Monthly Archives: October 2021

Philadelphia Area Weather Forecasts

WEEKEND WEATHER FORECAST

Update Sat @ 9:50 PM — Sunday’s forecast is unchanged. Some “interesting weather” appears to be developing for the late Monday through Tuesday time frame. A deep upper low will spawn a deep coastal secondary low Monday night through Tuesday. Stay tuned.

Update Sat @ 9:31 AM — The uncertainty in the cloud cover remains for Saturday. The short range, higher resolution models are more optimistic about brightening skies and some sunshine during the mid-day hours. But the 2AM runs of the the GFS and ICON have the clouds hanging in. I’m now leaning towards less sunshine, but with low confidence.

All models now show showers moving in from the west, starting as early as 2-6 PM. The model blend (NBM) which is supposed to reconcile these forecast differences has showers moving in about 4 PM.

NBM forecast for rain/clouds Sat 4:35 PM (Click on image for a larger view.)


Update Fri 10:43 PM— Tonight’s models just becoming available, continue with a huge range in cloud cover for Saturday. I’m staying with the forecast below. Showers as starting as early as 3-4 PM, lasting into the evening, appear to be more likely.

The general forecast for this weekend is easy if you’re not into the details. Basically it will be partly cloudy/partly sunny with a chance of light scattered sprinkles late Saturday afternoon.

The detailed specifics of the weekend forecast are much lower confidence than usual.

Much of the forecast is driven by upper air disturbances (described on Tuesday) which are difficult to pin down. There’s a wide range in the forecast levels of cloudiness, especially on Saturday. Additionally, over the past few days, the timing of the showers/sprinkles (if any) from an upper air disturbance has moved from the morning to the mid to late afternoon.

Saturday

Going with a blend of the NBM, HRRR, GFS and ICON models, it looks like Saturday morning will be cloudy. Clouds break for a mix of clouds and sunshine sometime between 11 AM and 2 PM. There’s a slight chance of widely scattered light sprinkles after 4 PM.

High Temp 61.8º sd 3.3º (NBM forecast, Blue Bell). The high sd (standard deviation) reflects the high uncertainty in the amount of cloud cover. (Average seasonal high is ~62°.)

Mostly sunny in the morning with a period of increasing cloudiness in the early afternoon, possibly breaking for some sunshine late afternoon.

High Temp 61.4º sd 2.1º (NBM forecast, Blue Bell).

Looking ahead

After months of heavy rainstorms, recent weeks have been very dry. Our next rainfall of any significance is not on the horizon. Monday was supposed to be somewhat rainy, but much of that rain will move to our north.

It’s interesting that the publicized NWS Climate Center long range winter forecast released yesterday had the above-average precipitation area just north of our area, with our area having equal chances of above or below average precipitation.

Some rain Monday night into Tuesday may provide some relief in the dryness. A deep upper low will spawn a coastal low Monday night through Tuesday. High winds and heavy rain are possible

Philadelphia Weather Outlook

THIS WEEK’s WEATHER OUTLOOK

Update Thu @ 7:40 PM— The models are still trying to come together with this weekend’s forecast. Today’s GFS and NAM have returned to the earlier forecast that our area will be in an area of jet stream level lift and upper air disturbances on Saturday, causing cloudiness and possibly some light sprinkles until after noontime. Greatest chances of sprinkles areas north of the city. Then breaks of sun and clouds for the rest of the day.

Sunday looks to have some sun early morning, then significant periods of cloudiness and some bright spots.

Here’s a meteogram based on the latest GFS model cloud and simulated radar forecast for Blue Bell, PA (remember that meteograms are only for a single location, but in this case, most of the PHL area forecast will have somewhat similar conditions) —

GFS forecast Sat and Sunday total cloud cover and simulated radar for Blue Bell PA. Cloudiness Saturday morning gives way to some bright spots and sunny breaks. Arrows suggest possible sprinkles of rain. (Click on image for a larger view.)

Update Wed @ 8:17 PM— Friday and the weekend continue to look better.

Two cold fronts move through and the expected drop in high temperatures to more seasonable levels is apparent from the NBM (model blend) forecast meteogram for Blue Bell—

(Click on image for a larger view.)

On Saturday, the upper air disturbance moves to our north and west giving us clouds in the morning but breaking for partly sunny skies in the afternoon. Any showers on Saturday will be mostly to our north and west and will be light.

Sunday starts cloudy with breaks of sunshine by afternoon.

Update Wed @ 9:51 AM—This morning’s NBM (model blend) has fewer clouds both Friday and Saturday. (Clouds move in later Friday afternoon and dissipate Saturday morning.) Last night’s GFS also moved in that direction. Will keep an eye on it.

Tues 8:39 PM:

We’re having some fantastic sunny, mild autumnal weather and it will continue through Thursday. Temperatures are well above seasonal averages (Avg Hi 62º Low 43º Blue Bell, PA)

A cold front will slowly move through on Friday with cloudiness and possibly some very light showers. The front will stall to our south. Upper air disturbances will move through on Saturday with cloudiness. It looks like we’ll be in the “left exit region” of the jet streak, assisting the development of light showers on Saturday from these upper air disturbances.

A secondary cold front will be pulled in behind the disturbance giving us more seasonable (read colder) temperatures for Sunday.