Category Archives: Winter Storm Outlook


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Sunday 12 PM Update: This morning’s models have become available. The trends—

Less QPF is expected today, rain amounts reduced from 1 inch to about 0.6 inches water.

For Monday, temps in the upper atmosphere support snow fallling during the afternoon, but surface temperatures remain too warm until evening.

The upper atmosphere cools after 2 AM tonight and any precip will fall as rain >> snow by afternoon.  The lower atmosphere temps are slow to chill down, complicating accumulation forecasts.

The NAM has a coating up to 1 inch (with the lower range more likely) by late Monday evening.  The GFS has surface temperatures too warm for any accumulation in PHL and the immediate suburbs. 

For snow, I tend to go with the NAM. 

Sun 7:30  AM Update:  The 1 AM run of the GFS and NAM has moved the start of the precip to about 9 AM.  The latest HRRR depicts the freezing rain  further north and west —

HRRR PTYPE forecast for 9 AM Sunday

As I said yesterday, with the precipitation onset being revised each model run suggests that the models are having difficulty with this storm’s forecast. 

That said, my rule of thumb — when the models are having trouble with the very short term forecast, it puts into question their forecast for the following day. 

The models are still predicting a mix of rain and snow during the day and evening Monday, significantly more snow to areas north of Doylestown.  Last night’s ECMWF is all on-board with that forecast.  Can we really trust it with Monday’s forecast?   Stay tuned.

…from last night—

Earlier this week, I described this weekend’s weather as “interesting” and indeed it is a complex scenario involving a secondary coastal low pressure system and an upper atmospheric low that will interact in complex ways. See my previous discussions about the systems involved, as the general scenario appears to be playing out as predicted.  Here are the latest model trends —

Freezing rain and sleet begins 5-7AM from the city just north and west, and changes to all rain from 9 AM to noon. The city proper, south and east will have mostly rain. Areas north and west will have a prolonged period of sleet and freezing rain.  The graphic below is the latest NAM model forecast of PTYPE.

NAM forecast for 10 AM Sunday. Green rain, red freezing rain, magenta sleet. White line is surface freezing temp

The other models are in general agreement.

After the changeover, rain heavy at times is expected. QPF values about 1 inch water.

Sunday night, cold air filters in in the upper atmosphere.

For Monday —An upper low trailing the surface low will bring additional precipitation on Monday. A changeover to snow and snow showers is expected during the day, however for the immediate PHL area and adjacent suburbs, surface temperatures remain above freezing until late afternoon, so accumulation will be highly limited here. Currently, the models are showing a coating to an inch by the time it ends late Monday night, with considerably more in far northwest counties.

There remains uncertainty about this Monday’s snow. Stay tuned.



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Tues PM Update:  The latest models forecasts are all very different, making it impossible to make a definitive forecast at this time for the time period late Saturday into Monday.

The only trend supported by the models is that start of precipitation has been pushed back into Saturday evening, instead of Saturday afternoon.

The general trend is for a secondary coastal low to form.

The GFS puts the low to our south, allowing cold air to move in late Sunday night.    It’s still looking like predominantly rain, except for the start and the end where snow may fall. 

The GFS has the storm lingering on the coast through Monday, as it waits for an upper low to catch up to it and become “vertically stacked”.

With so many different model forecasts, we’ll need to wait for things to clarify.

Tues AM Update: The storm forecast for the Thanksgiving weekend continues to evolve. The GFS is faster and brings precip (as wet snow) in late Saturday afternoon.

The ECMWF (European)  is slower and has the precip starting late Saturday night and continues into Monday.  It remains mostly rain for us before ending as light snow. 

The general scenario is    wet snow—> rain—> light snow     before ending.

Canadian CMC forecast 4PM Sunday showing coastal secondary low  formation.  (The wavy red line is the 500-1000mb “thickness” line which is often (but not always) the rain-snow line.)

Secondary coastal low pressure development is again in the picture which will affect how much snow falls towards the end of the storm and how long it lingers into Monday. 

…from yesterday—

A stormy weekend looking likely…

As mentioned over the past week or so, an outbreak of winter cold air will affect our area the first week in December.  The journey to this cold dip in the jet stream promises to bring an “interesting” weather event for a large area of the country including our area.

