#Philadelphia #weather #PAwx #PhillyWx

Friday into Saturday

Posted Friday 03/20/26 @ 9:13 AM — Based on this morning’s temperature trends, it wouldn’t surprise me if we reach 66-67º today as our high.

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Posted Thursday 03/19/26 @ 7:45 PM — It’s been a slow, but cold week weather-wise. Things change a bit late Friday as a fast moving low pressure system rides along the top of the upper ridge, bringing some rain to our area Friday evening.

Friday will be sunny in the morning and sunshine through high clouds for much of the afternoon. High temps 63º to 65º in the city.

Things turn rapidly cloudy around 4 PM, somewhat earlier to our northwest. Rain moves in about 5:30 PM – 7 PM from west to east. The rain moves out quickly perhaps before midnight.

According to the RRFS, some of the rain may be briefly moderately heavy in some areas and total rainfall over a few hours ranges from 0.35″ to 0.60″

Saturday will be sunny with light winds 6-10 mph from the north.

My regular Weekend Weather Forecast will be posted tomorrow late afternoon.

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NBM Model New v5 Now running in Parallel on NOAA’s Server

Posted Wednesday 03/18/26 @ 5:24 PM ——Scheduled for Operational release on April 15th, NOAA is now running the new model on their main server NOMADS as a ‘parallel’ run with existing version 4.3. Additional information can be found here.

There are numerous changes and advances in how NBM v5, an ensemble model, statistically evaluates its constituent member models. Of interest in the summer months are improved thunderstorm and tornado probabilities, as well as improved probabilistic rainfall estimates.

Of particular interest is its calculation of the WBGT (“Wet Bulb Globe Temperature”), a better way of expressing heat stress on the human body. The WBGT includes the direct effects of the heat of the sun and the cooling of wind, unlike the regular “heat index” which is just temperature and humidity. I am considering including the Wet Bulb Globe Temperature in my forecasts where appropriate.

Getting back to this week’s weather, there won’t be any immediate need for WBGT inclusion in this week’s forecasts. Temperatures continue to run below seasonal averages until the Friday. Here’s the temperatures (NBM 5) for the next several days—

The above is calculated from the NBM 5 mean temperatures PLUS the standard deviations. This has been my recent technique for NBM v 4.3. We may discover that adding the standard deviation may result in an forecast of temps with version 5 that is too high.

There looks be some light rain Friday night, ending Saturday morning with clearing by the afternoon.

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Cold but Moderating Temperatures

Posted Tuesday 03/17/26 @ 5:04 PM — The chill down expected should continue with moderation by the end of the week. There’s a deep cold trough over the Eastern US while the West has very warm temperatures.

By early or late Friday (depends on the model) a disturbance will move over us with some more rain. Timing varies with some models showing Friday and others showing early Saturday morning.

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Monday Storm Updates

Posted Monday 03/16/26 @ 6:05 PM — The latest RRFS backs up the HRRR with a fast, narrow cold front rain band (“NCFR”) moving through between 10 PM and midnight. (Near the city -11 PM) A quick burst of very heavy rain with wind gusts of near or above 60 mph!

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Posted Monday 03/16/26 @ 4:03 PM — Well, I wish I could take that previous post back. Honestly, I don’t know why the severe weather indices drop so low after 6 PM, yet the models continue to show a squall line with very high winds and heavy rainfall at 11 PM. Go figure! Or maybe we’ll see.

The latest HRRR (18z) continues to show a sharp, very fast squall line moving through around 11 PM with winds gusting 60+ mph—

The HRRR shows wind gusts near 60 mph just west of the city and approaches 70 + mph closer to the NJ shore before midnight.

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Posted Monday 03/16/26 @ 2:59 PM — Clarification: By severe weather, I mean tornadoes, hail, severe thunderstorms. It still looks likely that we’ll have very strong wind gusts approaching 60 mph in the evening.

