Monthly Archives: April 2023

Philadelphia Weather Outlook

THIS WEEK’S WEATHER

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Wednesday Forecast Update

Update Wed 04/26 @ 9:48 AM — Some showers are moving in a bit earlier than forecast by most of last night’s models (with the exception of yesterday’s NAM-NEST). Additional scattered showers expected especially mid to late afternoon and evening. There will be breaks of sun at times today.

(It’s interesting that the two most recent NAM-NEST model runs did not show the showers this morning, but yesterday’s 18z run did. Such is the state of precipitation forecasts.)

According to the latest HRRR (12z) most of the showers will develop during the late afternoon and evening and most of the activity (not all) will be a bit north of the city—

Today’s 12z HRRR total precipitation forecast for Wednesday into Thursday. (Click on image for a larger view.)

Wednesday Forecast Update

Update Tue 4/25 8:28 PM — No significant change in the forecast from the forecast made this morning (below). Inspection of this afternoon’s models suggests that a warm front type situation will be part of the trigger for the showers Wednesday. The showers will be scattered and there may be breaks of sun with any showers. Any sunshine will increase instability promoting the possibility of low level thundershowers.

It still looks like plenty of rain this weekend, as early as Friday, but there are timing differences now between the GFS and the ECMWF. So it may rain Saturday or Sunday or both days. Or some of the rain may be late Sunday, with rain-free periods during the daytime. Too soon to nail that down.

Update Tue 04/25 @ 10:14 AM — It does appear that some scattered showers and low-level thundershowers will develop during Wednesday afternoon. Clouds move in late morning and showers/thundershowers can be expected from about 1 PM to 8 PM.

The scattered showers will develop dynamically in-place, rather than moving in from the west as a upper air wave moves through.

Here’s the latest HRRR cloud/radar forecast—

Today’s 12z HRRR forecast for 4 PM Wednesday afternoon. Clouds (dark) simulated radar and accumulated precip (green contours) (Click on image for a larger view.)

FYI, the models continue to forecast a fairly rainy weekend, now including Saturday and Sunday. Updates later.

Focus on Wednesday and after

Update Mon 4/24 10:27 PM — This afternoon’s models still have some showers on Wednesday but the timing ranges from starting in the early afternoon or early evening in our immediate area.

This weekend’s weather still looks very ‘interesting’ as heavy rain is looking more likely on Sunday from a stalled surface low enhanced by a highly amplified jet flow. Unsettled weather is possible late Friday and even some part of Saturday from the same system.

Update Mon 04/24 @ 8:57 AM — Last night’s models are signaling a change in the forecast as early as Wednesday. An upper air low pressure system north of the Great Lakes will rotate a disturbance through our area on Wednesday —

Satellite WV image Monday morning with superimposed RAP model jet level windstreams (250mb) . (Click on image for a larger view.)

Here’s the latest NAEFS forecast for Wednesday showing weak low pressure over us—

Today’s 06z NAEFS (Click on image for a larger view.)

The disturbance is weak, but enough to set off some showers on Wednesday. So what appeared to be a quiet few day of weather has changed into something less straightforward.

Today, Monday, and Tuesday will be the nicest days of this week. (Today will have some periods instability cloudiness develop in the afternoon.)

Additional disturbances and a developing trough (labeled “The weekend” above) will affect us Thursday through the weekend.

Quiet and Cooler

Previously Posted Sun 6:50 PM —

An upper air disturbance brought some clouds late today, Sunday, after a beautiful, mostly sunny day in the immediate PHL area. Areas east of us in NJ had some lingering clouds from the system that gave us the rain last night.

Following this disturbance, cooler air will be moving in for much of the first half of this week. Not much happening Monday through Wednesday, as high pressure and cool temperatures control our weather. Mostly sunny, some instability cloudiness at times.

We may get affected by a disturbance moving to our south on Thursday, with a chance of some showers.

Things look ‘interesting’ for next weekend. A highly amplified upper trough will develop and the models are showing a coastal secondary low that may develop and linger. There’s fairly good agreement among the ECMWF and GEFS models for this to occur. Cool, breezt and very rainy weather is expected. The time frame, depending on the model, is as early as Saturday, possibly lingering through part of Monday. I’ll keep an eye on this during the week.

Here’s the current NAEFS forecast for next Sunday—

Today’s 12z NAEFS forecast for next Sunday. Cold air plunges south. The red 540 thickness line captures the general contour of the jet stream. Secondary low formation in the Virginias may linger as the upper pattern takes on a negative tilt. (Click on image for a larger view.)

Philadelphia Area Weather Forecasts

WEEKEND WEATHER FORECAST

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Update Sat 04/22 @ 5:11 PM — The storms are moving in somewhat ahead of schedule of the HRRR and RAP models—

RADAR at 5PM Blue contours are “lapse rates”, areas of greatest thermal instability. Bullseye over eastern Bucks county, the strongest storms may develop there. (Click on image for a larger view.)

