Monthly Archives: December 2020

Weather Updates

UPDATED FORECAST- CHRISTMAS EVE

Updated Forecast Cold front passage delayed. Rain may linger into Friday morning, changing to snow flurries.

As mentioned in my updated post last night, the trend has been for the cold front to move through later and later. The model forecast has moved the cold front passage later, from 1 AM Friday morning to about 4-5 AM Friday (GFS model) as low pressure tries to develop when the front hits the coastline. See the GFS below—

GFS surface forecast 5 AM Friday (Click on image for a larger view.)

(This low pressure development along the coast was first suspected to be possible several days ago, but the models until today did not show it.)

The models are predicting high wind gusts of 50-60mph during the night time hours and cold front passage.

Rain will start as early as 4 PM Thursday afternoon and become very heavy during the evening. A total QPF of 1.5-2 inches of water is possible by Friday morning.

The NAM-NEST suggests possible convective (thunderstorm) activity as the front moves through—

High resolution NAM-NEST 3AM  Friday Precipitation Rate (Click on image for a larger view.)

The exact end time of the precip has become less clear as there are some differences in timing of the frontal passage as well as the degree of low pressure development. I would not be surprised if the low pressure system shows greater development.

The NBM (model blend) has picked up on this low pressure development and keeps a forecast of rain and then snow flurries lasting into Christmas day.

High temperature Friday will occur in the morning and fall during the day. Friday will be cloudy, cold and windy.

With the front slowing down, I expect further changes in the forecast. Stay tuned.

Philadelphia Weather Outlook

Rainy and Windy CHRISTMAS EVE

Thursday evening, Christmas Eve, is looking quite rainy and windy. Friday cold, windy with clouds. The weekend looks to be dry and cold.

A very strong cold front will move through Thursday evening and night, Christmas Eve.

GEFS 300 mb jet stream winds at 7 PM Thursday.  (Click on image for a larger view.)

An extremely sharp dip in the jet stream will usher in a cold front, that most models have moving through between 1 and 2 AM Friday morning.  Preceding and accompanying the the actual frontal passage will be fairly heavy rain (1+ inch)  but also very high wind gusts. 

Rain starts late Thursday afternoon and gets heavier during the evening and night-time hours. Winds will pick up as the evening progresses. Wind gusts of 50-60 mph are predicted with the highest winds with the front passage about 1 AM Friday morning.

There was some thought over the weekend that low pressure might develop along the front, but all models have ruled out snow here with the exception of snow flurries.

Cold air filters in during the day Friday on brisk winds. The early morning temperatures will be the high temperatures of the day and temperatures will fall during the day. Wind chills in the teens.

Instability cloudiness will also be with us Friday and a snow flurry can’t be ruled out.

Saturday will be cold and still a bit windy. Highs will struggle to reach 30º. Right now it looks to be sunny, but it’s difficult to time upper air disturbances which may bring periods of clouds.

An approaching storm from the Midwest will bring some increasing cloudiness during the day on Sunday.