THIS WEEK’S WEATHER

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Friday Outlook

Updated Thu 08/10 @ 7:58 PM — Friday looks quite nice with temperatures in the low to mid 80s. Sunshine through high cirrus clouds for most of the day.


Thursday Update
Thu 7:47 PM Forecast Review —So how much rain did we get?

MRMS radar- rain gauge estimated total amount of rainfall on Thursday. (Click on image for a larger view.)

Most models over-estimated the rainfall today. Last night’s 06z HRRR came the closest in predicting the axis of heavy rain. This morning’s HRRR didn’t do as well, probably due to inadequate model spin-up time.


Thu 1:54 PM Forecast Review — Based on current radar trends and RAP data, I don’t see the severe weather occurring in our immediate area. Probably just 0.5-1″ in the immediate PHL area.
Radar about 1:20 PM (Click on image for a larger view.)

Updated Thu 08/10 @ 10:58 AM — Light sprinkles have moved in earlier than forecast. The main activity is still to our southwest—

Thursday Forecast Clarified

Updated Thu 08/10 @ 8:05 AM Severity Parameter Comparison Table for Today– Allows comparison to recent severe events.

Severity Parameter
(HRRR)
Sept 1 , 2021
Tornado Outbreak
(example of highly severe)
Recent
4-1-23
Tornadoes
(Example of severe)
Recent 08-07-23
18z HRRR Bow Echo
08-10-23
Today’s
06z
HRRR
&
Impact

Notes
CAPE
Joules/kg
3500-4200
⚑⚑⚑⚑
2100
⚑⚑
2600
⚑⚑
1200
⚐ ⚐
Helicity
m^2/s^2
1350
⚑⚑⚑⚑⚑
655
⚑⚑
726
⚑⚑
560
Vertical Shear
1/sec
40-46
⚑⚑⚑
40-45
⚑⚑⚑
30
32
Precipitable Water 2.7″
⚑⚑⚑⚑
0.83”
2.1″
⚑⚑

2.3″
⚑⚑
Heavy rain potential
Lifted Index
º K
minus 6º
⚑⚑
minus 9.3º
⚑⚑⚑
minus 9.9º
⚑⚑⚑
Minus 4.4º
HRRR Hail
inches
1.9
⚑⚑
1.4
1.8
⚑⚑

1.6

Peak Wind Gusts
mph
40-50
⚑⚑
40-50
⚑⚑
65mph
⚑⚑
32mph
Storm Motion Shear Vector
Alignment
Aligned – ~ 90º
⚑⚑
Almost aligned
⚑⚑
aligned
⚑⚑
Somewhat aligned
250 mb
Jet Stream Wind
mph
63
135
⚑⚑
74
95-104
⚑⚑
Severity Parameters Impact: ⚑ indicates favors Severity ⚐ indicates Possible Severity
↓ indicates works against Severity ⇩ Significant, but less impact

Update Thu 8/10 7:37 AM — Last night’s 06z models have come together to allow for better forecast accuracy. It appears that CAPE values will be higher than previously forecast. The chance for strong to severe thunderstorms has increased for today. The biggest issue will be heavy rainfall in some areas, on the order of 1-2 inches.

Per the GFS (which had the best timing for Monday night’s storms) showers have already moved into far western parts of Montgomery county, but are heading northward, away from the city.

The main activity moves into immediate Philadelphia area between 11 and noon. Peak activity is around 2 PM, lasting on and off until 5-6 PM.


Thursday Outlook- More Thunderstorms

Update Wed 8/09 11:11 PM — Tonight’s high resolution models have a wide spread of forecasts. Some have heavy rain east in NJ, some have heavy rain northwest of the city. You get the idea. We simply don’t know where the heaviest rain will fall. Many areas may only get about 0.6”. A few may get 1.5”. Severe storms look less likely, with CAPE values unimpressive.

I’ll try to pin it down tomorrow morning.


Updated Wed 08/09 @ 5:15 PM — A rapidly moving system will move in from the west on Thursday. Showers and thunderstorms are likely to move in from the southwest from mid to late morning and linger through much of the afternoon. Some moderate to heavy rain expected in some areas. Some strong storms also appear likely, but CAPE values are in the 500-600 Joule/Kg range rather than the 2500 range of this past Monday night.

The latest HRRR shows an area of significantly elevated severe parameters between 11 AM and 2 PM in the Philadelphia area, while the NAM-NEST shows the most likely areas for strong (to severe) storms will be south into Delaware and much of the NJ Shore during the later afternoon. I generally go with the HRRR model with these sort of things.

Here’s the 18z HRRR—

Today’s 18z HRRR shows significantly elevated vertical shear (color shading) and significant helicity (brown rounded contours). Storm motion vectors (short arrows) are aligned with the direction of the shear (longer streamlines), increasing likelihood of stronger storms. CAPE values (not shown) are not all that impressive, due to lack of sunshine. Still enough CAPE to allow for thunderstorm development. Lower CAPE may put a lid on the severity of the thunderstorms. White arrow shows expected storm track. (Click on image for a larger view.)

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WEEKEND WEATHER FORECAST

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Sunday Update – Monday Outlook

Updated Sun 08/06 @ 9:14 AM — A southerly to southeasterly flow of warmer and more humid air will affect our weather today.

A warm front is taking shape for Sunday night (after midnight) that will bring showers and thunderstorms between 1 AM and 6 AM. The warm front and cold front are easily visible on water vapor imagery.—

Satellite Water Vapor Image Sunday morning shows the warm front (1) and the cold front (2) that will affect our weather Sunday night (after midnight) and Monday evening. This water vapor image (channel 9) specifically looks at the temperatures of the mid and upper levels of the atmosphere; it does NOT capture reflected light from clouds. The orange areas are warmer. On Monday, we’ll be in the narrow area orange sandwiched be the warm front and cold front. (Click on image for a larger view.)

This is the sort of scenario where thunderstorm might wake you up tonight.

After a very humid and sultry day Monday, very strong thunderstorms are likely late afternoon into evening Monday. (5 PM through 10 PM)

Plenty of CAPE, instability and helicity will be present for some strong to severe storms with a slight-marginal tornadic possibility. Unlike the past two thunderstorm threats, this one does not look like it will fizzle out.

Stay tuned for updates.


Updated Sat 08/05 @ 5:54 PM — Today, we had clouds and sunshine through a significant shield of high level clouds. Some people might casually describe today as having been sunny, but meteorologically, it correctly goes down as ‘mostly cloudy’ in the official PHL airport record.

Sunday may start cloudy very early, but we’ll have more sunshine than today through high level clouds and there will be fewer mid level clouds. High temp 87º- Blue Bell, 88º-Philadelphia. Increased humidity.

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