Update Mon @ 8:18 PM — Comparing today’s 18z GFS, GEFS, Canadian GEPS, and ICON model’s s forecast at 7AM Saturday—

  • GFS 18z Forecast 7 AM Saturday

The above graphic shows that the latest version (18z) of the GFS is forecast to be further away from us than either the 18z statistical version (GEFS) or the the latest ICON. The Canadian 12z GEPS (not run at 18z) is very different with speed and position. Click on the above graphic to animate.

Update Mon @ 5:24 PM — I was just about to consider a separate web page for this possible upcoming weekend storm. But the very latest GFS 18z just became available and it has the storm taking an eastern track, mostly missing us.

GFS 18z showing eastern track of storm with precipitation missing us. (Click on image for a larger view.)

In this time frame, the statistical ensemble models are generally used, to try to minimize the “model of the hour” or “model of the day” effect. But it is interesting that there’s been this shift eastward again. Stay tuned.

Update Mon @ 11:45 AM —Here’s the latest on the potential Nor’easter for the Philadelphia area late Friday into Saturday.

While previously looking like it would miss us, the latest trend is for the storm to be closer to the coast. The GFS as well as the GEFS mean and GEFS mode have all trended westward. The potential for several inches of snow by Saturday morning is there. The Canadian models have different speeds but somewhat similar westward trends.

Here’s the very latest GFS forecast for 7 AM Saturday—

GFS forecast 7 AM Saturday showing precipitation rates. Anything west and north of the red 540 thickness line would be snow. The other thickness lines along the coast suggest NJ would have sleet and freezing rain. An evolving situation with much time for things to change. (Click on image for a larger view.)

This is an evolving situation with plenty of time for things to change in a multitude of ways before Friday. Stay tuned.

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A relatively quiet seasonably cold flow will change to a highly amplified flow by Friday. Energy in the northern jet is expected to dive south Friday and phase with the southern jet stream, creating an intense coastal storm. Multiple models are forecasting this storm and multiple models have the storm somewhat missing us.

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 Sun @ 6:37 PM — Today’s models have also backed off on any snow for Tuesday. Next interesting period to watch is Friday into Saturday.

Update Sat @ 9:33 PM —Tonight’s models has backed off to just snow flurries Sunday evening.

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Originally Posted Fri 5:04 PM


Sunny and continued very cold. Gusty winds in the morning, subsiding in the afternoon. High temp 28.6º ± 2.6º (NBM model Blue Bell)


An upper air trough axis moves through.

Some cloudiness in the early morning, especially north and west of the city. Then partly/mostly sunny. Continued cold. Gusty winds. Clouds move in late afternoon.

The GFS is forecasting an upper air disturbance possibly bringing a dusting/coating of snow late evening Sunday, towards midnight. The NAM has some light snow earlier in the evening.

High temp 32.3º ± 2.5º (NBM model Blue Bell).


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.

Weather… and Other Things 'Up in the Air'

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