Category Archives: Winter Storm Outlook


Fri 11 PM Update— Tonight’s models just becoming available suggest that widespread accumulations greater than 12 inches are possible with this storm.  I’ll update in the morning.

The coming snowfall for the Philadelphia area is now looking to be significant.  All models have snowfall starting Sunday afternoon and lasting into Monday evening as a warm front approaches, followed by a secondary coastal low pressure system.  Most models have all snow, but the GFS has a mix with some rain late Sunday night into Monday morning before changing back to all snow. 

The majority of the accumulation is expected during the day Monday.

The latest GFS model snowfall amounts are—

GFS snowfall forecast by Monday evening  (Click on image for a larger view.)

I’m posting the GFS because it probably best captures the snowfall range of all the models.

However, I think that these model-calculated snowfall totals may be the low end of the spectrum and it assumes that some of the precip will be mixed with sleet/rain for a period of time increasing compaction.    If there’s no mix to rain, the snow amounts could almost double in some areas.  Indeed, a straight QPF to snow calculation gives many areas over 12″   Many models are forecasting QPF values of over 1-1.5 inches of water.

The ECMWF, ICON, NAM, SREF and other models are similar, although they each have their snowfall maxima at somewhat different locations. 

I’ll be updating through the weekend.  Stay tuned.


Usually, I update this blog with the ‘the latest model runs’ and the forecast trends.  

As the time frame of the Sunday-Monday storm comes into the range of the higher resolution models, this  potential snow storm is presenting a higher level of variation and  uncertainty with each model addition rather than a clarification of the likely storm snowfall.   Usually, I’d supply some snowfall maps, but it’s a waste of time right now.  Here’s why:

  • First, the European model has backed off of its extreme snowfall amounts for our area. 
  • However, the German ICON model has gotten extreme with its totals. 
  • The GFS has remained somewhat consistent with a mix with rain on Monday. 
  • The Canadian Global and Regional GEMS remains colder with only snow, no rain mix
  • The NAM has come into forecast range and it has a mix with rain/sleet.
  • The SREF forecast has come into range and it’s somewhat higher with snow totals.

Rather than present snowfall maps, I’m going to present maps that capture the current uncertainty—

ICON Statistical Ensemble model showing large variations in the possible position of the coastal low. (Click on image for a larger view.)


Here’s the Short Range Ensemble Model (SREF) showing high spread in standard deviation of the surface pressure. High spread suggests high uncertainty in the low pressure position .

SREF ensemble Pressure/standard deviation Monday 10 AM  showing high spread in pressure values in the NE quadrant of the storm.  This could suggest faster movement or uncertainty in position/configuration of the surface low.  Equally uncertain is the high pressure in NY State.   (Click on image for a larger view.) 


All this said, here’s the large scale “Snow-Liquid” forecast from the SREF suggesting the statistical areas where highest snowfall is expected—

SREF snow-liquid forecast (correlates with snow totals)  This suggests where the highest snow accumulations will be.   I’m intentionally not putting numbers on the graphic.  (Click on image for a larger view.)


So, the takeaway is that we’re getting several inches of snow, starting Sunday afternoon and running through Monday; high uncertainty about specific snow totals in any one specific area.   Sometimes, the best forecast information is to indicate what we don’t know.

I’ll take another attempt at specific snow forecast this evening, when we have today’s model runs to review.   Stay tuned.




Thu 05:58 PM Update — The Sunday time frame is just coming into range of the NAM and Canadian Regional models (84 hours). 

The general trend with today’s models has been for more snow.  The ECMWF model has as much as  8-12 inches for our area, but it is on the very high end of the scale right now. I believe the ECMWF, despite its stellar reputation, is over-doing the snow.

ECMWF (European model) snow totals by early Tues morning.  (Click on image for a larger view.)


The latest GFS forecast shows significantly  less snow, the difference being a further east development and less intensification of the coastal low.  The GFS brings warm air in during the storm, with a mix of snow/sleet at times reducing accumulations.

GFS snow totals by Tuesday morning  (Click on image for a larger view.)


The NAM, Canadian regional GEM and the ECMWF have the snow starting late afternoon/early evening Sunday, while the GFS and model blend (NBM) have some light snow starting earlier in the afternoon Sunday with the warm front.  

