Category Archives: Winter Weather


Last night’s models left us in two camps with different predictions about the amount of sleet and rain that will mix in this evening, with the effect of reducing total snow accumulations. 

As explained in yesterday’s post, while the “main” model runs are done at 00z and 12z, many models are re-run six hours later at 06z and 18z.  At this time of the morning, we have the 06z (1AM) model runs to see what trends have developed.

Here are the trends—

  • Snow starts between 12 and 2 PM today
  • Change-over to sleet and rain 6 PM- 11 PM
  • High gusty winds 40-50 mph this evening
  • Change back to snow after midnight
  • Snow ends 7-10 AM Thursday morning.

The NAM and NAM-NEST continue with significant sleet and less snow,  while several other models which had similar sleet forecasts (Canadian GDPS and RDPS) have more snow now.

At this point in the model cycle, the NBM (national model blend) 50 percentile forecast is the one to hang one’s hat on and the model’s statistical blend allows us to see the two different camps.

First, let’s look at the 25 Percentile—

The NBM 25 Percentile (meaning that only 25% of its model components have this forecast as its maximum snow depth—

NBM 25 Percentile Snow totals  (Click on image for a larger view.)


The NBM 50 Percentile (half of its models are above this value, half are below) is most representative of the current GFS, HRRR, CMC and ECMWF forecasts.

This is most likely total snow accumulations to expect

NBM Snow 24 hour accumulation 50 Percentile. (Click on image for a larger view.)


The above 50 percentile is the most likely snow totals to expect with this storm by Thursday morning. 

I’ll keep an eye on things. Stay tuned


The most important weather model runs, based on global weather balloon (“radiosonde”) upper air measurements, are done twice a day— at 7 PM EST (00 UTC, also called 00z)  and 7 AM EST (12 UTC, also called 12z).

These new upper air measurements often account for any significant change in the model forecasts. The latest NAM-NEST has a decrease in snow totals due to significantly more sleet.

Here’s the latest 00z model data, based on these new upper air measurements.

The 00z HRRR (High Resolution Rapid Refresh) shows snow developing between 12 noon and 2 PM Wednesday and ending 7-10 AM Thursday. It will become very WINDY Wednesday night.

The HRRR continues the trend for sleet to mix in with the snow in a large portion of the area during Wednesday evening, and then change back to snow before ending.

HRRR sleet, rain snow precip type.  9 PM Wednesday  (Click on image for a larger view.)


Despite the changeover to sleet at times, the latest HRRR has increased snow totals by Thursday morning—

HRRR snow total forecast  (Click on image for a larger view.)


The 00z NBM (Model Blend) shows a similar snow total forecast—

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


Several models are showing wind gusts in excess of 50 mph Wednesday evening.

Speaking of model forecast changes, the NAM NEST just available has more sleet and much less snow

NAM NEST Snow  (Click on image for a larger view.)

I think the NAM-NEST may be too warm with too much sleet.   We’ll have to see the trends into tomorrow morning.

Right now I’m sticking with the HRRR and NBM snow forecasts above.


Here’s the latest information about the upcoming snowstorm for the Philadelphia area.   The timing of the storm has come within the forecast range of the high resolution models.  

Here is the current timing based on the latest HRRR (High Resolution Rapid Update) model— snow starts in our area, from southwest to northeast about 12-3 PM Wednesday.  Heaviest snow and mixed precipitation during the evening hours Wednesday.  Snow ends about 4 AM Thursday morning.

There’s still some uncertainty about warm area mixing in at certain levels, changing the snow to a mix at times and reducing total accumulations.  The snow accumulations for several models has reduced a bit in the immediate PHL area, despite the fact that the snow-water equivalent has remained the same. This indicates some mixed precipitation will reduce snow accumulations.

Here’s the current latest HRRR snow totals—

HRRR model Snow accumulation forecast by 7 AM Thursday (Click on image for a larger view.)


Here are some other perspectives: The German ICON model is somewhat similar—

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

The European Model (ECMWF), which is run only twice daily.  This is the latest forecast from last night (the new model will be available about 2 PM EST today)  shows somewhat less—

ECMWF Snow forecast  (Click on image for a larger view.)

The NAM just available continues to show the storm a bit further north, introducing warm air into the mix, reducing snow totals from the above values.

Those of you who have been long-time followers of this blog know that I’ve been a big fan of the NAM for snowstorms, but I never used its built-in snow depth forecasts, instead doing the calculation based on FOUS QPF, thickness and temperature data. By those measures, the above forecasts from the above HRRR look pretty good, maybe even on the low side.

So, I think with current model advancements, I’m going to hang my hat on the new NBM model and use the 50-90 Percentiles as the best forecast—

NBM 50 and 90 Percentile range Snowfall forecast (Click on image for a larger view.)

The next big updates in models will be this evening. The NBM 00z forecast becomes available at 8:30 PM EST. The HRRR, NAM between 8:30PM and 10 PM. Stay tuned for more updates…

Tue 10:47 AM Update — Some new models coming in suggest that the lower range of snow (50 percentile numbers above) may be the better choice.


The latest models have become available for the snowstorm forecast for Wednesday afternoon into early Thursday morning.  Current timing is that snow starts about 2 PM Wednesday and ends around daybreak Thursday.  A significant nor’easter is forecast by most models.   The latest GFS snow totals forecast is also representative of the Canadian and ICON models—

GFS snow totals by Thursday morning.

However, there has been a re-emergence of a complication that I had mentioned yesterday. The NAM, SREF and NAM-NEST have a wedge of warm air being drawn in at 3000 feet during the peak of the storm Wednesday evening, changing the snow to sleet or a rain-snow mix—

NAM model at 6 PM Wednesday.  Lavender line (white arrows) shows freezing temperature at 3000 feet north of the Philadelphia area.  This would result in a changeover to sleet and rain during the storm peak. (Click on image for a larger view.)

As a result, the NAM and NAM-NEST show snow totals much lower—

NAM Snow totals Thursday morning  (Click on image for a larger view.)

So there’s some real differences with the higher resolution models (that are just coming into forecast range) and the GFS, Canadian and current ICON models.

Most higher resolution models only forecast 48-60 hours in advance. They will begin to come into forecast range with this evening’s model runs and certainly by tomorrow late morning. Stay tuned.