THURSDAY’S SNOW UPDATE

Forecast Updated Wed 8:03 PM — This morning's update captured the trend towards lower snow totals.

Tonight's range models will begin to become available over the next hour (NBM, HRRR, SREF, RAP, NAM, NAM-NEST and HIRESW).  

The late afternoon trend continues with the main axis of precipitation falling from Philadelphia and south; these same areas will have the earliest transition to sleet/freezing rain.

This afternoon's Canadian Regional GEM and German ICON model show QPF values of 0.3-0.4 inches water (3-4 inches of snow) prior to the transition to sleet and freezing rain.

I hope to have an update by 9:30 PM.
Forecast Updated Wed 5:10 PM — Quick update. Precipitation values have reduced from yesterday's models.  3-4 inches is the more likely accumulation from the city north.  More details this evening.
Forecast Updated Wed 8:27 AM — The models are still predicting in the general 4-6 inch range of snow accumulation especially north of PHL, followed by sleet and freezing rain.   Some of the models that are run at 1 AM EST in the morning (the 06z Models) are showing a more southern track with less snow.   

Another issue with snow accumulation not to be ignored is the average temperature of the atmosphere up to 18,000 feet (as depicted by a derived parameter called the "1000-500mb thickness" ) is marginally too warm for most snowstorms in our region.   While it will be cold in the lower layers, the setup really supports sleet and freezing rain.    My real day job will take precedence over these forecasts today, but I'll be back this evening for an update.

 

Forecast Updated Tue 10:24 PM — Tonight's short range models have become available still show a heavy punch of snow in the morning, starting before 7AM Thursday followed by significant sleet and freezing rain. About 5-7 inches of snow before the changeover about 1PM.  Northwest areas of Montco and Bucks may stay all snow with higher accumulations.  

There have been some shifts in the model forecasts. The snow will start a little earlier than previously forecast starting as early 4-6 AM Thursday from southwest to northeast.

NAM-NEST snow accumulation entering at 5-6AM from southwest (Click on image for a larger view.)

 

The NAM and NAM-NEST are showing a heavier snow accumulation in the morning prior to a change to snow/sleet and eventually sleet.  

The Short Range Ensemble Forecast (SREF), which combines statistical variations of the short range models including the NAM, has quite a spread in the possible snow accumulations and snowfall timing, as depicted in this simple one location meteogram for Blue Bell—

SREF  Blue Bell showing large spread (uncertainty)in possible snow accumulations before a change to sleet The dotted lines are ± 1 sd     (Click on image for a larger view.)

 

Capturing the spread and uncertainty is the latest Model Blend  (NBM) snowfall totals—

NBM snow total percentiles. ***PLEASE NOTE THAT THE NOAA RELEASED A NOTICE TODAY THAT THEY HAVE FOUND ISSUES IN THE NBM PRECIPITATION STATISTICS. THE PROBLEM TO BE CORRECTED IN MARCH.*** (Click on image for a larger view.)

 

My preferred model for snowfall/sleet has always been the NAM model or a combined NAM-GFS blend.  Here’s the latest NAM (which tends to run high this much in advance)—

NAM 18z Water to Snow Equivalence conversion 10:1   (Click on image for a larger view.)

 

The NAM is showing an early burst of heavy snow during the morning.

Additionally, the NAM has an extended period of sleet and freezing rain on top of the snow after 12-1 PM.

This afternoon's GFS model has just become available as I write this.  The possibility of at least a 5-8" generalized snowfall is forecast with the snow -> sleet changeover delayed into the late afternoon.  

As usual, we have uncertainty with a coming snowstorm.  But the trend is for a more significant snow accumulation.    Stay tuned for updates.

 

 

THURSDAY’S STORM

Forecast Updated Tue 9:02 AM — A quick update. From the latest GFS model v 16 (scheduled to replace the existing GFS model and  become the operational GFS on March 16th)   A picture’s worth a thousand words—

Latest GFS version 16 Snow water equivalence with superimposed PTYPE. Snow accumulation by 8 PM Thursday  and predominant precipitation type by area.   (The current, operation GFS version 15.3 has slightly higher snow totals. )   We know things will change and in recent storms, the GFS has over-predicted snow. 

 

Snow starts between 6 and 9 AM Thursday. 

Next update early evening Tuesday.   Stay tuned.


 

Mon 09:59 PM Update — Tonight’s NAM has joined the rest of the models and now has snow at the start by 6-7 AM with 3-6 inches from the city north before the snow changes to sleet about 10AM – 1 PM.

NAM PTYPE Thursday 10AM  (Click on image for a larger view.)

 

Expect further changes.

 


From this afternoon…

A complex winter storm system will approach on Thursday morning and develop a secondary coastal low later Thursday.  As usual, there’s considerable spread regarding the precipitation type and amounts.  

I’m going to start with my own mantra—

“Never ignore the NAM model”

While the current GFS, ICON and Canadian GEM have about 3-5 inches of snow, starting about 7-9 AM Thursday morning , the NAM has it starting as all sleet, little snow, and changing to sleet- freezing rain by about 1PM—

NAM model showing freezing rain and sleet to our north (magenta)  by 1 PM.  Notice the position of the 32º freezing line (white) just running through Philadelphia. The green-blue-yellow is ‘simulated radar’ rain or freezing rain.   (Click on image for a larger view.)

 

The GFS has it starting as snow, but changing to sleet and freezing rain much later in the day Thursday.  The GFS shows 5+ inches of snow. 

If we follow the NAM mantra, the GFS is over-predicting the snow.

The Friday time frame is beyond the forecast range (84 hours) of the latest NAM, but the GFS  shows a secondary coastal low development with continuation of precipitation through Thursday night. The GFS doesn’t currently show much snow at the back end of the storm.

A very interesting storm system.  Stay tuned!