 PM and not looking to accumulate very much before ending by 7PM.