Updated Fri 07/28 @ 9:41 AM — The high temperatures and apparent temperatures (heat index) for today have been well-advertised. Here’s the latest NBM forecast—
Here’s the NBM Heat Index high (apparent temperature) occurring between 2:45 and 4 PM.—
(Click on image for a larger view.)
A few widely scattered showers about 7 PM; most areas dry.
Another Forecast Change?
Update Thu 07/27 @ 5:36 PM— So far there hasn’t been any storm activity in the immediate PHL area and that may not change. The cloud cover we had earlier put a lid on things developing so far.
The latest satellite and radar shows some activity developing to our west—
Satellite chan 13 and Radar at 5:21 PM. Disturbances to our west may reach here within the next 2-5 hours. (Click on image for larger view.)
The latest HRRR keeps this activity moving through the western suburbs and falling apart as it hits the city. The latest HRDPS has the storms moving through Philadelphia into NJ over the next 2-5 hours. Which is correct? Who knows, but the HRDPS has earned my respect in recent weeks.
Forecast Change
Updated Thu 07/27 @ 11:39 AM — A review of this morning’s models suggests a somewhat different picture and a different forecast. Significant cloud cover has reduced the previously rapid increase in temperatures.
Several models now keep the storms to the north and west of most of the immediate PHL area. Many areas won’t see any thunderstorm activity, according to the latest models.
The latest HRDPS captures this—
Today’s 12z HRDPS shows many areas from the city, south and east, will see no thunderstorm activity this afternoon or evening. (Click on image for a larger view.)
The 12z NAM-NEST shows a very different coverage of storms today—
Today’s 12z NAM-NEST also shows large areas unaffected by today’s thunderstorms.
So the previous forecast certainty of severe thunderstorms has become less than certain for many areas. Is the potential change the result of model spin up issues? Is it cloud cover reducing overall CAPE and temperatures and increasing CIN? I guess we’ll see.
Thursday- Potential for Scattered Severe Storms Likely
Updated Thu 07/27 @ 7:49 AM — The expected high heat and humidity for today has been well-advertised. As mentioned last night, the potential for scattered severe storms (which may form a line) appears to be in the likely category.
Based on a review of last night’s models (00z and 06z runs) a strong mid and upper level wave will move through our extended area between 2 PM and 6 PM with some activity still possible as late as 8 PM.
(There are timing differences in the models. The HRRR has a 4-6PM peak time frame, while the HRDPS has an earlier start time in the northwestern suburbs: 2 PM)
Heavy rain, strong wind gusts, potential hail and a slight-minimal risk of tornadic activity are all possible today.
One way the models estimate the severity is by estimating the height of cumulonimbus clouds (thunderheads). Referred to as “echo tops” in the models, (as measured by radar) typical very strong/severe thunderstorms show echo tops of 38-40,000 feet above ground. Several of last night’s models show echo tops of 45-48,000 feet!
06z HREF Echo Tops forecast for 5 PM Thursday. (Click on image for a larger view.)
Below is today’s Severity Table, based on the 06z HRRR. As mentioned last night, some (but not all) of the parameters are the most extreme I’ve seen this season. (Notably missing is jet stream wind energy.)
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 marginal effect ↓ indicates works against Severity ⇩ Significant, but less impact
As you can see, many parameters put us in the high probability of scattered severe storms. Most notably missing is jet stream wind energy; we’ll see how this impacts the forecast.
Hot Thursday – Potential for Severe Storms
Updated Wed 07/26 @ 9:50 PM —The hot air that’s been in the news affecting the Southwest and Central US this summer will be poking into our area Thursday and Friday.
Additionally, a strong disturbance in the mid and upper levels of the atmosphere will pass through our area between 2PM and 6 PM Thursday. The models are showing extremely high CAPE and precipitable water. Some of these severity parameters are the highest we’ve seen this season.
These disturbances are parameterized by the models as areas of strong positive and negative vorticity. It’s not often to see such strong vorticity couplets intertwined, as shown in the HRDPS below—
18z HRDPS 500 mb vorticity and geopotential height (~18,000 feet) at 5 PM. Very strong positive and negative areas of vorticity showing as intertwined couplets (the “conservation of momentum” in action.) . Notice how this strong upper wave distorts the usual straight line blue line of equal 500 mb height (“isohypse”) (Click on image for a larger view.)
Combined with this disturbance, scattered or clustered strong and severe storms will affect our area with localized heavy rain possible during the 2 PM to 6 PM time frame.
Looks to be very interesting. Updates tomorrow morning with the new HRRR.
Wed 5:42 PM Forecast Review — We weren’t as hot today as had been hyped on the TV forecasts, and even this morning’s NBM model (shown here) overshot the actual temperatures. Dew points were only in the high 50s and low 60s at the peak temperatures.
Here’s the RTMA temperatures at recorded at 3:45 PM —
RTMA model temperatures (actual or estimated preliminary) at 3:45 PM. (Click on image for a larger view.)
I do think we’ll get closer to the high heat values Thursday and Friday.
Wednesday Update
Updated Wed 07/26 @ 10:05 AM — The latest NBM high temperature for today with statistical spread (uncertainty)—
Today’s 12z NBM Max mean temperature forecast with statistical spread (uncertainty) contours. Areas of greatest contour density have the greatest statistical range of uncertainty. I’ve found the NBM tends to run somewhat low in summer here. With the NBM and summer high temperatures, generally add the uncertainty to get the likely temperature. (Click on image for a larger view.)
