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Today’s models continue with the idea of weak low pressure developing along the front as it moves through early Wednesday.

There are differences in timing and snow amounts, but the statistical models, the GFS and the European have increased snow totals to 3 inches. We’ve seen these forecasts change drastically, but I’m letting you know that some amount of snow is expected during the morning rush hour on Wednesday.   Stay tuned

On my previous forecast, I mentioned that I would keep an eye on the strong cold front  moving through Tuesday night into Wednesday morning.  I mentioned that snow showers were possible Wednesday morning.

The most recent model runs suggest a trend that the rain mixes with and changes to snow sometime after midnight Tuesday as the upper atmosphere chills down, supporting snow.

While strong cold fronts often scour out any residual moisture, the models are showing that some weak low pressure development occurs along this front as it moves through, with moisture lingering after the cold air moves in aloft.

Latest GFS model forecast for Tuesday 11 PM showing snow-critical temperatures (arrows) have moved southeast of us.  Cold enough for snow. Shading is simulated radar.

The GFS has about 1 inch of wet snow for the Philadelphia area ending about 9-11 AM Wednesday morning  The NAM about 0.8 inches.   We know that things change.   Stay tuned.


Little change in the forecast since my “outlook” was posted yesterday.

Fast moving high pressure will bring cold temperatures for Saturday.  Saturday starts with some cloudiness, but skies clear by early afternoon.  It will be cold.  High 38º (based on the NBM model) or 43º ( based on the EKDMOS).   I think the colder model will be correct.  It will be breezy.

Early Sunday morning, a weak warm front moves across with some clouds early.   Skies clear for sunshine by mid morning.  Milder but still below average temps.  (High 43º NBM or 45º EKDMOS)

Rain for Monday and Monday evening.  A strong cold front moves through late Tuesday. 

Rain, turning to snow showers early Wednesday morning.  Very cold Wednesday and Thursday.   I’ll keep an eye on the system for late Tuesday into Wednesday.

For you weather model fans, the NBM (National Blend of Models) gets an upgrade to version 3.2 next week Dec 17th.  The NBM combines several short and medium range model forecasts, including  the European and Canadian models and statistically weights them according to their performance over their first six hour forecast accuracy.   It’s run every hour.  It’s been very good at predicting rain vs snow and total precipitation.


High pressure builds over us for the weekend.

Saturday will have some cloudiness in the early morning, then sunny and cold.  High temps an unseasonable 38 (NBM) or 42 (EKDMOS).   Fairly cold Saturday night with temps in the 20s.

GFS Model forecast 7 PM Saturday. Click for larger image

Sunday may also start with some clouds in the morning, but sunshinse by late morning through much of the afternoon.  High pressure moves to our east allowing a return flow of somewhat milder air.  High 43 (NBM) or 47 (EKDMOS).

Rain on Monday.

A significant chill-down expected by the middle of next week as a highly amplified pattern develops over the US.


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Mon 11:40 AM Update: This morning’s other models (WRF-NMM, WRF-ARW and the GFS) all show barely a coating to a fraction of an inch of snow accumulating by the end of this storm around midnight in the immediate PHL area and adjacent counties.  The HIREF shows a stronger snow shower possible about 6 PM, not substantially adding to the total.

As I mentioned last night, the new GFS probably did the best with this storm overall.   The much-touted European ECMWF seems to have over-forecast the snowfall here.  

…from earlier this morning—

I was able to review the latest NAM, NAM-NEST and RAP.  All are consistent with the coating to an inch for Philadelphia and surrounding counties.   Below is the NAM snow depth forecast for late this evening—

NAM Snow Depth forecast (in inches) for midnight Monday.  (click for larger image) Most areas much less than an inch around here.

I don’t anticipate any more changes.

Storm Forecasts & Other Things "up in the air".