As mentioned, the storm consists initially of a “warm air advection type snow” on Sunday afternoon from the original low pressure in the Midwest. Then, the bulk of the storm’s precip comes on Monday with the expected secondary coastal low.  The Monday time frame still beyond the forecast range of all of the higher resolution models.

Stay tuned. 

I’ll update tomorrow morning when we have some newer model data.

from earlier this morning…

Last night’s models have become available.  Here’s the breakdown on current model forecasts— The GFS is more disorganized with the secondary low formation off the coast.  As a result, the primary low’s circulation continues to bring in milder air in our area through a low level jet circulation instead of having a colder northeast flow from the secondary—

GFS low level 925 mb jet flow 1 AM Monday shows SE flow (Click on image for a larger view.)

As a result, the GFS has a changeover to rain for some part of the storm in the immediate PHL area and suburbs, keeping the snow accumulations to 1-4 inches.

GFS snow totals by Tuesday morning. It’s visible where that low level warm air ‘ate away’ at the snow accumulation.  (Click on image for a larger view.)

The GFS sequence is snow (Sunday)–> rain mix (Monday morning)  –> Snow (Monday afternoon.) 

On the other hand, the Canadian CMC-GDPS and European ECMWF have it colder but develop the storm further east and north.  The have somewhat different snow totals and for different reasons— the secondary storm is stronger and colder, but is slower to develop and is further east.  They don’t show the changeover to rain, but they have less total precip for our immediate area.

The model blend (NBM) in recent years has been quite good (a recent exception being this past Monday).   Here’s its snow totals (average)—

NBM mean snow total Forecast by Monday eve (Click on image for a larger view.)


The current takeaway from all of this early prognostication is that the initial warm front snow starts as early as noontime on Sunday. About 1 inch of snow likely by Sunday evening. 

The coastal storm, depending upon track and development, affects us on midnight Sunday into Monday with additional accumulations. 

Look for more updates later today.


Wed 09:41 PM Update — As mentioned earlier, this possible snow event will be in two parts, a warm front type snow as early as Sunday followed by a secondary coastal low. The time frame is beyond the forecast horizon of the shorter range high resolution models.

FYI: The newer model runs of the major global models, the GEFS, CMC, and the ECMWF don’t become available until well after midnight. (The GFS model data for this forecast time frame becomes available about 11:10. ) Your favorite TV weather entertainer won’t have any meaningful updated forecast information this evening.

I’ll update tomorrow morning.

Wed 05:58 PM Update —There’s still much uncertainty regarding the amount of development and track of the secondary low late Sunday into Monday. Just about every model has some snow.  The latest European model is much slower with the development but brings a major snowfall to our area, especially areas just west of Philadelphia as late as Tuesday.   It’s way too early to post specific numbers.  Recent storm forecasts have over-stated the snowfall totals this many days in advance.  (The actual storm is well past the 84-89 hour range of the higher resolution models.)   But stay tuned.

BTW,  I saw this interesting article in the Washington Post.  

from earlier…

Cold air has built up in Canada and it will make its plunge into the eastern US over the next few days resulting in an amplified jet stream.

After several frigid days through Saturday, low pressure in the Midwest will try to bring warm air and moisture over us aloft (basically a warm front) on Sunday.  This should result in snow for us some time on Sunday afternoon or evening.   These scenarios (warm advection snows) can produce an inch or two of snow, sometimes more.

Of interest, several models have this Midwest low pressure system spawning a secondary coastal low late Sunday night—

GEFS model showing coastal secondary low formation Sunday night. (Click on image for a larger view.)

Depending upon the exact location of this secondary low, we could get additional snow.  Right now, the dip in the jet flow does NOT show an ideal track for development needed for heavy snow in our immediate area—

GFS model 300 mb jet stream flow. The dip isn’t sharp enough to bring the developing coastal storm close enough for heavy snow in Philadelphia.  (Click on image for a larger view.)

As we saw with today’s system, model storm forecasts several days in advance can easily change.  (The southward impulse (white arrow)  could phase more, causing a sharper bend in the flow.)   This one looks like it has potential, but the trend over the past day has been for a more eastward track.  

