Monthly Archives: May 2024

Philadelphia Area Weather Forecasts

WEEKEND WEATHER FORECAST

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Fine Tuning Monday’s Forecast

Update Mon 5/27 6:08 PM — The TV forecast I heard about an hour earlier was still emphasizing severe weather, tornados, hail this evening. I wish I saw evidence for this in the models. Some thunderstorms still possible with gusty winds.

Posted Monday 05/27/24 @ 10:39 AM — This morning’s HRRR (12z) shows some moderately elevated severity parameters (CAPE, Vertical Shear and Helicity) about 2 PM in upper Montgomery county.

Today’s 12z HRRR shows elevated moderately elevated severity parameters about 2 PM in upper Montgomery county. As is mostly the case, exact model placement of this area of potential severe weather is often never exact. (Click on image for a larger view.)

Posted Monday 05/27/24 @ 8:38 AM — Here’s the latest satellite water vapor image with superimposed MRMS radar—

Current satellite water vapor image with superimposed RAP model 500-1000 mb thickness lines (yellow), potential vorticity (fine violet contours) with superimposed MRMS RADAR. The area of clearing to our northwest will give us some bright skies and sun this morning. The impulse (white arrow) in West Virigina will bring more rain by about 2 PM. (Click on image for a larger view.)

Update Mon 5/27 7:32 AM — Last night’s model capture the following trends — We may see a brief period of sun or bright skies late morning. It appears that the heaviest rain will fall in far northern areas, closer to Allentown.

The showery rain with embedded thunderstorms will begin in the early afternoon and continue on and off through about 10 to 11PM. Wind gusts and localized heavy rain expected especially this evening, but severe weather will be most likely far north into Lehigh county and to our south in Maryland. Rainfall totals will be closer to 1 inch in the immediate Delaware Valley, rather than the 2-3 inches posted last night.

I’ll update if this morning’s models show significant changes.

Rainy Monday on Tap

Posted Sun, 05/26/24 @ 5:37 PM— Several rounds of rain will move through during the day on Monday, beginning before daybreak and continuing through mid-evening. The rain will be showery in nature, not a continuous rainfall, and there will be embedded thunderstorms.

The heaviest rainfall will likely occur early Monday evening with some heavier, stronger thunderstorms. While severe weather doesn’t appear to be likely, it never can be ruled out

The NBM and HREF show anywhere from 1.5 to almost 3 inches of rain, mostly in the immediate Delaware Valley area—

This afternoon’s HREF forecast total rainfall for Monday. (Click on image for larger view.)
This afternoon’s 18z NBM is somewhat more conservative with its total rainfall prediction. (Click on image for larger view.)
Increased Signal for Thunderstorms Sunday

Posted Sun 5/26 4:04 PM — Storms developed, just east of the I-95 corridor, this afternoon. That’s about as good models can get with this sort of thing.

Despite many days of forecasting heavy rain for Monday, some models have backed off from this. Will update later.

Posted Sun, 05/26/24 @ 10:21 AM— The latest NAM-NEST continues with a forecast of pop-up storms Sunday, between 4 PM and 6 PM along the I-95 corridor.

Update Sun 5/26 7:32 AM — Yesterday’s forecast for thundershowers didn’t materialize, as storms to our west dissipated as they approached our area last night and pop-ups didn’t occur as forecast during the daytime.

Unlike yesterday, there’s greater model consensus that some pop up storms will develop in and around the I-95 corridor during this afternoon. A stalled frontal boundary along with high CAPE values (approaching 2000 j/kg2) should provide enough of a trigger.

Update Sat 5/25 8:32 PM — We may still see some showers move in this evening, especially western suburbs, but I have to say that the Canadian models over-forecast the scattered shower potential for today.

The weak boundary moving through this evening will stall and move north as a warm front early Sunday. As that happens, some new showers and thunderstorms may spring up early Sunday morning but coverage will be limited.

As the warm front moves north, a mostly sunny Sunday is expected, but there may be periods of clouds at times in some areas. Limited, widely scattered showers are possible again Sunday afternoon and more so Sunday evening. It will be more humid. Monday still looks rainy in the morning with heavier showers and thunderstorms in the evening.

