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.
Winter storm hysteria has hit again in Philadelphia for the 2014-2015 winter season. From Hurricane Schwartz’s “shhhtorm alert” to Cecily Tinan’s “tracking the storm track”, you can hear the hysteria and sensationalism building for this “weather event”.
It’s going to be a coastal low. Much of the storm will be RAIN in and around Philadephia for much of the day. As always, the far northwest areas, near Allentown and Reading may have an earlier changeover to snow, although the amount of precipitation will be less there.
But this blog focuses on Philadelphia and its immediate suburbs. In this area, it will start as rain, likely heavy, until at about 2 PM, according to current models. It wil start mixing with and changing to snow after 2-3 PM, but temperatures at the surface will not be sinking below freezing until a bit later. This changeover will be occuring just as the precipitation will be tapering down.
With temps just approaching freezing at the surface, the very wet surfaces from the earier rain, and the intensity of precipitation winding down, expect a wet mix mix of about 1 inch by rush hour.
The real issue will be possible icy conditions after 7 PM.
In terms of accumulating snow totals, this is not going to be a big deal in Philadelphia. It’s very bad timing for the many people on the roads for the Thanksgiving holiday. In terms of bad travel conditions, this will be significant. In terms of accumulating snow for Philadelphia, this storm is being hyped right now.
The latest GFS and NAM models show a low pressure system moving up the coast Wednesday with heavy rain mixing with and changing to snow before ending Wednesday evening. Accumulations look to be light in PHL, but bad timing for any holiday travel. Stay tuned.