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Friday Weather Update
Fri 7:22 PM —Forecast Review —Today’s weather was a sobering and decisive event in my ongoing elusive attempts to predict the location and timing of thunderstorms. I must confess I enjoy reviewing the voluminous model data for each storm, trying to find the one parameter or model that would have given a very accurate thunderstorm forecast.

But today, I put in an unusually large effort to forecast the storms, but to no avail. The models were all pretty much wrong in one way or another. I reluctantly have reached and agree with the conclusion, “All models are wrong, but some are useful.” — George EP Box , noted British statistician.

Going forward, I’m going change my approach to thunderstorm forecasting.

Posted Friday 06/14/24 @ 5:16 PM — With all this prognosticating today, some heavy cells have popped up early and not where forecast!

Radar at 5 PM (Click on image for a larger view.)

Posted Friday 06/14/24 @ 3:18 PM — The current radar (MRMS) and RAP model depicts the current conditions that support the forecast for the heaviest thunderstorms to move off to our north. Check out the graphic caption for more info—

MRMS radar summary with superimposed RAP model mid level wind streams (blue streamlines) and vertical velocity pressure (“Omega”) (yellow contours) The main area of storms will follow the mid level blue contours to our north. The yellow negative Omega numbers in the yellow oval show the southern areas of precipitation will enter an area of downward vertical motion. Downward motion works against thunderstorm development. (Click on image for a larger view.)

Based on this, the forecast posted earlier this morning (below) appears to be supported. Of course when it comes to weather —a dynamic and chaotic process — things could change.

Posted Friday 06/14/24 @ 12:30 PM — The latest Canadian HRDPS model captures the forecast I’ve been trying to convey with my earlier update. It shows heavier storms to our north and west, but large areas of the immediate Philadelphia area receive no rain!

As many of my regular readers know, the model exact placement of forecast heavy rain is something that very often comes up short. But this trend where the heavier activity skips over Philadelphia and re-forms in NJ is something different models are all suggesting.

This morning’s 12z Canadian HRDPS total rainfall forecast for this event. Some areas receive over 1.5-2″ while large areas receive almost nothing. The HRDPS breaks down precip into “convective” and non-convective. The lighter green-yellow areas are mostly convective (thundestorm) rainfall. (Click on image for a larger view.)

Despite my forecast of low severity potential here in Philadelphia, always follow the the official NWS forecast warnings in these situations.

Friday’s ‘Storms’

Posted Friday 06/14/24 @ 8:18 AM — Last night’s models continue with the forecast of a highly broken line (maybe batch) of showers and storms affecting our area late afternoon into evening. The heaviest activity will remain north of us and some areas in the immediate Philadelphia area may see little in the way of rain or storms. A severe weather event here is low probability. A 3 hour time lapse of the 06z HRRR captures this—

06z HRRR instantaneous precipitaton rate from 5 PM to 7 PM. The models are predicting a secondary batch of showers moving through our area before midnight. (Click on image for a larger view.)

A second group of showers and possibly thundershowers moves through about 11 PM to midnight. We could use the rain, but this front, for reasons described in previous updates, likely won’t provide it. We’ll be entering a dry and hot stretch of weather starting late Monday.

I’ll update if this morning’s models move in a different direction.

Friday Weather Update

Posted Thursday 06/13/24 @ 5:30 PM —Several models show a disturbance moving in from the southwest this evening, possibly causing some showers and thunderstorms in Chester county, perhaps into the city. On satellite imagery, I see the disturbance but not any showers at this time. A low confidence event —

Satellite image (channel 13) shows the weak disturbance which several models show producing showers and thunderstorms later Friday evening. Not overly confident about this. (Click on image for a larger view.)

As for Friday, showers and thunderstorms move in ahead of the cold front as early as 4-5 PM far northwest suburbs and into the early evening (6-11 PM) in the city. As discussed in this morning’s update, the line of showers/thunderstorms will be broken and doesn’t appear to be overly severe, although a few spots show potential. —

This afternoon’s 18z HREF shows rain and areas of maximum upward helicity from 6 PM through 10 PM. This shows 9 PM. (Click on image for a larger view.)

Heaviest activity in central and northern NJ and in into Delaware and Maryland. The showers may linger and blossom a bit towards midnight before moving off for a nice weekend.

I’ll update Friday.

Thursday Friday Weather Update

Posted Thursday 06/13/24 @ 8:15 AM — Today, Thursday, will feel more like June here in Philadelphia, with temperatures approaching 90º in the mid to late afternoon and dew points moving from the comfortable mid 50s to the low 60s this afternoon.

Friday will see hot weather with highs 90-93º and dew points moving into the more uncomfortable low to mid 60s.

Friday‘s cold frontal passage timing continues to change back and forth, but has reverted back to the late afternoon/early evening, more specifically 6 PM to 9 PM. A line of showers and thunderstorms will move towards us, but it appears that this line will become broken as a result of these storms not having upper air support (no adjacent jet streaks) and another system, now affecting Florida, will be out int he Atlantic to our east, robbing the front of some upward vertical motion. Some areas may see a heavy thunderstorm, but many may not.

