Daily Archives: Thursday, December 30, 2021 5:26 pm

Philadelphia Weather Outlook

NEW YEARS WEEKEND WEATHER OUTLOOK

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Updated Fri @ 9:58 AM — Some higher-resolution data coming in for Sunday’s forecast. Rain in the early morning. Then a break in the showers early afternoon, with even some sunshine.

Temperatures in the 60s! Becoming very WINDY. A line of showers moves through 4-6 PM with the cold front. Temperatures drop with gusty winds. Showers may linger into after midnight, then possibly change to snow showers.

Updated Fri 9:29 AM — Last night’s NAM and Canadian GEM and European ECMWF have followed the GFS in predicting another area of low pressure developing. However, for snow lovers, this low pressure system will, at most, brush us as it departs northeastward. Currently, at most, a coating of snow possible at daybreak Monday. This coating is most likely possible in southern NJ, Cape May—

This morning’s NBM 12z Accumulated snow probability > 0.10 ” by Monday morning. Low amounts, low probability, limited areas. (Click on image for a larger view.)

Just not a large enough cold air mass or amplification of the jet stream for it to give us snow here. But I’ll keep an eye on this. My regular weekly, “Weekend Weather Forecast” will be posted this evening.

Update Thur 6:40 PM— The GFS forecast below for Sunday is currently an outlier, with the European (ECMWF) Canadian (GEM) and German (ICON) models have the precipitation missing us, exiting to our south. This is an evolving forecast.

Cold air, which has been conspicuously absent this past month, has been building up in northern Canada and indications are that it has reached enough critical mass to plunge (at least temporarily) down into the continental US and into our area by the end of this weekend.

A strong frontal boundary has developed separating the very cold air from unseasonably warm air. Low pressure systems have been developing and ejecting towards us in this warm sector, giving us the clouds and occasional showers these past few days.

Currently, the cold air is expected to plunge southward and then eastward. Depending upon how far southward vs eastward will determine the shape of the jet stream and the path of low pressure expected to develop on the front.

GFS forecast for Friday at 7 PM. Will the high pressure system move mostly southward (1) or more eastward (2)? The more eastward plunge (2) was forecast, but the trend has been more southward (1) over past days. The red line is 540 thickness line, a rough estimate of the contour of the cold air mass. (Click on image for a larger view.)

New Years Eve

Cloudy and mild. Rain doesn’t get here until 2-5 AM Saturday morning.

Saturday

Some rain in the morning. Heavier rain by mid afternoon.

GFS forecast 2 PM Saturday. The plunge seems to be more southward. (Click on image for a larger view.)

Sunday

This forecast is still evolving. This is preliminary. Some rain early, becoming WINDY. The front comes through either mid afternoon or later, somewhat influenced by the degree of low pressure development. If the low tracks northward, the usual rain-snow dividing line may set up here.

Here’s the latest GFS model, just available —

GFS forecast for Sunday midnight. Red thickness line is the rain snow line. Not clear which path this low will take. (Click on image for a larger view.)
Latest GFS forecast for 5 AM Monday morning. Some snow forecast for Philadelphia. (Click on image for a larger view.)

Stay tuned.