Monthly Archives: July 2023

Philadelphia Area Weather Forecasts

WEEKEND WEATHER FORECAST

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Sunday Rainfall Totals
MRMS based rainfall totals 24 hours 9 PM Saturday through 9 PM Sunday. Orange Contour numbers are in mm. 25.4 mm = 1 inch (Click on image for a larger view.)
Weather Update

Updated Sun 07/09 @ 4:04 PM— Far western suburbs have been having heavy rain most of this afternoon. The rain with embedded thunderstorms is slowly moving in our direction and should be in the immediate Philadelphia around 5 to 6 PM with additional activity moving in from the south.

Current Radar at 3:53 PM. An area of convection (circled) is developing down in the DC area will looks like it will be advancing into our area by 5-7 PM.

As had been forecast by several, but not all, models, the heaviest rainfall will likely be to our north and west.

The area of storms will depart our area by 10 PM, according to the latest HRRR.

Weather Update

Updated Sun 07/09 @ 12:29 PM — Latest GFS model total rainfall is similar to last night’s general model forecast —

Today’s 12z GFS total rainfall through midnight tonight. (Click on image for a larger view.)

This morning’s GFS and HREF still have the period 3 PM through 8 PM as producing the highest rainfall rates in the Philadelphia area and immediate suburbs.

Weather Update

Updated Sun 07/09 @ 11:10 AM — Thunderstorm activity has broken out way ahead of model forecasts, including . The complexity of this weather setup suggests last night’s models are not handling it accurately.

10:58 AM Radar. Significant thundershowers have broken out just to the west of the city, moving north-northeastward (Click on image for a larger view.)

Some of this morning’s models are much faster with the precipitation entering and leaving our area.

Sunday Heavy Rain Potential

Updated Sun 07/09 @ 9:15 AM — A complex system is approaching our area with strong jet stream energy and several impulses expected to affect us Sunday into Monday.

The complex system is shown via this morning’s water vapor image —

Satellite WV image shows multiple disturbances and jet level trough to affect us today through Monday. (Click on image for a larger view.)

The models continue to place the immediate Philadelphia area in a “doughnut hole” where heaviest precip seems to be missing, but I’m going to chalk that up to some modeling error at this time.

Showers and thunderstorms can break out any time early this afternoon, but peak time for rain and thunderstorms is about 3 PM to 10 PM. There isn’t much signal for any tornadic activity and excessive, heavy rainfall appears to be the main threat. Areas far south and far north of Philadelphia will have the most activity.

Sunday Forecast

Update Sat 7/08 10:55 PM — Some areas today received heavy rainfall with the thundershowers that popped up reasonably close to the model forecasts.

MRMS total rainfall that fell Saturday afternoon. (Click on image for larger view.)

For Sunday, a few scattered showers possible in the morning, but most models have the main area of showers and thunderstorms moving in between 3 PM and 5 PM.

Of interest, tonight’s models have the heaviest rain missing the immediate Philadelphia area and surrounding suburbs.

Updates Sunday morning.

Saturday Forecast Update

Updated Sat 07/08 @ 9:18 AM — This morning’s 06z models continue with a forecast of fairly widespread pop-up showers today in the immediate Philadelphia area and Delaware Valley.

These pop up showers/thundershowers will start as early as 1- 2 PM in and around the city and will persist until at least 8 PM. Rainfall in some may be heavy. PWATs will be in the 1.7″ to 1.9″ ranged. Here’s the latest HREF model with the forecast locations at 4 PM—

Today’s 06z HREF model showing 1hour rainfall with standard deviation (contours) at 4PM today. (Click on image for a larger view.)

The latest trend with the rain on Sunday is for the heavy rain to move in mid afternoon (3 PM) from the west into the the early evening hours. This is about 3 hours later than previously forecast.

Previously Posted Fri 7:59 PM —

Western areas had some fairly heavy rain Friday afternoon, but as had been suggested, the showers barely made it into Philadelphia.

Saturday will be similar to Friday with clouds giving way to a mix of sun and clouds. Thunderstorms again in the afternoon and evening hours. Details below.

A significant jet stream induced wave will bring stormy conditions for Sunday. An developing upper level large scale trough and a potent jet stream impulse will cause low pressure to develop. Significant lift will result in heavy rain.

Today’s GFS jet stream wind forecast for Sunday at 2 PM. The sharp bend in the flow near the Appalachians and the strong jet streak near Tennessee will cause low pressure and lift to develop in our region (red oval). This jet streak is strong considering it’s summer. (Click on image for a larger view.)

Saturday

Clouds and some fog in the morning, then partial sunshine in the afternoon. Like today, showers and thunderstorms will develop from 2 PM to 8 PM. The storms are currently favored to develop just east of the Delaware Rive during the afternon, but may spread westward into Philadelphia during the evening. Potentially heavy rain possible wherever the storms develop.