Wednesday Outlook
Updated Tue 07/25 @ 8:45 PM — For Wednesday, temperatures near 89-90º but dew points in the low 60s will keep heat indices close to the actual temperatures.
For Thursday, actual temperatures go well into the 90s with dew points into the upper 60s to near 70º. A disturbance may cause showers/thunderstorms Thursday afternoon or evening.
Thursday high temps—
Today’s 18z GFS max temperature forecast for Thursday. These are actual temperature. Heat indices over 100 in many areas.
Tue 4:02 PM Forecast Review —Little in the way of severe storms, but much more areal coverage and heavy rain than one would have guessed from the several forecasts I heard on the radio which seemed to be only forecasting ” scattered storms” this morning.
Radar with superimposed RAP model winds at 3:45 PM Tuesday. (Click on image for a larger view.)
Today’s thunderstorms were yet another example where the HRDPS model seems to have done much better than our seemingly more sophisticated HRRR model this summer season. Not sure why.
Tuesday Forecast Update
Updated Tue 07/25 @ 12:36 PM — The storms have broken out even earlier than 2 PM originally forecast—
Radar at 12:31PM with superimposed RAP model boundary level winds and IR satellite image. (Click on image for a larger view.)
Updated Tue 07/25 @ 11:26 AM — This morning’s HRRR forecast has much more scattered storms than the NAM-NEST or HIRESW or previous Canadian RGEM/HRDPS. I’m leaning towards greater coverage of storms today. Severe parameter levels are only in the moderate range, but it doesn’t preclude severe activity.
Satellite IR with RAP model overlays. The impulse that will set off our storms is shown in Maryland (yellow contours and white arrow) at 11:15AM. Some showers already breaking out in the Valley Forge area. (Click on image for a larger view.) (Click on image for a larger view.)
Tuesday Forecast Update
(readers- a half-baked forecast update posted about 15 minutes ago was accidentally published. It’s been deleted.)
Updated Tue 07/25 @ 8:48 AM — A review of the 2AM (06z) HRRR and Canadian RGEM suggests a more toned-down severe forecast. The storms will move in earlier, possibly as early as 2-3PM in the western suburbs and move east. They will be more scattered.
The HRRR model is showing significant areas of CIN (Convective Inhibition) that will persist, making any activity more scattered and less intense.
Additionally, despite the increase in jet stream winds, we are forecast to be in an area called the “left entrance region” of a jet streak, where upper level downward motion reduces the needed upward motion for thunderstorm development.
The latest (06z) Canadian HRDPS shows similar coverage as its forecast yesterday, but its severity parameters are not as high in the immediate PHL area.
The HRDPS shows greater overall rainfall and more coverage than the HRRR—
HRDPS total predicted rainfall by 7 PM tonight. (Click on image for a larger view.)
I’ll update later this morning, if things change.
Tuesday Outlook
Update Mon 7/24 9:47 PM — Tonight’s HRRR model is suggesting increased likelihood of severe weather Tuesday late afternoon. Updates tomorrow morning.
Updated Mon 07/24 @ 8:40 PM — An upper/mid level trough will swing through our area late afternoon on Tuesday. The usual high CAPE values and high instability parameters will be present.
Helicity is not all that high, so no tornadic activity expected. The standard “strong to severe storms” is the forecast at this time.
Expect thunderstorms to move in from the west as early as 3:30 PM far western suburbs peaking in the immediate PHL area about 4:30PM to 6 PM.
Today’s 18z HRDPS forecast “precipitation rate” at 4 PM Tuesday. Some heavy downpours in western suburbs. The HRDPS has it weakening as it enters Philadelphia, then regaining strength in NJ. (Click on image for a larger view.)
The distinguishing feature of tomorrow’s storms will be a very strong jet stream presence. This may increase the likelihood of severe weather and strong wind gusts.
I’ll post a severity parameter table tomorrow morning if it looks like it will be warranted.
Monday Forecast Update
Updated Mon 07/24 @ 10:50 AM — Last night’s early models didn’t capture these showers over Philadelphia this morning. Despite that, they are still maintaining a forecast of most (but not all) shower activity stays to our west.
Current radar/satellite—
Water Vapor Monday morning shows additional showers moving up towards Philadelphia from the south. A significant disturbance (circled) will move to our west (white arrows) this afternoon and evening. Showers possible any time, but mostly far western suburbs. (Click on image for a larger view.)
Of the models out this morning, the HIRESW-MEM2 is probably capturing the shower activity best today—
Today’s 12z HIRESW-ARW MEM2 rainfall forecast through 6 PM Monday. (Click on image for a larger view.)
Yesterday’s HRDPS did pretty well with its forecast (shown below). Unfortunately, I’m having technical problems downloading the last few runs of the HRDPS right now.
Update Sun 7/23 10:43 PM — Tonight’s early models are keeping the showers to the west of the Pottstown area during the daytime hours Monday.
Previously Posted Sun 9:22 PM —
Low pressure moving off the Carolinas and a wave in the mid and upper atmosphere will set off showers and thunderstorms on Monday. Most of the activity is expected west of the immediate Philadelphia area, but some pop up storms are expected here as well.
The latest HRDPS shows this total accumulation and coverage for the showers. Note there is little activity expected in NJ.
Today’s 18z HRDPS showing predicted rainfall through 10 PM Monday. (Click on image for a larger view.)
Timing these showers will be difficult, as always. Some are expected in the immediate PHL area early afternoon and again early evening.
