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.)
Update Tue 07/04 @ 4:56 PM— A quick update. Strong storms moved through eastern Bucks county this afternoon and blossomed as they moved across into NJ.
At 4:41, large cluster of strong thunderstorms with likely hail was moving across central NJ—
None of the models predicted this cluster.
Tuesday- 4th of July- Weather Update
Update Tue 07/04 @ 8:54 AM— Predicting the exact location of the storms over the past few days hasn’t been easy, nor is it ever; the model forecasts can be extraordinary, but they’re models of reality, not reality.
Today, Monday, we have an assortment of high severity factors that may lead to strong storms, but knowing where they’ll pop up is like trying to predict where the first bubble will form in a heated saucepan. The general model consensus is that they’ll be scattered, rather than organized.
The current HRDPS is slightly more aggressive with storms than the latest HRRR, but it gives the general location of where these pop up storms will develop—
0z HRDPS precipitation rate at 2 PM.
The timing of the storms can be anytime between 2 PM and 8 PM. The general trend from the HRRR and NAM-NEST is that the immediate Philadelphia area will be a likely place for these storms to develop.
HRRR Severity Parameter
Sept 1 , 2021 Tornado Outbreak (example of highly severe)
Recent April 1st 2023 Tornadoes (Example of severe)
Severity Parameters Impact: ⚑ indicates favors Severity ⚐ indicates Possible Severity ↓ indicates works against Severity ⇩ Significant, but less impact
It should be noted the last night’s HIRESW-FV3 did an incredible job with last night’s storm placement. The latest HIRESW-FV3 is forecasting very isolated thunderstorm development today.
Monday Update
Updated Mon 07/03 @ 3:13 PM — The 14z HRRR model has little in the way of thunderstorm activity in Philadelphia this afternoon and evening.
Here’s the HRRR—
14z HRRR shows the immediate PHL area free of showers/thunderstorms through 4 AM. (Click on image for a larger view.)
Monday Forecast Update
Updated Mon 07/03 @ 11:44 AM — This morning’s models again has the majority of today’s thunderstorms moving through to our far north and south. The immediate Philadelphia area doesn’t appear to be in area of main activity.
Today’s 12z HRRR precipitation rate at 7 PM—
12z HRRR shows much of the activity to our north and south at 7 PM.
The high resolution statistical model (HREF) shows most precip today missing the immediate PHL area—
12z HREF total precip through 10 PM Monday. (Click on image for a larger view.)
Previously Posted Mon 9:29 AM —
Monday Forecast
A slowly moving upper trough with areas of vorticity rotating into our area will provide warm/hot, humid conditions and the ongoing possibility of thunderstorms today.
The upper air scenario, as depicted by a satellite water vapor image, shows the trough —
Water Vapor image with superimposed RAP 250 mb wind flow, White arrows capture the upper flow. Numerous small scale disturbances will be rotating into our area today and gain on Tuesday. (Click on image for a larger view.)
Highest chance of thunderstorms over the immediate Philadelphia area will be between 5 and 10 PM, but scattered storms can be expected any time. High temperatures 88º Blue Bell and 91º Philadelphia.
