This morning’s latest NAM and GFS models got this forecast wrong. Even as the snow is falling, both predicted a QPF of less than 0.10 inches water! Hardly accounts for the snowfall.
Regarding the short range models, the NAM based HRRR (hourly forecast) also blew this one. The GFS LAMPS forecasts (also hourly) shows snow likely until 2- 3 PM, although I find the GFS LAMPS is often wrong about such things.
Forgetting about the models, using old forecasting techniques, pressures are dropping along the coastline, showing the development of a coastal low. (See graphic below.) Not sure if my trough explanation earlier was correct.
So a much larger storm than forecast. 3-4 inches in some areas seems likely, although my guess is as good as yours. Without some reliable numbers, it’s all guess work.
It does look like it’s ending in the western suburbs, based on current radar.
I’m wondering what the media forecast people are going to say about this one? Usually, they just do their thing, as though the coating to 2 inches everyone forecast was nearly correct.
The 1 AM runs of the NAM and GFS continued with the 0.11 and 0.06 QPF values. Even with snow-water values of 20:1, it appears that the NAM may have done better than the GFS with the snow totals. (Usually an average of the two gives a pretty good read, but not this time around.)
In my neck of the woods, northwest of the city, we already have about 2 inches of snow. I’m beginning to think this is one of those clipper systems that exceeds snowfall expectations, making the forecasts all off.
One of the things talked about by the central NWS forecast office a week ago was the possibility of developing an obscure setup called a “Trowal -Norlun trough”. There’s a deep low pressure system far off the coast today; the clipper system may have “connected” with this far offshore causing this band of snow enhancement we’re experiencing this morning. That connection or upper air trough is the Trowal-Norlun trough, resulting in enhanced lift and enhanced snow production.
With the short range HRRR still showing small amounts of QPF remaining, we’ll have to see how much snow actually falls. Clearly a wrong forecast.
The very latest NAM shows 0.06 inches water for PHL this morning but suggests the heavier band of precip (see graphic above.)
Latest radar shows the banding clearly. (See graphic below) Notice that the gridpoint for PHL airport is not in the heavy band.
I wouldn’t be surprised if areas in the banding get 3 or more inches from this storm.
This morning’s models still show the upper air clipper low moving through our area Saturday morning. The GFS has the system weakening as it approaches Philadelphia, with a QPF that is now about 0.06 inches water. The NAM doesn’t weaken the system and maintains a QPF of about 0.11 inches water.
The light snow starts about daybreak and ends about noon. Depending upon your model of choice and the expected high snow-water ratio of 15:1 or 20:1, we could get about 1-2 inches, but a mere coating is also possible.
I’ll update with tonight’s models. The early NAM data becomes available as early as 9:15 PM
Fri 5 PM: This afternoon’s models have the very light snow starting a bit later and ending early afternoon. The trend has been towards less QPF. The experimental NBM (National Blend of Models) has a QPF of 0.06 for our area, or about about a coating to 1 inch of snow.