Monthly Archives: December 2021

Philadelphia Area Weather Forecasts

WEEKEND WEATHER FORECAST

Happy New Year!

Update Sat @ 10:11 PM — It hasn’t gone unnoticed that the TV/radio weather people haven’t really mentioned anything about the snow possibility for our region on Monday morning. I’m truly perplexed by it. But I guess that’s why you’re here reading this blog.

I must mention that the NAM, NAM NEST and ICON models have shown zero snow until this evening. ( They now show snow in southern NJ only.) Very strange to have such a wide divergence of forecasts.

Here’s tonight’s HRRR model, which continues to show snow, as does the NBM and the SREF.

HRRR 00z
NBM 00z
SREF 21z “large scale snow” 3 hour parameter

With all these models, the most likely forecast is snow in Southern New Jersey and a rapid taper to zero just west of the city. The surprise would be if the SREF just above is correct.

Update Sat @ 4:18 PM — I have more interesting data to share and chew on regarding snow early Monday. The most recent run of the HRRR (which forecasts out 48 hours) has more than moved in the direction of the GFS regarding possible snowfall for Monday. Here’s the latest HRRR snow totals for Monday morning—

HRRR 18z forecast for Monday at 1 PM. Snow totals. (Click on image for a larger view.)

Several models, including the Canadian GEM, the Canadian GEPS and the ICON Ensemble (ICON-EPS) have moved in the direction of the GFS with accumulating snow, especially for South Jersey. Their snow totals have a similar profile, decreasing towards Philadelphia, with maximum snow of 2.0 inches in Cape May County.

The possibility of snow has been a ongoing possibility since since I mentioned it in Tuesday’s blog post. It’s been on again, off again. The above HRRR is the latest, possibly the most extreme possibility.

If you’re just joining this this blog forecast discussion, I recommend you read the background info below.

Stay tuned.

Update Sat @ 11:10 AM — The latest GFS model just became available minutes ago. Hard to believe (literally), but here’s the snow the GFS is predicting by Monday morning. I’m just throwing this out there —

GFS 12z 01-01-22 Snow accumulation by 10 AM Monday. (Click on image for a larger view.)

Update Sat @ 9:58 AM — The period between Sunday and Monday is just coming into range of the higher resolution short range models.

Saturday —rainy and mild.

Sunday- Cloudy with a thinner cloud deck into the afternoon. Very mild until the front moves through.

The front moves through between 3 and 5 PM. Gusty winds with highly scattered light sprinkles as temps begin to drop.

Despite last night’s GFS model’s snow prediction, the other models all keep any precipitation to our to our far south and east. Even the NBM has removed snow from its forecast. So no snow late Sunday night into Monday.

Updated Sat @ 8:10 AM — No significant change in Saturday’s forecast. The spotty light rain drizzle in the morning today will become more steady by the afternoon.

The trend for late Sunday is for any measurable snow to be in Delaware and southern NJ (Cape May).

The GFS still has measurable snow here by Monday morning. The higher resolution short range models have no snow in our neck of the woods. The GFS ensemble still shows light precipitation falling as snow before daybreak south and east of the city, but no accumulation around PHL. (As was the case last week, the GFS over-predicted snow here. I’m going to lean away from the GFS this time around.)

Previously Posted Fri 6:10 PM —

The mild weather continues through Sunday. A continuing complex scenario is expected play out Sunday into Monday as a cold front approaches from the west and low pressure develops as it stalls.

Saturday

Low pressure to our southwest will sweep another warm front through Saturday. Saturday will be rainy during the day and evening through midnight. There may be some pauses in the action, but a rainy day expected. High temp 59º ± 3.0º

Sunday

The advancing cold front approaches during the day. Showers in the morning, then mostly cloudy until late afternoon (~4 PM), when the actual cold front moves through and additional showers develop. Becoming WINDY. Additional showers during the evening. High temp 59.5º ± 5º (high spread/high uncertainty!) NBM model Blue Bell.

I’ve been focusing on the Sunday evening period since Tuesday’s blog post and and it’s been on again off again regarding low pressure development. A new wrinkle has developed with today’s models—

The cold front appears to stall late Sunday night into Monday morning. Yet another low pressure system develops before daybreak Monday to our south. The precipitation shield appears to expand enough to cover our area with light snow.

The GFS and the ensembles (GEFS and SREF) show a light snow shield over us early Monday morning. Totals appear to be less than 0.5″ near Cape May NJ.

GFS forecast for Monday 5 AM. Violet areas show snow. Notice the red line (540 thickness line) along with other “critical thickness” lines in magenta and purple (determinants of average temperature at certain key levels of the atmosphere and “PTYPE” or precipitation type. (rain vs. snow vs sleet vs. freezing rain) The thickness lines are displaying much waviness, indicating potential for low pressure development. (Click on image for a larger view.)

The GFS has light snow only from 4-8 AM with just coating amounts in the immediate PHL area. The ensemble forecasts continue the possibility of light snow later into the morning with additional weak waves on the front.

NBM (Model Blend) forecast snow totals Monday late morning. (Click on image for a larger view.)

This continues to be a complex scenario and a low confidence forecast. I’ll keep an eye on this. Stay tuned.

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.