Sat 9:00 PM—The latest GFS continues with a very cloudy but relatively rain-free Sunday.

This weekend’s forecast has been anything but straight-forward, as there have been huge differences in the major models for several days. I’ve put extra focus on Center City and the Philadelphia Broad Street Run forecast time frame.

Things are coming more into focus, with the Sunday period coming within the range of the higher resolution models. (Most models only have a forecast range of 48-84 hours, with the majority only 48-60 hours.)

Overview: High pressure over the North Atlantic will nudge down over us tonight as a back door cold front. An easterly flow will develop around this high pressure system. A coastal system with tropical characteristics will attempt to move up the coast, but will be blocked by the high pressure system.

Saturday

An easterly flow will develop over our area, bringing in moisture and clouds. The models differ with the amount drizzle, but several models have some bright spots in the morning, followed by low clouds and widely scattered areas of drizzle by afternoon. High temp 70.2º sd 1.8º NBM model, Blue Bell. Winds ENE 7-13 mph

Update Sat @ 10:01 AM This morning’s HRRR still shows some sun for Saturday afternoon.

Update: Fri 7:37 PM Friday night’s RAP and HRRR show showers before daybreak Saturday, then sunshine through high clouds Saturday morning into early afternoon. Then clouds move in during the mid to late afternoon. No rain, according to the RAP and HRRR during the day on Saturday.

Update: Friday’s GFS has some sunshine through high clouds Saturday afternoon. Not as cloudy as the other models.

The increasingly easterly flow around the high and the pressure gradient with the blocked low will result in some heavier drizzle and light rain Saturday night, mostly after midnight.

Sunday will be cloudy. The models continue to diverge with the amount of rain on Sunday. The GFS and GEFS have come somewhat closer together, with light rain/drizzle in the early morning before 8 AM, then tapering off during morning.

Total amounts of rain are light, regardless of the model. (The NBM has less than 0.17 inches for center City Philadelphia after 8 AM. ) The GFS and ICON continue with MUCH less, on the order of 0.03″.

High temp 69.7º sd 1.9º NBM model, Blue Bell. Winds light ENE with occasional gusts to 13 mph.

Sat 9:00 PM—The latest GFS continues with a very cloudy but relatively rain-free Sunday. There is still a low range of precipitation forecast by other models, but I continue to favor the GFS model’s very cloudy but relatively dry forecast.

Sat 8:51 AM— The latest GFS model from early Saturday morning (06z) continues with a relatively rain-free forecast for Sunday.

I’m leaning towards those GFS lower rain amounts after 8 AM.

(Heavier rain expected east, towards the Jersey Shore.)

The latest GFS (18z model run= 2 PM EDT) just became available as I post this. Still low amounts in Philadelphia.

Updated Sunday forecast on Sat @ 6:30 PM highlighted below

Overview: The good weather blocking maintains itself for most of the weekend. Moisture aloft in the form of some high, thin cloudiness both Saturday and Sunday. An overall beautiful weekend, weather-wise.

The blocking pattern that has been providing the extended period of fair weather will be breaking down and a return flow around departing high pressure will bring in moisture initially in the form of high clouds. Showers approach late Sunday evening.

Saturday

Sunny in the morning. Mild. Moisture aloft will move in to our area, with sunshine through high cloudiness Saturday afternoon. High temperature 76.4º ± 2.1º (NBM Model Blue Bell) Light winds.

Seasonal average high temperature this time of year is 69º (Blue Bell) and 71º (Philadelphia)

Sunday

High pressure continues to move off to our east as more moisture moves in aloft. Warmer air moves in.

While Saturday’s forecast overstated the high thin cloudiness based on the GFS, most models have increasing cloudiness on Sunday per the paragraph below.

The models are forecasting a varying degrees of cloudiness, but the current trend is for some early morning cloudiness, then sunshine, then increasing cloudiness after 3-6 PM, especially western suburbs.

There is some degree of uncertainty regarding the amount of cloudiness on Sunday. This afternoon’s NAM and NAM-NEST bring in some showers late afternoon and early evening. Likely too fast.

High temperature 80.4º ± 1.8º (NBM Model, Blue Bell) Light winds.

Overview: The deep low that brought the cold front through Thursday night will move off into Canada, leaving our area in an upper trough of cooler air.

This upper jet level trough will usher in a weak secondary cold front Saturday afternoon.

Behind the weak front, high pressure builds in for Sunday.

Average season high temperatures this time of year is ~72-73º

Saturday

The jet trough will usher in a weak secondary cold front Saturday afternoon causing some considerable high clouds (cirrus) and some considerable mid-level cloudiness in the far northwestern suburbs. (Basically, more cloudiness expected the further northwest you are.)

Sunny skies in the morning and early afternoon, some high to mid-level cloudiness in the late afternoon. More cloudiness in locations far north and west of the city. High temp 75.0º ± 1.1º NBM Model, Blue Bell. Light winds.

Sunday

High pressure builds in. Sunny skies. High temp 71.4º ± 1.1º NBM model, Blue Bell. Breezy at times.

Next Week

Next week, much of the country moves into a much warmer upper air ridge, although the northeast remains in a cooler upper northwesterly flow. Except for some impulses moving down, no significant storms.