Category Archives: Winter Storm Outlook


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, SREFHIRESW 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.


Update Fri @ 10:47 AM — This morning’s GFS (12z) just available shows even less snow by the changeover time of about 10 PM— less than 1 inch. Even less will be left by Monday morning after a night of rain.

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Of course, I’ll be updating the storm forecast with my regular weekly feature “Weekend Weather Forecast” Friday evening.

Update Thu @ 10:20 PM — Tonight’s NAM, just available, continues with the same forecast for Sunday evening into Monday. A transition to rain around Philadelphia about 10 PM.

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Update Thu @ 5:10 PM — (some additional changes highlighted below.) The storm forecast for Sunday evening into Monday has just come into range of this afternoon’s NAM model which forecasts up to 84 hours ahead. While its forecast doesn’t extend into Monday, it shows what we can expect for the start of the storm.

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It’s not clear whether there will be much snow on the back side of the system, but the trend is that there won’t be much.

It should be noted that the GEFS (ensemble) still maintains more of a coastal storm. So there’s a glaring lack of agreement with this storm. Things could change.

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Update Wed @ 9:03 PM — I’m going to need to change the title of this posting tomorrow to “Storm Late Sunday into Monday”.

This afternoon’s (18z) GFS and GEFS models have become available.

The GFS and ICON continue with a western track and a significant influx of warm air a few hours into the event. Any light snow at the start changes to sleet and rain and with a change back to snow possible but most of the moisture has move northeastward. So little accumulation in the immediate PHL area, according to the GFS.

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