The Phillies game may just finish ahead of the storms. It’s going to be close.

Update Sat 04/22 @ 10:18 AM — The latest 12z NBM, HRRR, RAP and NAM-NEST models have become available.

Here’s the trends. Storms move in between 6 PM (western suburbs areas) and 8 PM.

Heaviest rain in western suburbs and in New Jersey. The immediate PHL area appears to fall into a lull in activity of the storms.

Heaviest rain—

12z HRRR total rain accumulation. This corresponds well with the greatest chance of severe weather. The ‘lull’ in activity around PHL shows. It should be noted that forecasts of exact placement of the heaviest rains are not possible with the current state of the models. It’s provided to give a sense that these storms will not have very uniform in coverage and intensity. (Click on image for a larger view.)

As for severity, below is my HRRR Severity Table—

Based on this table, I see tonight’s storms as typical thunderstorms with a chance of locally severe.

I don’t see tornadoes as likely. Largest hazard will be possible localized hail and strong straight line wind gusts.

I should indicate that the latest NAM-NEST has much higher CAPE and helicity values than the HRRR. I’ve found that the HRRR is more reliable for these sort of events and I’m going to ignore the more severe NAM-NEST forecast at this time.

Sept 1 , 2021
Tornado Outbreak
(example of highly severe)		Past
April 1st 2023 Tornado
Outbreak
HRRR 12z
Today’s
12z HRRR Forecast		Impact
CAPE 3500-4200 Joules/kg2100 550⚐ ⇩
Helicity 1350 m^2/s^2655460 ⚐ ⇩
Vertical Shear 40-4640-4525 ⚐ ⇩⇩
Precipitable Water 2.7″0.83”1.1″ ⚐
Lifted Index minus 6º Kminus 9.3ºminus 2.3↓ ↓
HRRR Hail Parameter 1.91.41.3⚑
Peak Wind Gusts 40-5040-5035-40⚐
Aligned Storm Motion and Shear Vectors- ~ 90ºAlmost alignednon-aligned ⇩
Jet stream level – wavyHighly cyclonicwavy⚐ ⇩
Jet Stream 250 mb Speed – 63High 13594-100⚐ ⇩
Severity Parameters Impact: ⚑ indicates strongly favors Severity
⚐ indicates Possible Severity
↓ indicates works against Severity ⇩ Significant, but less impact

Update Sat 4/22 @ 8:09 AM — Quick update. Last night’s models show the following trends: A further increase in CAPE and some areas of moderate helicity, especially in Delaware and areas north of Trenton. Some areas with strong to severe storms now appears possible.

The line of storms moves in between 6 PM and 8 PM, about an hour or so later than forecast last night. (Earlier in western suburbs.)

I’ll update later this morning before 10:30 AM with this morning’s 12z models.

Update Fri 4/21 9:52 PM — With tonight’s HRRR just becoming available, I want to underline the possibility of a one or two widely scattered pop up storms during the mid afternoon due to increased instability.
The timing of the main line of rain and storms is mostly unchanged. The HRRR also shows a modest increase in severity parameters. Updates tomorrow morning after 10:15 with the new HRRR and NAM-NEST.

Previously Posted Fri 7:55 PM —

A strong cold front will move quickly through our area Saturday night with showers and possibly some thunderstorms. Cooler high pressure builds in for Sunday. (Average highs this time of year are 66º-67º)

Satellite water vapor shows the position of the front and upper level low this morning—

Water Vapor image shows upper low and position of the cold front from this morning. (Click on image for a larger view.)

Saturday

The clouds associated with the approaching cold front will move in during the early afternoon. Thickening clouds by 4 PM. It will become fairly windy and gusty in the afternoon.

It is possible that a few stray sprinkles may break out during the mid afternoon, ahead of the main line.

The main line of showers and possibly some thunderstorms moves in between 5 and 6 PM, earlier far western suburbs. Wind gusts near 30 mph. Greatest coverage of thunderstorms- western Chester County. Severity parameters are only in the moderate range.

Today’s 21z HREF Simulated Radar Forecast for6 PM Saturday. (Black-grey is cloud cover) (Click on image for a larger view.)

While any thunderstorm can produce locally heavy rain and strong wind gusts, no extreme weather is expected with these storms at this time.

NBM Wind/Gusts Forecast—

NBM wind and wind gust forecast Blue Bell, PA (Click on image for a larger view.)

The line of showers and storms will be fast moving and precip should end about 2 AM. Total rainfall about 0.5 -1”, greatest amounts in far western suburbs.

High temperature 77º sd 1º ( NBM model location Blue Bell, PA)

Sunday

Becoming sunny and breezy.

High temperature 64º sd 3º ( NBM model location Blue Bell, PA)