Category Archives: Updates


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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.


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Tonight’s models show the southern storm brushing southern Delaware on Sunday.  (Interestingly, the new FV3-GFS had hinted at this all week.)

Sunday will be mostly cloudy with considerable mid and high level clouds. Temperature highs only 36.


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Saturday morning’s NAM has backed off on the showers in eastern PA but has a slight chance for widely scattered showers in central NJ Saturday morning. Lingering periods of cloudiness in the morning still expected.  

Tonight’s NAM data is just becoming available and it has the front lingering over our area during Saturday morning.  This is a large change in the forecast. So there’s a chance of lingering light, scattered showers during the morning hours tomorrow, Saturday. Skies improve in the afternoon.

(GFS data not available until much later due to daylight saving time, so I can’t compare models at this time)

A weekend forecast  that appeared high confidence, now appears to have some new uncertainty.


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This morning’s models haven’t changes significantly. Widely scattered showers are now in Delaware.   The short range models have the rain moving into the immediate  Philadelphia between noon and 2 PM, earlier to the south.   The rain continues into the late evening, heavy at times.

The axis of the heaviest precip shows some variation among the models, with areas just to our south and areas of NJ having the heaviest.   It’s very difficult to pin point the exact placement of the heaviest precipitation.

Sunday is currently looking relatively dry, with only very widely scattered showers; most of the area rain free.