However, of interest is the German ICON model whose current forecast extends to 120 hours (Monday 1 AM).  It has very heavy snow tracking just south of our area late Sunday—

ICON model Snow Precipitation Rate Monday 1AM  (Click on image for a larger view.)


Using some new software, here’s the GFS projected snow totals for this potential storm—

GFS projected snow totals  (Click on image for a larger view.)


Stay tuned.


Mon 05:46 PM Update — This is complex forecast.  Snow is NOT the issue. Precipitation type (sleet, freezing rain, rain)  is a question.

There are  differences between various models, especially regarding surface temperatures and temperatures below 6000 feet between 6 AM and 11 AM.  There are different forecasts for precipitation type. 

The NAM/NAM-NEST group have temperatures below 32º while the GFS/RAP group have it a above freezing.   So the NAM group has more freezing type precip (sleet/freezing rain) after daybreak.

In the old days (before I  had direct model data), I would use the NAM FOUS data for my forecast.  The FOUS shows little snow for our area, but maintains a longer period of sleet/freezing rain/rain into the mid morning hours.  So, I’m leaning towards the NAM group for this forecast.  The graphic below captures the parameters of the NAM FOUS data—

NAM NEST PTYPE forecast with 32ºF isotherms 9 AM Tues  (Similar NAM FOUS)  (Click on image for a larger view.)


Both model groups have the majority of precip in the immediate PHL starting later,  after 8 AM, although it starts earlier far southwest of the city.

Areas southwest of the city will have precip start before daybreak; these  areas are more likely to get a coating of snow before the changeover.    

If things change, I’ll update between 10 and 11 this evening


Mon 01:35 PM Update — A quick update.  This morning’s GFS has virtually NO snow accumulation for the immediate PHL area.   Surface temperatures rise above freezing early, so most areas won’t see any freezing rain either.   Just light rain.

GFS model 8 AM Tuesday PTYPE (note that these areas become all rain quickly)  (Click on image for a larger view.)


Thursday’s expected storm will miss us entirely, according to the latest GFS.

Sun 10:46 PM Update — Tonight’s models suggest several hours of light sleet and freezing rain after daybreak Tuesday morning following a light coating of snow that falls before daybreak. It’s not clear if the models are over-predicting this freezing precipitation potential.  We’ll need to see tomorrow’s models to get a better idea of the thermal profile on Tuesday.

Sun 04:53 PM Update — Today’s models continue to downplay the snow for Tuesday.

NAM NEST snow totals Tuesday. Philadelphia has a coating

 Warm air moves in at 2000-3000  feet, resulting in a period of sleet and freezing rain instead of snow after daybreak Tuesday.  Little accumulation in and near the city. 


The storm for Thursday, while more developed, appears only to brush us with some snow.  Again, an inch is the most likely amount.

From Saturday evening…

The upcoming week looks to be a relatively active weather week compared to the past few weeks.


The storm expected to pass to our south on on Tuesday is showing less development and less moisture. Additionally, it now appears to move in later Monday evening and gradually exits during the day Tuesday.

Warm air appears to be brought in about 2000-3000 feet above ground level.  The predominant precipitation type will be sleet/freezing rain, then light rain on Tuesday instead of snow, after a small accumulation Monday night.  Total accumulations have reduced to under 1 inch for most of the immediate Philadelphia area—

GFS snow totals 10 AM Tuesday  (Click on image for a larger view.)

It appears that light rain showers, may linger on and off into the day Tuesday, tamping down or melting some of the small accumulation we receiving Monday night.  The main issue may be early Tuesday morning with some icy areas and perhaps some freezing rain in some areas—

GFS Precipitation Type and Surface Temp Tuesday 7 AM  (Click on image for a larger view.)  Please note that the GFS has been over-predicting freezing rain in recent storms.


Yet another storm is now showing moving to our south on Thursday. This storm is expected to brush us with light snow Thursday morning, however there is a possibility of greater intensification with this system—

GFS Precip forecast Thursday 10 AM (violet = snow)  (Click on image for a larger view.)

Currently, only about 1 inch of snow is expected with this Thursday storm.

Stay tuned.