ECMWF forecast for 8 PM Friday evening. Frontal position drawn. (Click on image for a larger view.)

The severe weather parameters in the CAM models (Convection-Allowing Models such as the HRRR) often don’t show possibility of severe weather until 24 to 12 hours before the event, so things may change with tomorrow’s forecast. Right now, things don’t look too severe.

Total rainfall will only be in the 1/2″ range, with some locally higher amounts.

This upcoming weekend looks very very nice with cooler temperatures and low humidity. Heat returns after Monday.

Following Friday’s frontal passage and rainfall, and after this coming weekend, we’ll be entering into a generally hot and very dry pattern for the following weeks. This may be the last generalized rainfall we receive for almost two weeks.

Additional Forecast Changes Wednesday thru Friday

Posted Tuesday 06/11/24 @ 4:20 PM — We had significant cloudiness today at times, as forecast in my update yesterday. However the shower activity has been much less than forecast, continuing a recent trend where the model forecast total precip is over-stated.

Here’s the current radar showing a paucity of rain. —

MRMS radar capture 4:12 PM. Additional showers may develop through 8 PM. (Click on image for a larger view.)

The latest models show more forecast changes through Friday.

First, tomorrow, Wednesday, will be cooler than previously forecast, likely staying in the low 80s. Some clouds at times, but there will be significantly more sunshine than today and more sun than had been forecast yesterday.

Thursday warms up, but not as hot as previously forecast, likely staying just under 90º. A few pop-up widely scatted thundershowers possible mid to late afternoon. Most areas dry.

The big forecast change is for Friday, where a cold front will be moving through during the mid to late afternoon into the evening. (Previously, the front was expected late evening into the night.) Thunderstorms, are possible late afternoons and early evening. Rainfall forecast is on the light side.

Highs will be in the near 90-92º, but there’s great uncertainty in that temperature due to uncertainty with clouds and the timing of the front. We may not get that hot. The standard deviation (uncertainty) is as high as ± 5º with some ensemble models and it’s likely trending towards the cooler range.

Forecast Change for Tuesday, Wednesday

Posted Monday 06/10/24 @ 7:02 PM —This afternoon’s 18z models have added showers and thundershowers late afternoon Tuesday from the city eastward into NJ as well.

Posted Monday 06/10/24 @ 4:54 PM —Today’s models are now showing several upper air disturbances moving through our area early Tuesday morning and again late Wednesday morning through Wednesday afternoon. These disturbances will allow a warm front to move through about the same time frame.

Sunshine, but more cloudiness than previously forecast for both Tuesday and Wednesday, along with light widely-scattered showers, near Philadelphia before daybreak Tuesday morning and mostly in NJ Wednesday afternoon.

18z NAM-NEST forecast low cloud cover (blue) forecast for noon on Tuesday. Some areas have sun, some have clouds. This is a snapshot and different areas will have clouds and sun at different times. (Click on image for a larger view.)

A rumble of thunder also possible, especially near the Jersey Shore, according to the ECMWF and Canadian models on Wednesday afternoon. Any showers will be light. Many/most areas will not see any rain, but considerable cloudiness at times.

End of Week Heat

Posted Monday 06/10/24 @ 9:00 AM — Following spectacular days Monday and Tuesday, seasonably warm temperatures return Wednesday and hot days are expected Thursday and especially Friday.

Friday is currently forecast to be especially hot—

NBM forecast high temperatures Friday. Dew points in the mid 60s will make the heat noticeable. (Click on image for a larger view.)

Unfortunately, I don’t see much rain over the next week, with the only chance Friday night as a cold front moves through. That amount doesn’t currently look to be very high—

NAEFS probability of rainfall greater or equal to 0.10″ Not very high. (Click on image for a larger view.)

Previously Posted Sun 4:50 PM —Cooler than average and beautiful weather expected here through at least Thursday. The NAEFS forecast map for Monday captures the main features—

NAEFS forecast for Monday evening shows a cool northwesterly upper flow around an mid-upper level low pressure system in Canada. The 576 thickness line (500-1000mb thickness) in dark orange represents the ‘edge’ of very hot tropical air, currently suppressed to the south, but poised to move northward by the end of the week. Of additional interest is an area of disturbed weather taking shape in the Gulf of Mexico (shown as a ?) which may be an area of impending tropical storm development. (Click on image for a larger view.)

By the end of the week, hotter air begins to approach our area with temperatures possibly near or above 90º by Thursday or Friday.

Another cold front moves through later Friday into Saturday with a brief cool down for next weekend. Hot weather expected the following week.

NAEFS forecast for next Saturday, showing cold front moving through. Increased activity in the Gulf of Mexico. (Click on image for a larger view.)

Except for the cold front passage next Friday, things will be dry. It will take a tropical system to give us substantial rain with this pattern.