Previously Posted Fri 3:24 PM —

This weekend’s weather will be dependent upon a a series of frontal passages that include a stalled cold front returning as a warm front on Sunday morning and then stalling again. Passing along that front will be a series of disturbances, that along with daytime heating instability, will trigger isolated or scattered thunderstorms. Plenty of sunshine will also add to the unstable mix.

Exact timing and placement of these frontal passages will be difficult to forecast. After reviewing numerous models, I’m leaning towards the Canadian RGEM and HRDPS which did exceptionally well with last season’s thunderstorm forecasts. Adding credence to their forecast is the GFS, the experimental RRFS, and the ICON EPS which seems to have a somewhat similar forecasts.

Not in support of this forecast is the latest HREF which downplays showers significantly in our area for Saturday and Sunday. The ECMWF is similar for Sunday’s forecast, but not so much for Saturday. So a difficult forecast to pin down.

Saturday

Sunny in the morning, sunshine through thin high clouds in the afternoon. Increasing cloudiness late afternoon. A chance of widely scattered thunderstorms especially west of Philadelphia after 2-4 PM and more organized showers and thunderstorms moving in from the west after 8 PM.

Canadian RGEM shows widely scattered showers/thunderstorms as early as 4 PM. (Click on image for a larger view.)
Experimental RRFS shows showers/storms moving in from the west about 10 PM Saturday. The RRFS does have a tendency to over-predict thunderstorms. (Click on image for a larger view.)

NBM high temperatures: Blue Bell, PA 86º Philadelphia, PA 87º
uncertainty (based on standard deviation): low

Sunday

Becoming mostly sunny after some morning residual showers/storms. The front that moved through Saturday night stalls and moves back as a warm front. Instability will result in pop-up showers/thunderstorms during the afternoon .

NBM high temperatures: Blue Bell, PA 85º Philadelphia, PA 86º
uncertainty (based on standard deviation): low

Monday

Showers and thunderstorms beginning daybreak or early morning and continuing off and on. There may be a taper in the afternoon. Heavier rain and storms beginning Monday evening.

NAEFS model statistical “mode” version with overlayed GEFS model “bias-corrected” precipitation forecast for Monday at 2 PM (Click on image for a larger view.)

NBM high temperatures: Blue Bell, PA 78º Philadelphia, PA 80º
uncertainty (based on standard deviation): high uncertainty


Philadelphia Weather Outlook

THIS WEEK’S WEATHER

Things have stabilized somewhat with my family medical situation, and I will continue posting thoughts and commentary about the current weather over the coming days. For the time being, postings won’t be as frequent or comprehensive as they have been in the recent past.

Weekend Looking Better

Posted Thursday, 05/23/24 @ 7:35 PM— Timing and position of various disturbances over the weekend continues to be in flux. The gist of this morning’s update still holds, but it would not surprise me if things change radically, either for better weather overall, or worse for Saturday and Sunday. Monday still looks rainy.

Additionally, the weather conditions will be highly location dependent. With a front that will stall to our south, distances of 30-50 miles east-west or north-south of Philadelphia can have very different weather at different times of the day both Saturday and Sunday.

Posted Thursday 05/23/24 @ 12:45 PM — The timing of various disturbances moving across the area will impact the sensible weather this weekend, at least through Sunday. A lingering frontal system will affect mostly South Jersey on Friday with showers. Philadelphia may see sun on Friday.

Another disturbance approaching on Saturday will result in scattered showers/thundershowers late Saturday into Saturday evening in Philadelphia. The Jersey Shore will likely be sunny through high clouds on Saturday. The showers move into NJ Saturday night.

Sunday looks quite nice both in Philadelphia and the shore. There may be some clouds at times at the shore.

Monday looks cloudy and rainy. The heaviest rain in the early morning, but lingering cloudiness and scattered showers for the rest of the day with heavier rain developing again during the evening.

GFS forecast for 8 AM Monday. Heaviest rain for the 3 hour preceding 8AM. Scattered showers throughout the day. (Click on image for a larger view.)

