Category Archives: Winter Storm Outlook


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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.

Winter Weather Forecast

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Forecast Update Fri 11:30 AM
This mornings NAM data just available shows a QPF of 0.15 inches water. This morning’s GFS has a QPF of 0.11 inches water.

Based on the average, we’re back to 1, possibly 2 inches of snow, starting about 9-11 PM and ending about daybreak on Sunday.  Considerably less the further north one is from Philadelphia. 

There’s been tremendous consistency and continuity with the models and the storm track. I’ll update tonight with my usual Weekend Weather Forecast about 10:45 PM.

Forecast Update Fri 10 AM
This mornings NAM data just available shows a QPF of 0.15 inches water. The GFS data is not yet available this morning, but last night’s 06 UTC (1AM EST) run showed a bit higher- 0.22 inches water for Philadelphia.

The forecast remains on track for 2 inches, possibly 3, in Philadelphia with lesser amounts to the north. Light snow starts early evening Saturday and ends during the morning hours on Sunday.

There’s been tremendous consistency and continuity with the models and the storm track.

I’ll update tonight with my usual Weekend Weather Forecast about 10:45 PM.

Forecast Update Thurs 12 PM
This morning’s models (NAM, GFS, FV3-GFS) are in remarkable agreement with a forecast of light snow starting early Saturday evening and ending sometime during the day on Sunday. Most of Sunday should be snow-free with the exception of light flurries.

The total QPF is about 0.18 inches water, which translates into about 2-3 inches of snow at these temperatures.

Today’s GFS is down-playing intensification with throw-back snow later on Sunday.

There is increasing agreement among the models that the low pressure system will track to our south. We’ll be on the northern fringe of the precipitation shield.

Over-running moisture ahead of the low cause light snow to fall late Saturday afternoon or Saturday evening.  About 1 inch or so is expected in the general Philadelphia area by Sunday morning.  (Total QPF water equivalent is 0.10 inches)  The NAM and GFS are in agreement, as is the FV3-GFS.

Sunday will be cold but little if any snow is expected during the daytime hours. Any snow will fall to our south in Delaware.

The GFS has some intensification and development of the low as it reaches the coast Sunday evening.  It shows some throwback moisture, allowing some additional snow Sunday night. An additional 1 inch of snow is possible Sunday night.

GFS forecast
GFS forecast Animation for Sunday

As everyone who reads my blog knows, we really can’t make an accurate call on QPF and snowfall amounts until 12-18 hours before the event.  Stay tuned.


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Forecast Update Wednesday 11 PM
Tonight’s NAM has become available and its forecast range takes us to Sunday morning. Like tonight’s GFS, it has us in the northern fringe of the precipitation shield. QPF values are about 0.16 inches water, which suggests 2-3 inches of snow, less north of Philadelphia. The outlier remains the FV3-GFS which has a more northerly track. Still too early to hang one’s hat on a specific accumulation.
Forecast Update Wednesday 12 PM
This morning’s models continue with the idea that Philadelphia will be on the northern fringe of this storm, whose northward movement will be blocked by high pressure in the northeast.

We are just coming into range covered by the NAM model (84 hours out) and it has some light snow breaking out north of the main low pressure system.

Based on current forecasts, 1-2 inches are possible at PHL with even lower amounts in Montgomery and Bucks counties. (Don’t take these numbers literally…it’s just a current estimate.)

Tonight’s NAM will probably give a better handle on things. For Philadelphia and north, this is a minor snow. Heavier amounts in southern Delaware and possibly Cape May County NJ.

For those wondering, I’m trying a format where updates are placed in these “update boxes” embedded in my earliest post about this weather event. Generally, when I make these embedded updates, I don’t rebroadcast it to Twitter.  Let me know if this works for you.

Forecast Update Wednesday 8 AM
Last night’s models have shown greater consensus with the predicted storm for the weekend.

GFS forecast
GFS forecast (from 06 UTC run) for Sunday morning.

With the exception of the FV3-GFS, all keep the storm blocked to our south. Little or no snow expected in Philadephia and areas to the north with the current predicted track and development of the storm.

I will continue to watch this. Still waiting for input from the NAM model.

Forecast Update Tuesday 11 PM
Tonight’s models just available have the storm sliding to our south with only the northern edge of the snow field affecting Philadelphia. Several models (Canadian and ECMWF European) have it almost missing us. The snow, if it occurs, will begin later Saturday afternoon.

Will have to wait until the NAM model is in range for a better handle on this snow potential. The NAM only predicts 84 hours out.

The National Blend of Models shows only a 50% chance of any snow for Philadelphia. Still too uncertain at this time.

Forecast Update Tuesday 11 AM
This morning’s GFS and FV3-GFS still back to showing a coastal storm that affects our area Saturday night into Sunday. Still much uncertainty with the forecast in this time frame. Stay tuned.
Forecast Update Tuesday 8 AM
Last night’s models (with the exception of the FV3-GFS) have begun to show the blocking effect of high pressure, which would have the effect of keeping the anticipated storm to our south, mostly missing us. This was the original prediction, until the GFS and others went on the coastal storm band wagon yesterday. Still too much uncertainty with the forecast in this time frame.

Much too early to be sure, but both yesterday’s GFS and today’s GFS FV3-GFS show the possibility of a coastal storm bringing snow to our area, most likely on Sunday.  This system is also predicted by the ICON and Canadian models, although these models suppress most of the precipitation to our south.

A high pressure system will be bringing much colder air to our region this weekend.  As mentioned in yesterday’s post, the configuration and position of this dip in the jet favors a storm track that blocks northward movement any low pressure system to our south.  Apparently, the newest GFS and FV3-GFS  have the blocking effect less pronounced.

This is first time since our snow in November that any system has appeared to threaten any real snow for us.

The time frame is over 6 days away, well beyond the 3-5 day window of meaningful statistical likelihood.  (Unfortunately, the government shutdown has shut down some of the experimental statistical model sites that I usually count on)

Mon 11pm Tonight’s GFS continues to show a possible snowstorm for Phladelphia and South Jersey late Saturday into Sunday.

Stay tuned.


Watching possible coastal storm for Sunday

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There’s still wide model spread with the coastal storm that’s expected late Sunday.

Most of the models had been suppressing this to our south, but last night’s models, particularly the GFS-FV3 and the Canadian CMC are showing trends towards it moving up the coast, giving us snow.

Most of the statistical models (EKDMOS and NBM) and the operational European (ECMWF) keep a low chance (30%) of this happening, but this needs to be watched.  (The chances have been increasing.)

Tuesday night’s models keep the storm just to our south.