Category Archives: Philadelphia Area Weather Forecasts

Philadelphia Area Weather Forecasts

WEEKEND WEATHER FORECAST

#Phillywx #Philadelphia #weather #PAwx

Sun 9:36 AM Forecast Review — The total rainfall yesterday fell short of several model forecasts in most areas. The system, originally expected to linger into Sunday, cleared out before daybreak, reducing totals. Retrospectively, the GEFS and NAEFS ensemble models had more accurately forecast the rain totals. Here’s the MRMS rain gauge and radar estimate for total rainfall yesterday—

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

The forecast update posted last night still holds. Sunshine, with instability cloudiness expected this afternoon.

Sunday Forecast Update

Updated Sat 10/14 @ 9:54 PM — Rain will be ending about 3 AM and skies clear around sunrise. Mostly sunny in the morning, sun and periods of cloudiness in the afternoon due to instability.. Windy. Highs 59º Blue Bell, 61º Philadelphia.

Saturday Forecast Update

Updated Sat 10/14 @ 9:51 AM — Showers have moved through Philadelphia and a steadier rain is on the way by late morning.

The latest water vapor image shows the likely placement of the secondary low pressure expected to form off the coast—

Current Water Vapor (9:46 AM) image with superimposed RAP model thickness lines (yellow), potential vorticity (fine white contours) and superimposed MRMS radar. Notice the fine white contours of Potential Vorticity over the “black band” of the water vapor image in the red box. This is the likely location of the secondary low formation. (Click on image for a larger view.)

The latest NBM (12z) just available shows its latest estimate of total rainfall with this system —

Update Fri 10/13 9:49 PM — The earlier start of the rain Saturday, between 7 AM to 9 AM, is now forecast by additional models.

Previously Posted Fri 6:01 PM —

As forecast this past week in “This Week’s Weather”, a low pressure system will approach from the west early Saturday and a secondary coastal low will quickly develop along the coast later Saturday through Saturday night.

Here’s the ECMWF forecast for 2AM Sunday morning, showing the secondary coastal low—

ECMWF forecast for 2 AM Sunday morning (Click on image for a larger view.)

A generalized 1″-1.3″ rainfall is forecast for most of the area through early Sunday morning.

Saturday

Cloudy with rain moving in from the west anytime between 8 AM and noon.

(There’s been a wide spread of times for the rain to start here, depending upon the model. This afternoon’s high resolution models have closer to a noon start time. With this sort of thing, I tend to lean towards the GFS which has an 8-9 AM start. ) Increasingly windy.

High temperature 55º ( NBM model— location Blue Bell, PA)
High temperature 57º ( NBM model— location Philadelphia, PA)
uncertainty ± 2.3º

Sunday

Any remaining showers ends early morning. Clouds break for some sunshine 2-5 PM in many (but not all) areas. Windy.

High temperature 58º ( NBM model— location Blue Bell, PA)
High temperature 59º ( NBM model— location Philadelphia, PA)
uncertainty ± 1.8º

Winds
NBM Wind Meteogram for Blue Bell, PA (Click on image for a larger view.)
Philadelphia Area Weather Forecasts

WEEKEND WEATHER FORECAST

#Phillywx #Philadelphia #weather

Sunday Update

Updated Sun 10/08 @ 11:34 AM — My forecast already included clouds and isolated sprinkles today. It appears that a significant wave has developed to our southwest and we may have considerably more cloudiness than forecast at times. Breaks of sun possible and still a chance of isolated sprinkles as well.

Sunday morning WV image with superimposed RAP model thickness lines and potential vorticity contours (white). You can see why they call it a “wave” here. (Click on image for a larger view.)

Sunday

Update Sat 10/07 9:47 PM —

Sunny in the early morning. Periods of clouds and sun in the afternoon. There’s a chance of isolated instability sprinkles in the mid to late afternoon, especially north and west of the city. Windy.

High temperature 60º ( NBM model— location Blue Bell, PA)
High temperature 62º ( NBM model— location Philadelphia, PA)

Sat 3:57 PM Forecast Review — Yes, the model forecasts have been a bust today. More specifically, the rain totals have been a mere fraction of what was forecast, even this morning. Here’s this morning’s NBM rainfall forecast—
Today’s 06z NBM rainfall forecast for Saturday. (Click on image for a larger view.)

