Monthly Archives: April 2023

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)

Philadelphia Weather Outlook

THIS WEEK’S WEATHER

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Fri 5:45 PM Forecast Review — The weather’s been nice and beautiful and so the focus has been on the uncertainty in high temperatures for today. It took awhile for things to warm up to the 80s.

Here’s the RTMA for 3 PM showing the preliminary observed high temperatures for the day—

Today’s 3PM RTMA showing preliminary high temps for today. This morning’s NBM was very close to what was observed.

Friday Forecast Update

Update Fri 04/21 @ 8:22 AM —Large differences remain with the forecast high temperature today. The reason for these forecast temperature differences are slight differences in the origin of the forecast surface wind flow.

The latest RAP model wind streamlines captures the differences: where the easterly flow originates from the northeast, it will be cooler. Where the easterly flow originates from the southeast, it will be warmer—

RAP 09z model. Follow the easterly windstream origin and the reason for the differences in the high temp for today becomes apparent. (Click on image for a larger view.)

In contrast to the much cooler RAP model forecast, the Canadian high resolution model (HRDPS) forecasts temperature much higher for us today—

Today’s HRDPS shows much higher temperatures due to a more southerly wind component forecast. It should be noted that the HRDPS temperature forecasts have ALL been too high for us this season so far. (Click on image for a larger view.)

The NBM model blend is right in the middle of the two forecasts above—

07z NBM high temp for Friday (Click on image for a larger view.)

For Saturday, the rain moves in between 5PM and 7 PM. Details in the regular “Weekend Weather Forecast” later tody.

Update Thu 4/20 @ 7:55 PM — Friday is going to be another beautiful day. There has been a reduction in the high temperature forecast for Friday, as expected from last night’s forecasts.

Previous days this week, highs of 85º looked like a certainty for Friday. The latest GFS has reduced its high to about 82º for Friday, while the high resolution mesoscale models (HRRR, NAM-NEST, HREF) have us only in the mid 70s tomorrow.

Not surprisingly, the NBM is forecasting about 80º (with a high standard deviation of about 3.5º). That’s still a high level of uncertainty for next day temperatures. (As usual, my temperatures refer to location Wings Field, Blue Bell, PA.)

Here’s the latest NBM high temps—

Today’s 19z NBM high temperatures for Friday. (Click on image for a larger view.)

As for Saturday, the forecast is for the rain to move in about 6 PM or so.

Huge High Temperature Uncertainty Friday

Update Thu 4/20 8:01 AM — Last night’s models continue with an incredibly large spread in high temperatures for Friday, due to the uncertainty of the easterly wind flow and possibly some low level clouds.

The HRRR and RAP keep us in the low to mid 70s as a high, while the global models (GFS) are in the low to mid 80s! The NBM model captures this — its high temperature standard deviation is almost 6º. That means that of the approximately 40 models that the NBM statistically weights, there’s a range of ± 6º in the forecast highs of 67% of the models. It also means that 33% of the models have even larger differences.

(Typical standard deviation in the NBM high temperature a day in advance is about 1-2º!)

This all boils down to high uncertainty in the forecast high for Friday.

I wouldn’t mind some wonderful 80º temperatures, but I’m leaning towards betting on the lower end of the range right now.

Update Wed 4/19 10:59 PM — Possible big change in the Friday forecast. Tonight’s NAM-NEST shows an easterly wind flow with much cooler temperatures. Just one model, but don’t be surprised if Friday is much cooler than forecast earlier.

Update Wed 04/19 @ 7:53 PM — A strong upper air ridge will be in place over us for the next few days.

For Thursday, we”ll get into the mid to upper 70s and there may be some high thin cirrus cloudiness at times as moisture move up along the upper air ridge.

Current Satellite Water Vapor. The arrows show the path of the disturbances moving far north of us on Thursday—

Temperatures will continue to warm and Friday will be very warm for April.

Friday’s forecast high temperatures—

Latest NBM model high temperature forecast for Friday. Temperatures in and around the city are predicted to reach near 85º but there’s a very large spread (standard deviation of 4º- 5º) among the models indicating high uncertainty. So while 85º is the mean temperature, it could be higher and it could be lower by as much as 4º-5º (Click on image for a larger view.)

The forecast for the weekend is clarifying. A strong cold front moves through Saturday night with some possibly moderate to heavy rain. It looks like the precipitation moves in about 4-7PM.

The latest GFS model has rain just entering the western suburbs at 6 PM Saturday. (Click on image for a larger view.)

It should clear out for Sunday with much cooler weather.

