Overview: We’re entering a [much needed] tranquil period of weather for the next several days, as the upper air flow flattens out (“zonal flow”) by Sunday and surface high pressure moves into a position to provide southwesterly flow of warmer air.
Saturday
High pressure almost over us will provide mostly sunny skies. As the upper air flow flattens, a weak upper air disturbance may bring a brief period of cirrus cloudiness during early afternoon, mostly NW of the city. High temp 78.2 ± 0.8º (NBM model Blue Bell PA)
Sunday
High pressure moves to allow warmer southwesterly flow. Mostly sunny, but some ripples in the mid level flow may result in brief periods of high clouds at times. Breezy with some gusty winds at times. High temp 78.2 ± 0.8º (NBM model Blue Bell PA)
NBM Wind Forecast
The southwesterly flow will eventually bring back some summer-like temperatures early next week with highs forecast in the mid-upper 80s and dew points in the upper 60s.
On the very long range horizon, a tropical system may organize off the southeast coast and move up next weekend.
Thu 12:05 PM Forecast Review — The thunderstorms ahead of the front fell apart just as they approached Philadelphia about 1 AM, as the energy got transferred to the developing low pressure system that formed along the front.
Last night’s early models that prompted my “Huge Forecast Change” update, the HRRR and RAP, were a little too far west with the rain that developed this morning. The later NAM and HIRESW models did a better job with the westward extent of the rain.
I was off-duty by the time these came out.😉
UPDATE 9:50 PM – HUGE FORECAST CHANGE: Tonight’s 00z models are becoming available. (The 00z models is one of the two model daily runs that include direct upper air measurements done with weather balloons [radiosondes].)
If you’ve been following the forecast, the timing of the frontal passage has been pushed further up with each model run. The latest HRRR and RAP 00z models shows the front significantly stalling as it tries to move through the Philadelphia area tonight.
The latest: low pressure is expected to develop along this stalled front and move up over Philadelphia and southern NJ overnight and through tomorrow. Most of the thunderstorm activity may stay to our west overnight or just make it into Philadelphia about 1-3AM.
Additional rain expected to develop with this low pressure system over our area in the early morning hours and through much of tomorrow. The forecast has changed significantly. Too early to put it all together. But the RAP and HRRR area forecasting rain for much of early Thursday! (This afternoon’s GFS had hinted at this change.)
00z HRRR Radar/ surface pressure forecast for 11 AM Thursday (Click on image for a larger view.)
The higher resolution HRRR keeps the heavy storms just to our west tonight. The RAP (which uses GFS input data), has the storms making it to PHL, followed by significant rain on Thursday.
Update Wed @ 3:53 PM — Latest HRRR 18z available. Forecast still on track. A couple of isolated storms late this afternoon. Rain and thunderstorms, possibly severe, move into the immediate PHL area around 11PM – 2 AM, earlier north and west, in line with this morning’s NAM and HIRESW models. Rain and storms last through the night. Helicity and shear values lower than previous model runs, making thechance of tornadoes in our area unlikely. Still strong/severe storms possible with total rain amounts somewhat higher, near 1 inch, possibly 2 inches.
Strong storms are expected tonight. Most of the forecast remains unchanged from last night’s post. As usual, there are differences in the models from this morning.
To make things easier, I’m going to do a straight HRRR model forecast and then indicate where things don’t all agree.
Most of the thunderstorm activity breaks out between 9 PM and 1 AM with the maxima around 11-1 AM. (The latest HRRR does not show much activity in the afternoon, BUT the HIRESW models continue to show scattered storms this afternoon after 3 PM.)
The difference in forecasts between having strong/severe thunderstorms and extreme events like tornadoes are physical dynamics that are parameterized as helicity and vertical shear in numerical weather prediction models.
During the period between 9 PM and 2 AM, the HRRR has notable areas of moderately high helicity and high wind vertical shear. (As per my post last night, the helicity values are 350 m2/s2 compared to last week’s 1000 m2/s2 values; these are significantly less.) My personal feeling is that strong/severe thunderstorms are likely,but tornadoes are possible but not likely.