Here’s the current estimated actual measured rainfall that occurred over the past 24 hours based on radar and rain gauges (MRMS estimate) —

MRMS based total 24 hour rainfall period ending 3 PM. based on combined radar with actual rain-gauge measurements (Click on image for a larger view.). (Click on image for a larger view.)

The model forecast guidance usually impresses me. Not over the past three days. As I’ve mentioned over the years, the presence of a tropical system nearby seems to affect the accuracy of the models. Not sure why that is, and it’s just an observation based on my many years doing this stuff.

I had mentioned earlier this morning that the HRDPS did well.

HRDPS (06z) forecast for total rain Saturday. (Click on image for a larger view.)

Well, not perfect by any means, but the rainfall totals were more in line with what we received and it even placed the 0.00″ area fairly close to where no rain was received. Yet areas in NJ received no rain, despite some significant rain being forecast.

Saturday Update II

Updated Sat 10/07 @ 10:00 AM — So, which model has done the best with this system? It appears the Canadian HRDPS has done very well. It even shows the break of sunshine we had over the past 3 hours.—

HRDPS cloud cover at 10 AM. (black/grey is cloud cover). (Click on image for a larger view.)

While we’re on an HRDPS roll, the model has very light total rainfall for much of our area—

06z HRDPS total rainfall through 2 AM Sunday morning. (Click on image for a larger view.)

Updated Sat 10/07 @ 9:33 AM — Here’s the latest WV/Radar image—

WV MRMS radar with RAP model “most unstable” CAPE (yellow contours) . While the front has moved through, the rain/showers with this system is sill to our west and will be approaching. Model forecasts suggest most of the rain will be to our northwest. (Click on image for a larger view.)
Saturday Update

Updated Sat 10/07 @ 9:16 AM — The model forecasts over the past two days haven’t been that stellar and today’s forecast continues that trend. The front has just moved through the immediate Philadelphia area, based on wind shifts in the RTMA—

RTMA wind shifts at 9 AM. (The RTMA model is run every 15 minutes with a 15 minute computational time delay.) Colors show wind gusts entering western suburbs with streamlines showing the wind shift to the northwest. (Click on image for a larger view.)

Additional showers expected, but much of the heavier rain is moving off to the northwest.

Previously Posted Fri 5:52 PM —

An upper level trough will drive a cold front through Friday night into Saturday morning—

Water vapor image Friday afternoon shows upper and mid level of the atmosphere. (FYI, It doesn’t show clouds.) Areas of vorticity encircle the upper trough. This trough is forecast to close off and become a ‘closed upper low’ around Quebec province. Disturbances (areas of vorticity) rotating around this low (blue arrows, white contours) will set off cloudiness at times this weekend. (Click on image for a larger view.)

Saturday

The front comes through the Philadelphia area around daybreak Saturday with showers and wind gusts. Areas of vorticity behind the front will set off additional showers into the afternoon. There may be a quick break of sun, but mostly cloudy conditions with rain showers expected. It appears that one of those disturbances moves through early afternoon, with some rain showers. Unsettled is the term often used to describe this scenario. It will be windy. The rain may last until 4-5 PM, according to this afternoon’s models and skies will clear west to east.

(In a departure from recent forecasts, this afternoon’s HRRR shows some areas of heavy rain here Saturday. Right now, I’m seeing it as an aberration. I’m inclined to lean towards the HRDPS model, which has an area of heavy rain in northern Montgomery and Bucks counties Most other areas have lighter precip, less than 0.5 inches. )

High temperature 68º ( NBM model— location Blue Bell, PA)
High temperature 69º ( NBM model— location Philadelphia, PA) uncertainty ± 1.6º

Sunday

Continued windy. Sunny early in the morning. A considerable amount of instability cloudiness develops late morning into afternoon.

High temperature 60º ( NBM model— location Blue Bell, PA)
High temperature 61º ( NBM model— location Philadelphia, PA) uncertainty ± 1.7º

Winds

18z NBM wind meteogram for Blue Bell, PA (Click on image for a larger view.)