Peak thunderstorm probability at 6 PM (NBM model)

High temperature 8.6º sd 1.5º ( NBM model— location Blue Bell, PA)

Sunday

Cloudy. Light rain develops in the morning and becomes heavier and more widespread by early afternoon. Rainfall may be very heavy, especially north towards Allentown. 1.5″ to 3 plus inches possible. Becoming windy. Thunderstorms especially during the afternoon and evening. Rain may continue into the night as a cold front moves through. Windy conditions.

Severe weather possible. Stay tuned.

GFS total rainfall forecast through Sunday. (Click on image for a larger view.)

High temperature 82.9º sd 2.6º ( NBM model— location Blue Bell, PA)

Philadelphia Area Weather Forecasts

THIS WEEK’S WEATHER

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Friday Weather Update

Updated Fri 07/07 @ 9:26 AM — Another quick update. Latest NBM and HRDPS is a bit faster with the showers and thundstorms moving into Philadelphia. As early as 2-4 PM, although peak is 4-7 PM.

Todays’ 12z NBM shows showers/thunderstorms at 4 PM (Click on image for a larger view.)

Updated Fri 07/07 @ 8:32 AM — A mid level disturbance in western PA this morning will slowly move eastward today, triggering showers and thunderstorms.

This morning’s satellite Water Vapor image with wave-type disturbance in white circle. (Click on image for a larger view.)

The showers/thunderstorms will be mostly west of a line from Reading to Allentown during the afternoon, but last night’s models suggest some may make it into Philadelphia between 4 PM and 10 PM. It does not appear likely that the activity will move far into NJ

An advancing trough will bring partly sunny skies Saturday with showers possible again in the afternoon and evening. Sunday looks to be wet with potentially heavy rain as low pressure develops ahead of the trough.

Check back early evening for my weekly “Weekend Weather Forecast”.

Friday’s Weather

Updated Thu 07/06 @ 8:19 PM —A few thundershowers popped up in Reading and Allentown this afternoon, with a few others in Montgomery and Bucks counties—

Current WV image with radar and RAP jet stream level wind streamlines superimposed. A weak dissolving front is in central PA. For Friday, this weak frontal remnant will be a focus of greater coverage of thunderstorms late Friday afternoon and evening, mostly in far northwestern suburbs, . (Click on image for a larger view.)

Some fog is possible Friday morning.

The weak frontal remnant will be a focus of greater coverage of thunderstorms late Friday afternoon and evening, mostly in far northwestern suburbs. It’s unclear how far the storms will travel eastward into the Philadelphia area late tomorrow, but several models have them falling apart. The ECMWF and HRDPS has some heavy rain that makes it into the immediate suburbs.

I’ll need to update tomorrow morning.

Thursday’s Weather

Updated Thu 07/06 @ 8:33 AM — Expect similar widely scattered thunderstorms to pop up again today, somewhat more westward than yesterday’s showers. The models are relatively consistent with the placement of these thundershowers. They can occur any time between 2 PM and 9 PM today, most likely time frame is 4-7 PM.

HREF forecast placement of today’s thundershowers—

Today’s HREF forecast location of pop up showers. As always, these locations should not be taken literally. Basically expect any showers that pop up to be near and west of the Delaware river. The ECMWF, not shown, has the majority of showers a little further southward. (Click on image for a larger view.)
Upcoming Weather Thur-Sun

Updated Wed 07/05 @ 7:59 PM —Satellite Water Vapor images can provide the visual clues into model forecasts, and of course, the models incorporate upper air data from these images for their forecasts.

The current pattern of scattered afternoon thunderstorms is being driven in part by a small upper air cyclonic trough (1 below) over Pennsylvania. More info in the caption below—

Water Vapor at 7:46 PM with superimposed RAP jet stream level wind stream lines. (1) The upper level weak trough (white arrows) is adding some lift for thunderstorms today and for Thursday. (2) An approaching weak cold front will move through on Friday with more significant rainfall. (3) A developing large trough descending down from Canada will interact with disturbances (4) late Saturday and especially Sunday. Possibly heavy rain possible on Sunday. (Click on image for a larger view.)

Widely scattered thunderstorms expected again Thursday afternoon and early evening. These will mostly be in the western suburbs of Philadelphia, possibly extending into the city. More widespread showers and thunderstorms likely later Friday. Heavy rain possible Sunday.

Wednesday Weather

Updated Wed 07/05 @ 11:29 AM — High humidity and some weak upper air disturbances will cause scattered thunderstorms to develop any time from 2 PM through about 8 PM. Low vertical shear will cause any pop up storms to collapse under their own downdrafts.

The latest HREF shows most of the possible thunderstorms will be in NJ. This also supported by the ECMWF. The NAM-NEST is showing areas of high CIN (convective inhibition), so we’ll see if these actually form—

Today’s 12z HREF ensemble model shows most of the activity in NJ. (We’ve seen from recent days that the models aren’t able to exactly forecast the exact location of storm development in this scenario. (Click on image for a larger view.)

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