Monthly Archives: November 2023

Philadelphia Area Weather Forecasts

THIS WEEK’S WEATHER

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Friday Update

Update Thu 11/23 9:38 PM — A weak cold front will move through mid day Friday with a few hours of mid and high level cloudiness.

Details for the weekend tomorrow. Currently, it appears that the showers will arrive Sunday evening between 6 and 8 PM. Initially, they’ll be quite light.

Possible Weekend Forecast Change

Updated Wed 11/22 @ 7:58 PM — The next few days are expected to be mostly sunny, but there’s been a shift in the forecast for Sunday.

Some background first:
Earlier this week, a coastal storm was expected to form in the Friday to Saturday time frame. Model runs by Tuesday had the storm moving out to sea, well to our south, not affecting our weather.

Today’s GEFS, NAEFS, ECMWF, ICON and GFS now have this coastal system moving up the coast late on Sunday and interacting with an approaching cold front late Sunday afternoon into evening, possibly during the Eagles game.

The timing varies, with the ECMWF having some rain moving in during the mid afternoon Sunday, while the GFS has the rain developing after 8 PM or so.

Here’s this morning’s ECMWF forecast for 7 PM—

Today’s 12 z ECMWF forecast 3 hour accumulated rain forecast for Sunday at 7 PM (Click on image for a larger view.)

There are currently timing differences, with the GFS slower and later with the rain here. This system has been tough to nail down and I would expect further changes in the forecast.

Wed 9:49 AM Forecast Review — We had the heavy rain as forecast, although totals were somewhat overdone by the HRRR (but more accurate with the development version of the RRFS). The banding of heavier rain set up close to the forecast with a strip of 2″ rains right through the immediate Philadelphia area.

Here’s the MRMS total rainfall for the past 24 hours—

MRMS rain guage – radar based estimate of rainfall received. Color shading is in inches. Line contours are in mm (25.4 mm= 1 inch) (Click on image for a larger view.)

Here’s yesterday’s 12z RRFS—

Yesterday’s 12z RRFS shows the 2.1″ band right through Philadelphia. Not bad! Note that my color coding is very different than the MRMS /AWIPS. (Click on image for a larger view.)

As for Wednesday, several models continue with the idea of mostly cloudy, but a few breaks of sunshine can’t be ruled out. It will be mild. There’s the possibility of a few widely scattered sprinkles about 8 PM this evening.

Sunny for Thanksgiving Day and into the weekend. The trend is for lower temperatures here into the next week or so.

Wednesday Outlook

Updated Tue 11/21 @ 8:36 PM — The rain moved in on schedule and the heaviest rain is expected between 11 PM and 2 AM tonight. Winds are also expected to peak in the same time frame.

Here’s the current situation at 8:30 PM—

Current satellite water vapor image with superimposed RAP model 500-1000 mb thickness lines (yellow), Mean Sea Level Pressure isobars (black contours) potential vorticity (fine white contours) with superimposed MRMS RADAR. The heaviest rain is still to our south and will move in later this evening. (Click on image for a larger view.)

The rain ends about 4-5 AM Wednesday and skies will be cloudy early morning. There will likely be some brief periods of sunshine early to mid morning, but instability cloudiness will be with us for most of the day Wednesday. There’s even a slight chance of a widely scattered sprinkle.

It will be very mild! High 59º-61º, but somewhat windy.

Thanksgiving Day will be sunny, but cooler. high 51º-53º

Tuesday Rain Update

Updated Mon 11/20 @ 5:30 PM — We’re back to an earlier start of the rain on Tuesday, with the GFS, HRRR and NBM all shows rain moving in from the southwest, as early as 12:30 PM in Chester County and moving into the rest of our area after 1-2 PM.

1 PM HRRR simulated radar forecast—

18z HRRR simulated radar forecast 1 PM Tuesday afternoon. (Click on image for a larger view.)

The latest HRRR cranks out some impressive rainfall and wind gusts later Tuesday approaching 40 mph —

18z HRRR total rainfall through Wednesday 7AM . (Click on image for a larger view.)

It looks like the rain will end about daybreak Wednesday. We may see sunshine as early as 9-10 AM. but instability cloudiness will quickly move in with considerable cloudiness by afternoon.

