Monthly Archives: July 2021

Philadelphia Area Weather Forecasts

WEEKEND WEATHER FORECAST

Updated Monday forecast, Mon 07:40AM, highlighted below

Sun 06:50 PM Forecast Review — Sunday’s showers is a perfect example of  why I began this blog post from Friday with

 

“exact forecasts with an upper closed low are fraught with difficulties, inaccuracies and uncertainties.” 


A mid-level upper air disturbance rotating around the upper low, unpredicted by most of the earlier models today, caused the scattered showers Sunday afternoon.  I don’t think weather models, as good as they can be, can do any better with forecasts and upper air closed lows.  

Looking back at this morning’s models, the HIRESW-ARW2 did predict the showers, but it didn’t become available until after 10:20 AM.  Had I actually looked at the HIRESW-ARW2, I don’t think I would have updated my forecast; there were too many models not showing showers.  

Updated Saturday forecast, Sat 08:00AM, highlighted below
Updated Saturday forecast, Fri 11:10PM, highlighted below

Our weather early this weekend will be affected by a cyclonic flow of air around an upper closed low pressure system. This flow and the upper closed low pressure system’s cold air aloft results in instability clouds and showers.

As my regular readers know, exact forecasts with an upper closed low are fraught with difficulties, inaccuracies and uncertainties. Such may be the case this Saturday. The upper low’s effects are expected to diminish by Sunday.

(Today, Friday, is a perfect example. Some areas are having showers and some areas have sun breaking through clouds. This morning’s models have been unable to pinpoint the proper timing and placement. )

I’ve compared current radar with six different models run this morning. The HREF has had the highest skill in the forecast location of the current rain this afternoon. So I’m going to lean on that model for Saturday.

Here’s the current HREF forecast for Saturday at 1 PM (there’s a chance of showers in many areas) —

HREF forecast for 1 PM Saturday showing rain (bright green contours) and clouds (blue shading) Upper low center (L) Arrows show upper air cyclonic counterclockwise circulation around upper low. (Click on image for a larger view.)

So, it looks like considerable clouds and showers develop Saturday afternoon. It won’t be raining all afternoon.

The upper low moves away on Sunday and Monday, but still a low chance of showers Sunday late afternoon.

NBM – 1 Hour Rain Probability meteogram, Blue Bell. Anything over 15% probability for a 1 hour period is a good chance. (Click on image for a larger view.)

Considerable cloudiness, perhaps some sunny breaks at times with scattered showers, especially in the afternoon. Breezy. High 73.2 ± 3.2º (NBM Blue Bell)
Update:Tonight’s models have reduced the showers Saturday afternoon. Following morning clouds, look for considerable sunshine in the afternoon, except far western suburbs.

Sat Morning Update: Despite my optimistic update from last night, the early morning model runs (06z) have reverted back to the original forecast of plenty of showers developing throughout the day, especially late afternoon.

NBM 1 hour rain forecast Saturday 3 PM—

NBM Saturday 3 PM – 1 hour rain and cloud (dark shading) (Click on image for a larger view.)

The upper low moves off. Some cloudiness in the morning, breaking way to a mix of sun and occasional clouds. A chance of a shower late afternoon. . High temp 78.4 ± 2.1º NBM Blue Bell

Sunny and warmer. Some models show widely scattered showers between noon and 3 PM, although this may be model noise. High 86.0º ± 3.1º (high spread!). High 87.6º ± 1.6º NBM Blue Bell.

Philadelphia Weather Outlook

THURS HEAVY RAIN & WEEKEND OUTLOOK

With a complicated forecast, let’s try a different approach. I’m going to use two different model’s meteograms showing precipitation for Blue Bell, PA. (remember that a meteogram only shows the weather for one specific location; the Jersey shore’s meteogram may look much different for the same time frame.)

Both show heavy rain (1 inch) starting about 2 PM Thursday afternoon.

The ICON shows significant rain again on Saturday. The model blend does not. (Not shown here, the GFS has showers in the late afternoon Saturday.) Both show Sunday and Monday essentially rain free.

ICON model—

ICON model rain meteogram for Blue Bell, PA Is it over-doing the rain on Saturday? (The GFS only has rain in the afternoon on Saturday.) Why the ICON model? It did well with last night’s rain, while the GFS model totally over-predicted last night’s rain. (Click on image for a larger view.)

Important clarification: the meteogram times indicate prior 1 hour accumulations. If you were to take those times as literal (and you shouldn’t), it would mean that the actual rain start times could have been as much as 1 hour earlier. Accumulation also does not show light rain that might not have accumulated enough to show in the scale of the meteogram.

NBM —

NBM meteogram for Blue Bell PA (Click on image for a larger view.)

So after some heavy rain Thursday and early Friday, much of the Fourth of July weekend may be relatively rain free. (Considerable clouds on Saturday with possibly some showers in the afternoon, according to the GFS.)

Sunday and Monday look much better weather-wise.

It will be cool on Saturday (high in the 70s) and still below average highs on Sunday (about 80º)