Winter Weather

2 PM Radar shows snow falling heavily much further north than had been predicted by the models.
Weather Update Fri 12:30 PM
The latest HRRR (High Resolution Rapid Refresh) and NAM high resolution show about 1 to 1.5 inches snow in Philadelphia and about 2 inches of snow in southern Delaware and Chester counties. Less in northern suburbs.

As mentioned last night, this clipper was insignificant until last night’s model runs showed there might be some actual QPF associated with it.

from this morning–

Weather Update Fri 11:30 AM
The NAM probably did the best with the QPF last night. This morning’s models (GFS & NAM) continue to show about a coating to 1 inch of snow, ending about 2-3 PM. (The National Blend of Models did the worst, where it was still showing no snow here even with this morning’s runs.)

from last night–

An upper air disturbance (an area of vorticity) will move across our area, staying mostly to our south.  The models have not been very impressive with this disturbance until tonight’s model run. The NAM has a QPF of 0.10 inches of water while the GFS has 0.03.

The National Blend of Models has nothing for Philadephia. Not surprising—  these clipper systems have a way of being a bust, but occasionally they surprise.

So, flurries or a coating to an inch is possible in and south of Philadelphia for late morning into afternoon on Friday. Even less north.


Weather Update Tues 7 PM
The remaining QPF for snow is 0.06 inches water before ending between 10-11 in Philadelphia. Whatever accumulation you have at 7 PM, you can probably add another 1/2 inch of snow to the totals. The NBM did the best with the timing of the changeover. The GFS had a QPF that was likely too high and this morning’s NAM QPF was way too low.  The thermal profile of the NAM probably did the best.

The actual arctic front moves through between 12 noon and 1 PM Wednesday.  A coating to an inch of additional snow is possible from snow squalls.

Weather Update Tues 12 Noon
Using a specific geographical point for weather data, I’ll be using Wings Field, Blue Bell, PA. The latest NBM (National Blend of Models) has a QPF of 0.30 inches water, closer to the GFS. The NBM shows a changeover to snow (possibly mixed with rain) about 5 PM. We’re looking at about 2 inches of snow by the time it ends about 10 PM. Significantly less east and south.
Weather Update Tues 11 AM
This morning’s NAM and GFS show the differences remain, with the NAM now 0.15 inches water and the GFS 0.34. We’ll have to see which is right.

The 1 AM (06 UTC) runs of the models are available this morning and each model is consistent with its own previous run.  The NAM is warmer and has less QPF (0.28 inches water), falling mostly as rain, changing to wet snow about 5-6 PM, ending about 10PM.  Total snow is about 1 inch in PHL but increases to 3 inches in areas west and northwest.  (Chester counties, Upper Montco).

The GFS and FV3-GFS are colder and have a higher QPF of 0.48 inches water.  The GFS has snow starting earlier, then a mix of rain and snow, then more snow, ending about 11 PM.  Total snowfall about 2.5 inches in northwest Philadelphia increasing to 4–5 inches the further west you go.

Both models have lower accumulations in Center City, closer to the Delaware and east into NJ.

The GFS has precipitation starting as early as late morning, instead of mid-afternoon (NAM).

It’s unusual to have such a wide range in QPF and the predicted temperature profiles from the surface to 18,000 feet are unusual in that it’s very cold aloft (supporting snow) but the models, especially the NAM, have a layer of warmer air below 5000 feet that will affect the precipitation type.  The GFS is colder below 5000 feet, and as a result, is predicting more snow.

I’m still inclined to go with the GFS, but honestly, I’m not sure.  This is one of those situations where sometimes knowing the possibilities is the best you can do.


Weather Update Monday 10:50 PM
Tonight’s GFS just became available. QPF value about 0.38 inches water. The GFS is colder than the NAM and shows most of the precipitation falling as wet snow from PHL, west and northwest. The GFS supports my feeling that a changeover to snow might occur earlier. Snow accumulations would be 2-3 inches, especially west and northwest of the city if the GFS is correct.
Weather Update Monday 10 PM
Tonight’s NAM just became available. QPF value about 0.28 inches water. The models are consistent that the upper atmosphere is cold enough for snow, BUT temperatures in the lowest 5000 feet are expected to be above freezing during much of the afternoon, so the changeover in and around PHL will be about 6 PM. According to the NAM, about 1 inch of snow when it ends about 9 PM Tuesday night. I still think it’s possible the changeover to snow might start earlier with higher accumulations. The latest NBM has a mix starting between 5 and 6 PM.
Weather Update Monday 5:10 PM
As soon as I posted the forecast below, the 1PM run of the GFS model has raised the snow totals to 3.5 inches. Read the rest of post immediately below to see the context.

So, I’ve been trying to get a handle on the rain –> snow precipitation that is forecast to accompany an arctic front passage late Tuesday afternoon.

Let’s first go with the models and their algorithmic snow predictions. The NAM has a total QPF of 0.28 inches water falling as rain changing to snow before ending in the evening.    The GFS maintains the highest QPF of 0.53 inches water falling as rain changing to snow before ending.  The European ECMWF has a QPF of about 0.40 inches water.  The National Blend of Models (NBM) which has been fairly accurate recently has a QPF of 0.32 inches water.

So how much snow?

The uncertainty with this weather event is determining when the changeover will occur.    Most of the models have the rain -snow changeover occurring late in the period, about 6-7 PM, after most of the precipitation has fallen as rain. 

The models are showing a total snow accumulation of about 1 inch, possibly 2, in the Philadelphia area, somewhat more west and northwest of the city.  I think that’s a good guess, especially since the initial surfaces will be wet, melting some of the first snow that falls.

I hate to go against the models,  but based on the thermal profiles of the NAM and GFS which are cold enough for snow except near the surface, I think it’s a possibility that the changeover to snow may occur earlier than the preconfigured snow algorithms are currently forecasting, raising the amount of accumulation.  (I’ve also noticed that the GFS predicted surface temperatures were too high today. Add dynamic cooling and we would get more snow.)

So you can expect the 1, possibly 2 inches, but don’t be surprised if another inch or so falls, if the the snow changeover occurs earlier than the models forecast.

I guess we’ll see … that’s what makes this so interesting.  Stay tuned!

One has to wonder why there’s no “flash-freeze” hysteria with this snowfall. If anything, this event will have a lot more icing than the last one.


Tonight’s model data just becoming available.  Some large model differences have emerged regarding Tuesday’s snow.

The NAM has rain starting in the afternoon, changing to snow in the evening and quickly ending by 10 pm Tuesday night.  Total snow less than an inch.  QPF values about 0.31 inches water.

Tonight’s GFS continues with the idea that there will be low pressure development along the front.  It has precipitation starting early, changing to snow earlier and lingering.  QPF values about 0.63 inches water and over 3 inches of snow possible.

The important takeaway from this divergence is that there’s more uncertainty with this snow than previously thought.  Stay tuned.