Tuesday Rain Update

Updated Mon 11/20 @ 10:02 AM — There has been some uncertainty with the onset of the rain in Philadelphia on Tuesday. This morning’s trend is towards a somewhat later start, about 3-4 PM, earlier in southern Chester county. The latest 12z NBM (which tends to lag when showing changes) is still showing a 1-2 PM start.

The total rainfall forecast based on this morning’s 12z HRRR—

12z HRRR total rainfall through 7 AM Wednesday. (Click on image for a larger view.)

I’ve been playing with the still experimental RRFS and it shows similar totals, although placement is different—

Today’s 06z RRFS TESTBED model totals. (Click on image for a larger view.)

The big story will be the WIND around midnight Tuesday into early Wednesday morning. Gusts are being forecast to 40 mph! Combined, with heavy rain, it will be an interesting 12 hours.

Previously Posted Sun 8:38 PM —

Low pressure is developing to our southwest and will move into our area Tuesday afternoon with rain. The rain may be heavy Tuesday night.

Current satellite water vapor image (Sunday evening) with superimposed RAP model 500-1000 mb thickness lines (yellow), jet level wind streamlines (orange contour-arrows) potential vorticity (fine white contours) and superimposed MRMS radar. The system in the middle of the country will move towards us and the rain will start Tuesday afternoon. (Click on image for a larger view.)

There has been some spread in the timing onset of the rain on Tuesday. Earlier forecasts were going with 4 PM. Recent models have some light rain as early as noon. Others delay the rain until 1-2 PM. The 1-2 PM start looks best at this time.

Rainfall totals are expected to be moderate to heavy with totals 1.6 +”. Here’s the latest NBM total accumulated rainfall forecast—

Today’s 18z NBM model accumulated rainfall Tuesday through Wednesday. (Click on image for a larger view.)

The heavy rainfall forecast has been consistently high. (We really need the rain!)

The speed of the system is still somewhat uncertain. The general consensus is that the rain ends Wednesday morning but clouds linger for much of the day. It will be quite windy.

Thursday, Thanksgiving Day, looks sunny but chilly. Highs upper 40s to low 50s.

There’s another chance of rain in the Friday through late Saturday time frame due to a coastal storm. Much uncertainty with the details right now.

Philadelphia Area Weather Forecasts

WEEKEND WEATHER FORECAST

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Additional information about the RRFS model has been posted.

The forecast for this weekend has been essentially unchanged for several days. As described earlier this week in This Week’s Weather, the frontal system moving through tonight will transfer its energy to a coastal low in New England. Much of the dynamics that go into precipitation will be lifting away from us and we’ll have very little rain.

The latest forecast from the NBM shows a bit more rainfall tonight than previous forecasts—

Latest NBM model forecast 00z , just available. Accumulated rain tonight (Click on image for a larger view.)

The front comes through tomorrow morning around daybreak with clouds breaking for sunshine by 9-11 AM and winds picking up late morning.

With a strong pressure gradient from the intensifying low pressure off the New England coast, our winds will be the big story on Saturday and to some extent Sunday.

NBM Wind Meteogram for location Blue Bell, PA (Click on image for a larger view.)

As mentioned last night, I’ve begun playing with the RRFS model still under development. Here’s its wind forecast for the weekend—

Today’s 18z RRFS wind forecast for Blue Bell PA. Somewhat more windy the forecast by the NBM. (Click on image for a larger view.)

Saturday

Cloudy early, then sunshine breaks out about 9-11 AM, earlier far west. Windy. High Temperatures will occur about noon, then decline

High temperature 55º ( NBM model— location Blue Bell, PA)
High temperature 57º ( NBM model— location Philadelphia, PA)
Uncertainty ± 1.7º which is average.

Sunday

Sunny and still somewhat windy. For the start of the Philadelphia Marathon 37º at 7 AM. By 10 AM 47º Still windy with gusts to about 18 MPH by 11 AM.

High temperature 53º ( NBM model— location Blue Bell, PA)
High temperature 55º ( NBM model— location Philadelphia, PA)
Uncertainty ± 1.3º which is higher confidence.

A large storm expected Tuesday through much of Wednesday with rainfall about 1.3″ in much of the area.