Posted Monday 03/16/26 @ 2:33 PM — I’ve been looking at the latest RAP, HRRR, and RRFS hourly data.

Despite the fact that the front is coming through about 10 PM-midnight, all models show the highest chance of severe weather from NOW to about 6 PM.

All severe indices drop off rapidly after that point. That’s somewhat different than what I heard about a squall line moving through later this evening just ahead of the front (which still may happen).

Always listen to the NWS when it comes to warning and watches, but I’d thought you’d want to know what I see coming down the pike.

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Monday Storm Update

Posted Monday 03/16/26 @ 10:21 AM — The strong cold front continues to press towards us. More showers and possible thunderstorms are poised to move in before the main front arrives about 10 PM to midnight.

This system lacks a great deal of thermodynamic punch with CAPE values in the 300 joule range and Lifted Index only minus 2º. What it lacks in thermal potency is somewhat made up for by high kinetics- areas of very strong vertical motion, high helicity and high vertical shear.

As the surface front passes through about 11 PM, the an area of the jet streak called the “right entrance region” will be near us. This again is not the most potent area of a jet streak but still can pack a punch. Adding to the mix is a slight ‘negative tilt’ of the jet.

So, the kinematics are very potent for this storm, implying high winds and potentially severe storms. If we get some sunshine (not clear that we will) this will add the missing thermal instability ingredient that is currently lacking.

Here’s my “severity parameter table” based on the latest HRRR RRFS and NBM data and compared to known severe outbreaks and last week’s severe event that didn’t happen—

Severity Parameter (HRRR) RRFS	Sept 1 , 2021 Tornado Outbreak (example of extreme severe)	4-1-2023 Tornadoes (Example of severe) HRRR	03-11-26 RRFS 18z Severe Predicted But Wasn’t	TODAY 03-16-26 HRRR RRFS NBM	Notes
CAPE Joules/kg	3500-4200 ⚑⚑⚑⚑	2100 ⚑⚑	450 ⚑	700 ⚑⚑	West of PHL
Helicity m^2/s^2	1350 ⚑⚑⚑⚑⚑	655 ⚑⚑	650 ⚑⚑	650 ⚑⚑⚑
Vertical Shear 1/sec	40-46 ⚑⚑⚑⚑⚑	40-45 ⚑⚑⚑⚑	23-25 ⚑	26-33 ⚑⚑⚑
Precipitable Water	2.7″ ⚑⚑⚑⚑	0.83” ⚐	1.8″ ⚑⚑⚑	1.5″ ⚑⚑
Lifted Index º K	minus 6º ⚑⚑	minus 9.3º ⚑⚑⚑	minus 3.1º ⚑	minus 2.3º ⚐
HRRR Hail inches	1.9 ⚑⚑	1.4 ⚑	1.6″ ⚐	1.7″ ⚑
Peak Wind Gusts mph	40-50 ⚑⚑⚑⚑	40-50 ⚑⚑⚑⚑	25 mph ⚑	67mph ⚑⚑⚑⚑⚑
250 mb Jet  Streak  Position	Left Exit region ⚑⚑⚑⚑	Left Exit region ⚑⚑⚑⚑		Right Exit region ⚑⚑
250 mb Jet Stream Wind mph	63 ⚑	135 ⚑⚑⚑	67 ⚑	180 ⚑⚑⚑⚑
Max Updraft Velocity m2/s2			135 ⚑⚑	111 ⚑⚑⚑⚑ __ 3 ⚑	Berks County __ PHL
Max 600 mb DZDT			50 m/s	0.9	19 ⚑
CWASP (NBM)				63 ⚑⚑⚑	NJ  Early Afternoon

So what can we say about all of this—

• The highest probability of any tornadic activity or very severe has several time points and likely locations: NJ early to mid afternoon. Berks county early evening.
• There will be several periods of showers and thunderstorms, some severe- early afternoon and again between 6 PM and 11 PM.
• Should we get any sunshine in the afternoon, the missing thermal instability component may cause a higher incidence of severe weather.
• The largest concern is high winds, likely straight line, approaching 70 mph!
• Total rainfall about 1-1.5″, although some models have reduced that somewhat.