Wednesday thru Friday

Update Tue 04/18 @ 5:35 PM — We had less cloudiness and higher temperatures Tuesday than forecast yesterday. I might add less wind as well. The clouds were mostly north of Quakertown

Wednesday through Friday looks good with warming temperatures. A few upper air disturbances on Thursday may bring a mix of clouds and sun. Friday looks even nicer and much warmer.

Temperatures will become summer-like again by Friday!

NBM (19z) forecast temperature/dew point with standard deviations meteogram for Blue Bell PA (Wings Field). Mean High temperatures shown. (Click on image for a larger view.)

The forecast for the weekend is in flux. The ECMWF has the cold front with shower/storms late Saturday. The GFS is slower and has the showers/storms into Sunday.

Tuesday Forecast Update

Update Tue 04/18 @ 12:01 PM — This morning’s models have backed off on the clouds for today with the exception of areas north of the city.

HRRR low level cloud forecast for 2 PM Tuesday. (Click on image for a larger view.)

Update Mon 04/17 @ 6:10 PM — The secondary cold front moved through late this afternoon with just some clouds. Colder air is moving in and will be with us through part of Wednesday.

Tuesday will be relatively chilly compared to recent days. Highs will be in the mid 50s. It will be windy and there will be a fair amount of instability cloudiness developing late morning through much of the afternoon, especially in areas from the city northward.

More sun on Wednesday, still windy. Temperatures will return to the 70s on Thursday. Friday looks good.

Weekend Early Outlook- part of the weekend (likely Sunday) looks to be quite interesting as heavy rain and thunderstorms as surface low pressure develops along a strong cold front.

Previously Posted Sun 8:53 PM —

A cold front moves through Sunday night about 1 AM with light scattered showers. No severe weather. Cooler air filters in on Monday,

Clouds linger Monday morning, then sunshine from about 10 to 3 PM.

Additional cloudiness with a secondary cold front late afternoon. Colder air for Monday night and Tuesday and quite windy. Very windy Wednesday. (Average seasonal high is about 65º-66º)

Moderating temperatures by Thursday.

(Next weekend looks a bit wet.)

Here’s the latest NBM temperature meteogram for Blue Bell, PA for Monday through Wednesday—

NBM Meteogram Temperature Dew point Max Temp with standard deviation (Click on image for a larger view.)
Philadelphia Area Weather Forecasts

WEEKEND WEATHER FORECAST

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Sunday Forecast Update

Update Sat 4/15 10:00 PM — Lingering clouds Sunday morning, breaking for periods of sunshine and clouds by late morning or early afternoon. Clouds move in towards evening and showers expected with a cold front after midnight. High temperature 78° but we may reach 80° with enough sunshine. Noticeably cooler Monday.

Update Sat 4/15 5:56 PM — Many areas did not receive any rain or storms today. The models are never able to predict exactly where storms will form and scenarios like today’s where the forecast is contingent on the position and track of an upper air low often result in the least exact precipitation placement forecasts.

Here’s what was happening at 5 PM—

Update Sat 04/15 @ 11:46 AM — It should be apparent that today’s weather is not going as forecast. The position and track of the upper air low is further north and west of what was forecast yesterday. As a result, the lcoation and timing of the showers and thunderstorms will be different than forecast. Current trends suggest that the heaviest showers and thunderstorms will be further north of our area. In fact, the precip in the immediate PHL area is looking fairly light.

Update Fri 04/14 @ 10:01 PM — The latest HRRR has become available and it gives a good idea about the scattered nature of the rain tomorrow and expected accumulations—

Friday night’s 00z HRRR accumulated rain by 8 PM. (Click on image for a larger view.)

Effective today, Twitter will no longer accept auto-posting from WordPress blogs like mine. I suggest Mastodon and the link on this page going forward.

Previously Posted Fri 9:04 PM —

Saturday Forecast

As mentioned in this morning’s update, the upper low that been bringing heavy rain to the southeastern US will be moving directly over Pennsylvania on Saturday. Showers and thunderstorms are expected as early as the mid morning hours Saturday and showers and thunderstorms will become more widespread during the afternoon hours.

Any storms are NOT expected to be severe, but localized moderate to heavy rain is possible for some locations. The exact locations depends upon the location of the upper low’s circulation, something the models aren’t perfect at doing.

The new NBM model v4.1 has a new forecast parameter, “thunderstorm coverage”. Here’s the NBM’s prediction for the greatest density of thunderstorm coverage tomorrow—

NBM 21z forecast “thunderstorm coverage” at 4 PM Saturday Greatest amount of thunderstorm activity in dark purple, but all areas especially north and west of the city have some possibility of thunderstorms. (Click on image for a larger view.)

High temperature on Saturday 72º sd 2.2ª (Blue Bell, PA)

Sunday

As for Sunday, clouds lingering in the morning. Some sunshine by afternoon.