Monthly Archives: January 2021

Winter Storm Outlook

UPDATE ON SNOW

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.

Winter Storm Outlook

MORE ACTIVE WEATHER

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.