Category Archives: Winter Storm Outlook


Update Fri @ 4:54 PM — I sure hope my forecast is correct. The temperature is dropping. The latest MRMS shows the back end of the precipitation near upper Montgomery County is changing to snow. Most areas still rain or what’s called “cold stratiform rain”

MRMS at 4:41 PM The MRMS shows actual precipitation type: rain vs snow. Snow is shown appropriately as white. *Click on image to animate .* (Click on image for a larger view.) Super-imposed are current observed temperatures.

Update Fri @ 3:34 PM

“All this rain, I hope it doesn’t turn to snow”

It won’t, at least around here. I’m sticking with the NBM and Canadian RGEM models which clearly show the precipitation departs before the cold air comes in—

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Update Thu @ 11:13 AM — The new GFS model’s snow forecast just became available—

GFS 12z forecast Snow totals by 8 PM Saturday. (Click on image for a larger view.)

I’ll try to make sense of these giant differences later this afternoon. Right now I’m inclined to go with the GFS, but that could change.

Update Thu @ 10:17 AM — This morning’s early models are becoming available and their higher resolution forecast ranges are taking us to 1 PM or 7PM Saturday timeframe.

The dichotomy between the NAM and GFS still exists. Here’s the very latest NAM snow forecast—

NAM 12z snow forecast through Saturday 7PM (Click on image for a larger view.)

Despite the new NAM forecast, the other new models entering into the picture this morning are tending towards the much lower snow totals of the GFS. (about 1 inch in Philadelphia and less west)

The new GFS comes out just after 11 AM. I’ll take a look and update. Stay tuned.

Update Thu @ 8:43 AM — A quick update with last night’s 00z and 06z model trends. The GFS remains as the eastern outlier with the NAM back with a significant snowfall. European ECMWF still shows 4-6″.

Here’s current snow totals by model (for grid point: Blue Bell PA)

Current model snow forecasts by model for Blue Bell PA. (areas east will have more; this is only one point data)

More updates later… stay tuned.

Update Wed @ 11:11 PM — A quick look at tonight’s GFS just available shows virtually no snow in Philadelphia from this storm . Only eastern NJ. Stay tuned.

Update Wed @ 9:47 PM — This afternoon’s Canadian RGEM and German ICON are very similar to the earlier GFS with low snow accumulations in the immediate PHL area and westward.

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Update Wed @ 3:54 PM — A quick update. Today’s GFS, GEFS and Canadian GEM are all pointing to an eastern storm track, much lower snow accumulations. The SREF just available has backed away from extreme totals of the NAM as well.

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Update Wed @ 8:02 AM — A quick update:

Note that this morning’s GEFS models are tracking a bit more eastward, resulting in a rapid decrease in snow totals from Philadelphia westward.

Here’s the latest NBM percentile snow totals by Saturday afternoon. Note the tremendous range between the 25th percentile and the 75th percentile.

The 50th percentile is the larger number. (Percentiles are tricky concepts….the 50th percentile indicates that half of the models have this snowfall amount or less; half of the models have more. It’s NOT the model average, nor is it necessarily the most likely amount.)

NBM model snowfall percentiles. 25, 50 and 75th percentile. The 50th percentile, also referred to as the median, is the larger number. (Click on image for a larger view.)

Updates later this morning. Stay tuned.

Update Tue @ 11:18 PM — Quick update. Tonight’s GFS moves the track further east, significantly reducing snow totals. Updates tomorrow.

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Update Tue @ 5:25 PM — With this afternoon’s models, there’s been some interesting trends. The GFS has trended a bit closer to the coast compared to the GEFS as well as its position forecast from this morning. That said, it keeps most of the snow in NJ, with a fast taper to an inch or so in Philadelphia.

I did get a chance to look at limited data from the European ECMWF, which currently forecasts a deeper storm than the GFS, closer to the coast. It has a significant snowstorm for New Jersey into the Philadelphia area. I’m not ready to get on the ECMWF band wagon.