Posted Wednesday 05/22/24 @ 4:18 PM — Recent model runs have accelerated the passage of the cold front. Originally expected to move through late Thursday, the latest models have showers, perhaps with some thunder, moving through Thursday morning. No major storms are expected. (There’s even a chance of some showers moving in this evening (Wednesday), especially western sections.

Showers may linger through Thursday afternoon and evening as the front slows down.

There’s much uncertainty in the weekend forecast. The Canadian global model has significant rain for us over the weekend. NOAA has been doing planned maintenance on their main server today, and I don’t have the latest NAEFS.

The weekend may be saved by the changes in timing of the next disturbances. It’s possible that shower activity will occur during the evening Saturday instead of the daytime hours. This especially true at the shore.

Sunday does still appear to have showers in the afternoon as does Monday.

Here’s the latest ECMWF forecast for 2 PM Monday—

12z ECMWF forecast for Monday at 2 PM Notice the northern extent of the dark orange contour (576 thickness line) into North Carolina – Very hot air is streaming northward. (Click on image for a larger view.)

Posted Tuesday 05/21/24 @ 3:57 PM — What’s really catching my interest over the past week is the extraordinarily high temperatures for May I’m seeing in Florida and Texas. Such severe high temperatures this early in the season is of great concern.

NAEFS model statistical “mode” version forecast for Sunday evening. Temperatures in excess of 100º in central FL! (Click on image for a larger view.)

There is evidence that these severe temperatures will be in our area sooner rather than later this upcoming summer season.

Posted Tue, 05/21/24 @ 11:00 AM— The next notable weather events this week and the upcoming Memorial Day Weekend will be a cold front that moves through later Thursday afternoon/evening and stalls east-west, becoming a focal point for low pressure development.

First, Thursday’s cold front will have plenty of thermodynamic energy for thunderstorms, BUT a weak jet flow and a weak anticyclonic curvature to the upper air winds may limit the development of locally severe weather. Most models are consistent with the heaviest rain and storms to occur to our north and to our south (in Maryland), somewhat skipping the immediate Philadelphia area. Total rainfall in the immediate PHL area will also be limited to under 1/2 inch of rain, as currently forecast.

The Memorial Day Weekend weather outlook is looking less than stellar. The cold front will stall to our south and areas of low pressure are expected to bring several periods of rain/showers Saturday through Monday. It’s too early to time these rainy periods.

Here’s the latest NAEFS forecast for Saturday evening—

If the current model forecast verifies, it appears that the upcoming weekend will be a repeat of the cool, wet pattern we’ve had for several weeks, with persistent low pressure systems moving just to our south. (Click on image for larger view.)

There’s still time for the weekend outlook to improve, but this is how it appears to be shaping up right now.

Previously Posted Sun 7:07 PM —

We finally have some sun and the first part of this upcoming week should feature sunshine and increasing temperatures.

The current water vapor image captures all the elements for this coming week’s weather—

Sunday: Current satellite water vapor image with superimposed RAP model 500-1000 mb thickness lines (yellow), Mean Sea Level Pressure isobars (black contours) potential vorticity (fine violet contours) with superimposed MRMS RADAR. The orange-red area show higher temperatures and the orange arrow depicts the general direction of the warmer flow towards us. Two areas of low pressure (with white arrows) are visible; one in the center of the country will move up north and west of us. The low in Georgia will move out to our south and east. Also depicted with a blue arrow is an influx of colder air in the Northwestern US. This air will eventually move in behind a cold front late Thursday or Friday. (Click on image for a larger view.)

Temperatures will likely get into the mid to upper 80s by Thursday.

Here’s the current NAEFS forecast for Thursday afternoon. A cold front will move through, likely with thunderstorms, and the front may stall east west to our south. —

NAEFS model statistical “mode” version with overlayed GEFS model “bias-corrected” precipitation forecast for Thursday afternoon. (Click on image for a larger view.)

The upcoming Memorial Day Weekend is likely to start reasonably nice (but cool) but with an increasing chance of showers and clouds from maybe Sunday and more likely into Monday. Much uncertainty, but this is how things look now.