Monthly Archives: December 2020

Winter Storm Outlook

MONDAY & Wednesday SNOW UPDATE

2 Comments

The latest models are just becoming available this evening. Most models have it too warm for snow in the immediate Philadelphia area.  Some snow accumulation is possible far western areas.  The NBM (National Blend of Models) 75 percentile snow accumulation captures it well. (75% of the constituent models and their statistical members are equal or below this accumulation amount)—

NBM 75 percentile snow forecast for Monday. (Click on image for a larger view.)

 

The latest HRRR and NAM, just available, is consistent with this NBM forecast. There may be some snow mixed with rain in areas that show zero accumulation towards the mid afternoon.

Rain starts around 6-8 AM Monday and ends about 3-4 PM.  It may briefly mix or change to snow in the western suburbs as it ends.

As for Thursday—

 

The models continue to show a major storm for Wednesday into Wednesday night.  It will be a stormy day by Wednesday afternoon with high gusty winds.  (At one point today, it appeared that warm air might move in, but the newest models are showing significant snow accumulation for Wednesday afternoon and night.) Here’s the latest GEFS snow totals by late Wednesday night—

GEFS Snow Totals (Click on image for a larger view.)

 

Here’s the NBM 75 Percentile snow totals—

NBM 75 percentile snow totals by late Wednesday night.  (Click on image for a larger view.)

 

Very clear is the sharp cut off in NJ as temperature will be too warm for much accumulation there.

I’ll be updating during the week. Stay tuned.

Winter Weather

MONDAY SNOW UPDATE

…


So, this all started a few days ago when the soon-to-be released GFS model version 16.0 had predicted some light non-accumulating snow for Monday when the other models were predicting all rain.

Since that time several (but not all) models have jumped on the light snow band wagon. There’s still much uncertainty about the temperature profiles on Monday. Many models show a change over to snow, but not all show accumulating snow in the immediate Philadelphia area Monday.

At this point in time, it’s best to move over to the new National Blend of Models (NBM) which runs hourly and statistically blends about 20 different models (including the much-touted European Model) with a complex statistical weighting based on each model’s accuracy over the immediate previous six hours.

One of the features of the newly release NBM version 4.0 is the statistical handling of many parameters, including snowfall.

The most recent NBM model shows rather unimpressive snow totals for most of us on Monday. (Again, it’s a blend of models. Current models aren’t all on-board with snow accumulation.)

So let’s try something new—snow prediction via statistical percentiles.

Here’s the total snowfall predicted in the 75th percentile (75% show this amount or less)—

NBM 75th percentile  (Click on image for a larger view.)

Here’s the total snowfall predicted in the 90th percentile (90% show this amount or less)—

NBM 90th percentile snowfall (Click on image for a larger view.)

As you can see, most of the immediate PHL area won’t see any accumulation on Monday, although precipitation will fall as rain changing to snow.

In contrast, let’s look at the very latest high resolution NAM-NEST model forecast—

Tonight’s NAM-NEST snowfall forecast for Monday  (Click on image for a larger view.)

I would go with the NBM model, somewhere between the 75th and 90th percentile amounts above.

 

As for Thursday, the already highly-advertised snowstorm still continues to threaten. I’ll be covering this during the week. Food for thought is the current ICON model—

German ICON model snow forecast  (Click on image for a larger view.)

For a change of pace, also of interest is the Canadian Global model—

Canadian Global GDPS Snow forecast for Thursday

The ICON model, which I recently found access to, has been pretty good in recent months. The Canadian model tends to over-state snowfall in recent storms last year.

Stay tuned.