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—

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 .

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

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.

 

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.

 

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—

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.

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)—

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.