HRRR forecast relatively helicity (contours in red box) at 10 PM, although MUCH LESS than last week’s storms. Orange shading = CAPE (Click on image for a larger view.)
As for wind shear, there is a concerning level of high vertical shear about midnight in areas north of the city. This would translate into severe thunderstorms with high straight line wind gusts, hail and heavy rain.
While I have boxed in the area of highest shear, strong storms could form in other areas.
HRRR vertical wind shear at 12 AM midnight. Severe thunderstorms likely in the red box. (Click on image for a larger view.)
Heaviest rain areas—
HRRR total rain forecast predicted rain maxima. (Click on image for a larger view.)
It should be noted that the NAM and HIRESW version models don’t agree on the above HRRR forecast. The principle differences for these models is that they have the heaviest rain and storms staying mostly to our north and west and have weaker showers/thunderstorms moving in after midnight and continuing until about 3-4 AM in the PHL area. They also have some scattered storms in the late afternoon today.
Update Wed 7:45 AM— Good news. Last night’s models have backed off on the helicity values. Risk of tornadic activity low. Other changes: the first “wave” of scattered storms in the afternoon (described in main post) has reduced in likelihood. The main area of showers/thunderstorms moves through around midnight to 1 AM. CAPE values are further reduced at that hour of the night. So extreme weather less likely. Still strong storms expected. Rainfall about 0.6-1.2” possible, locally higher. Wind gusts about 30 + miles per hour. I’ll update later this morning when the newest models become available.
Update Tue 10:25 PM —Forecast below remains on track. Peak helicity about 8 PM [far] northwest of the city. Second wave of thunderstorms closer to midnight.
A strong cold front will move through after midnight on Wednesday. Thunderstorms, some very strong or severe, will be preceding the frontal passage.
The current models suggest that scattered thunderstorms, some severe, may break out earlier than the main batch of rain. So the storms will come in two waves.
First wave:Widely scattered storms will develop as early as 3-4 PM in areas from Philadelphia north and west. These widely scattered areas of storms ahead of the main front may continue through about 7 PM.
The second wave,the main batch of storms and rain, comes through from about 9 PM through 1 AM. (Remember, I use Blue Bell, PA as the model location grid point.) Rainfall may be heavy in some areas: 1-2 inches of rain.
Following last week’s devastating tropical rain and thunderstorms (and tornadoes) , it’s important to see current model differences between last week’s severe weather and tomorrow (Wednesday).
CAPE values are much higher tomorrow compared to last week, but within range of typical strong/severe storm events.
Regarding tornadic activity, peak helicity values tomorrow are currently forecast less than 350 m2/sec2, significantly below last week’s storms.
(Last week’s tornadoes were developing at helicity levels more than two to three times as high: 800-1000 m2/sec2.) Peak helicity values occur with the second batch of rain and thunderstorms, although there is some significant helicity at 5 PM north of the city. Again, these helicity values are 1/3 that of last week’s Ida event.
I’m providing the following CAPE/Helicity graphics as examples. The location and timing are likely to change in future model runs. Don’t take these locations too literally.
HRRR Helicity and CAPE at 5 PM. Darker is higher CAPE. Contours are Helicity, Peak helicity less than 320 (Click on image for a larger view.)HRRR model Peak CAPE and Helicity at 10 PM (Click on image for a larger view.)HRRR peak Helicity/CAPE at midnight. Peak Helicity 350 (Click on image for a larger view.)
Tornadic activity, if any, will be late afternoon or early evening, with the first batch of energy and again with the second batch of energy, in the 9PM to midnight time frame. But since we’re dealing with life-threatening possibilities, always follow the warnings of the National Weather Service.
I’ll be updating this info tomorrow morning. The next HRRR model run of consequence will become available around 9:35 AM. I’ll try to update then.