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Update Tue @ 9:01 AM — Last night’s model runs (00z and 06z) of the global models moved the coastal storm decidedly eastward, away from us. That said, there’s still highest uncertainty (standard deviation) in our direction.

Graphics below show consistent eastern track, with snow shield just brushing our area. Uncertainty graphic (darker colors) show similarity with greatest uncertainty westward and northeastward—

  • GEFS forecast for Saturday 7 AM

So this storm was never shown to hug the coast and current forecast track is consistent between models and with previous runs. Still, there is uncertainty as shown by high standard deviations to the east and northeast. Some snow still possible, as we get the northwestern fringe with current track. Stay tuned.

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Previously Posted Sun 9:07 PM —

GEFS statistical mode forecast with bias-corrected precipitation—

  • Wide cold trough over us on Wednesday through Thursday

Right now, we would be on the very western fringe of the precipitation (snow), with the main storm missing us. As is always the case with a forecast 5 days ahead, things are likely to change. Stay tuned.


Update Thu @ 7:25 PM — One more thought about the forecast gone wrong….. Most models got the onset timing correct and the end of the precipitation correct. They also got the general QPF (quantity of precip falling) correct. They went very wrong on the thermal profiles.

As for the weekend, as previously mentioned, the models have been in consistent agreement with the storm early Saturday staying to our south.

We’ll have very cold weather this weekend. High temps likely won’t exceed freezing. At times, there will be cloudiness due to an upper air trough moving through with additional weak cold fronts/upper air disturbances. Otherwise dry.

The next period of interest is Tuesday, where secondary low pressure may develop off of the coast. This period also has the potential to be a mixed-precipitation event with icy conditions or sleet. Just a possibility now.

Look for my regular, weekly “Weekend Weather Forecast” Friday afternoon or evening.

Thu 08:35 AM Forecast Review —  So…most models got this forecast wrong.  BUT there’s always something to be learned. 


First, which models did the best here?  As was the case a few weeks ago, the ICON  and the new Canadian GEM model correctly predicted the temperatures warmer and more precipitation as rain.

Canadian GEM model from Wednesday evening forecast for 8 AM today correctly predicted the above freezing temperatures and precipitation type being mostly rain (green) vs snow (blue). Note its 32º line is also closer to what we saw today.  (Click on image for a larger view.)


Of our major models, the NAM and NAM-NEST were the closest. 

The HRRR, RAP, GFS, SREF,  HIRESW and HREF  groups had the 1-3″.   Of course, since the NBM is a composite or ensemble model of other models, it steered its snow totals towards the greater population of high snow accumulation models.   Our new GFS v 16 model has been very disappointing.

Another thing to learn with the NBM:  the 25 percentile snow totals seems to be surprisingly accurate lately here.  I have to admit, yours truly ignored the low-end NBM percentile and I discounted the warmer Canadian GEM and ICON as total outliers last night. 

(BTW, last night’s ICON and GEM had predicted most of our precip as rain with just a coating possible.)

I may have to add to my mantra: “never ignore the NAM, GEM and ICON” 


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Update Wed @ 9:27 PM — Tonight’s early models have become available.

Rain about 3 AM with the arctic cold front passage will change to snow from northwest to southeast between 6 and 8 AM.

Snow continues until about noon.

Of interest is the trend of this afternoon’s NAM and the new NAM and NAM-NEST which just became available.   It has less than an inch throughout the region. The reasons are:

  • The ground will start off wet.
  • The ground will be warm, above freezing, to start
  • Snow-water ratios will be low until it gets colder
  • Compaction of wet snow will occur initially


Latest NAM Model forecast (Click on image for a larger view.)

Regulars here know my mantra: “Never Ignore the NAM”

The latest NBM model is below and it captures most of the models forecasts from today. —

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Another impulse in the Gulf of Mexico may also slide to our south on Saturday. Again, we appear to be on the northern fringe.

In both cases, the currently forecast jet configuration is positively tilted and significant low pressure development near the coast is not currently forecast.

I’ll keep an eye on things. Check back during the week.