Forecast Update Sat 10 PM
Tonight’s NAM has moved down the QPF to 0.37 inches water for Philadelphia. The graphic with the tight gradient below is still a realistic forecast for snowfall — areas in far Northeast Philadelphia will have less snow than areas near the airport. Areas central to Philadelphia can expect 3-4 inches.

The accumulation will be extremely location dependent, south to north.

Here are some changes— light snow continues into Sunday evening in areas south. Far South Jersey May have over 1 foot of snow.

An unusual forecast is unfolding, with a tight gradient of precipitation and larger possible snow totals. With these tight gradients, I usually go with the NAM model.

This morning’s NAM had a QPF of 0.92 inches water at the airport, rapidly diminishing to very low amounts at Trenton.  This seemed like an error.  This afternoon’s NAM run is still running much higher than other models, with a QPF of 0.42 inches water at the airport and a tight gradient of decline.  I don’t usually use graphics for snow totals but the sharp gradient in amounts can only be depicted graphically:

NAM Forecast
NAM Forecast QPF through Sunday afternoon

This morning’s models also depicted the sharp gradient, but the QPF values were about half of the NAM prediction.

Based on the NAM, 2-5 inches will fall, depending on location north to south.

Snow will start between 10 and midnight tonight and will continue to about 1 PM on Sunday.  It’s possible that tonight’s model runs will back off on the high QPF values, but you’re reading this blog to find out what the possibilities are.


Forecast Update Sat 11 AM
So this morning’s GFS continues with a QPF of 0.15 inches water at Philadelphia airport, dropping off rapidly as one goes further north.  The NBM has a good chance of snow continuing until at least noon on Sunday.

NAM showing storm total QPF  The NAM shows intensification near the coast.  An error??

We will have to assume that the NAM’s very high QPF is a modeling error, but we need to keep in mind that there’s a chance that it’s not and that higher snow totals than currently predicted may occur just south of our immediate area.  Always interesting!

Forecast Update Sat 9:55 AM
Wow, this morning’s NAM has a QPF of 0.92 inches water at the Philadelphia airport, dropping off rapidly to 0.12 inches water at Trenton Airport.

This is either a modeling error or a significant change in the northern-most position of the heavy snow. It would mean the same graphic below, but with the darker blue being 1 foot of snow!

NAM model total snow accumulation by Sunday morning with darker blue 1 foot at Philadelphia airport and well over a foot in Baltimore.  Two inches in Trention!

Will have to wait for the GFS model due out in about 45 minutes to assess.

Last night’s 1 AM model runs (06 UTC) display slight differences.  Most of the QPF differences are the result of differences in model resolution and the fact that we’re trying to predict an “edge” or “boundary” area condition – where moisture meets dry colder air.  Models have a limit in accuracy in such boundary conditions.

Another way of putting this is that we’re ‘trying to split hairs’. This is  going to be a minor weather event for our area.  It doesn’t merit the designation “snow storm” and there are times when snow flurries or fast moving snow squalls put down heavier snow than what’s expected here.

Put another way, when we get rain, does anyone really know or care if we get 0.06 inches of rain vs. 0.11 inches?

Now that I got that out of my system, let’s talk about the current model predicted QPF:

The latest NAM still comes in high at 0.14 inches water.  The GFS comes in at 0.06, the NBM at 0.04 and the ECMWF at 0.15.  (For data point KPHL- Philadelphia airport)

This graphic probably captures the best precipitation breakdown:

ECMWF model total snow accumulation by Sunday morning.

Philadelphia is cut in half – 0.15 at the airport,  0.08 in Mt Airy 0.04 in Langhorne.  (Multiply by 10 or 12 and you get the snow total in inches.)

My preference in these situations is the NAM or NAM/GFS average with the slightly higher snow total,

Other differences are with the start time of the light snow.  The GFS starts early, about 8 PM while the NBM is later, after 10 or 11 PM.

We’ll see if anything changes.