Updates if needed later this afternoon. Stay tuned.

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Storm Update

Posted Sunday 03/15/26 @ 8:41 PM — The showers and storms are just beginning to take shape far to our south. Just to “play” I’ve put together a severity table for this evening. No severe weather is expected , but some thunderstorms are possible about 3 AM- rush hour. Some ingredients are there for strong storms, but other parameters are inhibitory.

Severity Parameter (HRRR) RRFS	Sept 1 , 2021 Tornado Outbreak (example of extreme severe)	4-1-2023 Tornadoes (Example of severe) HRRR	03-11-26 RRFS 18z Severe Predicted But Wasn’t	03-15-26 HRRR RRFS NBM	Notes
CAPE Joules/kg	3500-4200 ⚑⚑⚑⚑	2100 ⚑⚑	450 ⚑	660 ⚑⚑	in NJ
Helicity m^2/s^2	1350 ⚑⚑⚑⚑⚑	655 ⚑⚑	650 ⚑⚑	700 ⚑⚑⚑
Vertical Shear 1/sec	40-46 ⚑⚑⚑⚑⚑	40-45 ⚑⚑⚑⚑	23-25 ⚑	33 ⚑⚑⚑
Precipitable Water	2.7″ ⚑⚑⚑⚑	0.83” ⚐	1.8″ ⚑⚑⚑	1.3 ⚑⚑
Lifted Index º K	minus 6º ⚑⚑	minus 9.3º ⚑⚑⚑	minus 3.1º ⚑	plus 2º  ↓
HRRR Hail inches	1.9 ⚑⚑	1.4 ⚑	1.6″ ⚐	0.96″ ⚐
Peak Wind Gusts mph	40-50 ⚑⚑⚑⚑	40-50 ⚑⚑⚑⚑	25 mph ⚑	37 mph ⚑⚑
250 mb Jet  Streak  Position	Left Exit region ⚑⚑⚑⚑	Left Exit region ⚑⚑⚑⚑		Right Exit region ⚑
250 mb Jet Stream Wind mph	63 ⚑	135 ⚑⚑⚑	67 ⚑	80 ⚑⚑
Max Updraft Velocity m2/s2			135 ⚑⚑	1 ↓
Max 600 mb DZDT			50 m/s	0.9
CWASP (NBM)				50 ⚑

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Monday – Strong Storms Possibly Severe

Originally Posted Sun 4:17 PM — A warm front moves over our area tonight (Sunday night). A strong cold front moves through Monday evening.

Very mild and moist air will create thunderstorms, heavy rain through Monday night. There is the possibility of severe weather.

Following some heavy rain and possibly strong to severe storms about 3 AM Monday morning the rain will taper during the morning hours. There will be a break in the action by noon before more storms move in later Monday afternoon and evening.

Despite the break in the overall rainfall, isolated areas of severe weather may develop during the late morning and early afternoon.

Here’s the RRFS forecast for 5:00 PM—

The main event will move in between 6 and 10 PM. Heavy rain and strong storms.

During our last severe storm threat, I tried out the SWEAT index (Severe Weather Threat Index). I wasn’t all that impressed with its sensitivity and specificity.

The latest NBM version 5.0 to be released in three weeks expands on the use of a new composite severe index. In weather model parlance, it’s called the CWASP. (Craven-Wiedenfeld Aggregate Severe Parameter.)

I’ve been playing with the pre-release NBM 5.0 data. Here’s it currently shows a maximum coverage and intensity around 11 AM Monday.

I’ve found that these severe weather parameters and timing of severe storms tends to change on the day of the event. Stay tuned. I’ll update later this evening if there are large changes.

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