Let me cut to the chase— most of our immediate region will have rain as the predominant precipitation type (PTYPE).  That said, a number of thermal changes will occur during a rather prolonged low pressure passage.

Originally, the low pressure system was expected to affect us on Sunday. The timing has changed— precipitation will arrive on Saturday afternoon.  Thermal profiles are suggesting a possibility of wet snow at the start late Saturday afternoon or evening.

Current GFS Forecast for Saturday 4PM- Precipitation Type (green – rain, purple – snow, pink – ice)  (I tend not to rely too much on the built-in model PTYPE forecasts)


Below is the latest GFS Ensemble Forecast for 1 AM Sunday—

GFS Ensemble forecast 1 AM Sunday showing critical snow temperatures in our region (Lavender, blue,  light blue, white lines)  The model shows much of our area will be rain by 1 AM.

This precip will change to rain overnight and continue into Sunday.

As it stands now, much of Sunday will have heavy rainA changeover to light snow or flurries is possible towards the end of the storm Sunday evening.

As always, there are differences in the models.  The European ECMWF has a bit more snow for us Saturday late afternoon and Saturday night before a changeover to rain. The Canadian CMC  is a bit warmer.

The forecast for this weekend has changed considerably over the past day.   I expect more changes.   Stay tuned.


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As often happens with the afternoon “off-hour” runs of the models, the forecast can be pushed off course.  Such was the case with this afternoon’s NAM. One of these days, I’ll stop looking at them.

Tonight’s latest NAM and GFS models have become available. The trend is towards colder temperatures and higher QPF values.

The NAM has a QPF of 0.89 inches water and the GFS has 0.73.

If this were a straight forward forecast, this would be a 7-10 inch snowfall, even for Philadelphia.  However, there’s a few glitches.

The temperatures at critical levels of the upper atmosphere are a bit warm for snow. We’ve had similar data where we have had snow but other times, there was significant sleet and rain in the mix, making the snowstorm forecast a bust.

Here’s my take. Having seen the new NBM correctly predict precipitation type, I’m leaning on it heavily for this forecast, along with NAM thermal profiles.

A mix of rain sleet and snow will begin about 3-5 PM The mix will continue, leaning towards a shift to more sleet and rain between 7 and and 11pm in the area south of the white line in graphic below.

Transition line snow – sleet rain

Areas northwest of this white line will be all snow. Total snow accumulation In those areas about 5-8 inches by daybreak Monday.

Areas south of this white line will have a considerable amount of sleet and rain in the mix for at least 1/3 of the total storm QPF, during the heaviest precipitation time. This will significantly reduce total snow accumulations. The sleet line moves back south after midnight, as shown in my earlier afternoon post.

It’s an educated guess but the area from the Delaware river north to that white line may have 2-4 heavy inches. I think the warmer temperatures aloft will play a big role in keeping those numbers lower than they would be otherwise as a result of the sleet rain mix.  Very cold temperatures follow later Monday.

I will update again with tomorrow morning’s data.


This afternoon’s NAM model just became available. This is the “off hour” model run that does not include new weather balloon upper air (radiosonde)  data.

Here are the changes—Temperatures are warmer and precipitation ends earlier, about 4 AM Monday. Precipitation starts about 4 PM Sunday, probably as snow or a mix.Temperatures at the surface are above freezing, so the first amounts won’t accumulate very much.

Temperatures at critical levels of the atmosphere become too warm for snow about 8:00 PM in Philadelphia and move north.

8:00 PM Critical Temperatures

The warm air aloft moves north to a position shown below by midnight. This is the northern extent of the sleet/rain line.

Areas  north and west of the white line below are all-snow for the storm: 

Midnight Sunday critical temp- Maximum northward movement- white line is transition line

By 2 AM, cold air line moves back through Philadelphia:

Sunday 2 AM critical temp
So this means that between 8:30 PM and 2 AM, areas below the white line in the middle graphic are a mix of sleet and rain instead of snow.

This will seriously limit the snow accumulations in the immediate Philadelphia area. Combine this with marginally freezing surface temperatures in the 32-33 degree range means limited snow.

This is an off-hour run.  We really will need to wait until Sunday morning’s model runs for the final call.