THIS WEEK’S WEATHER

Sunday, July 14, 2024 8:41 pm GFS

#Philadelphia #weather #PAwx

Weekend Outlook

Posted Friday 07/19/24 @ 9:47 AM — Last night’s models continue with a similar forecast for the weekend— an increase in humidity, temperatures and moisture aloft, a mix of clouds and sunshine. The only change for Saturday– a chance of an isolated shower closer to the city about 2 PM; most areas dry.

For Sunday, a further increase in temperatures and humidity. The ECMWF and RGEM continues with a chance of isolated showers near the city during the afternoon; most areas dry.

Check back later this afternoon for my regular Weekend Weather Forecast.

Friday & The Weekend Outlook

Posted Thursday 07/18/24 @ 7:26 PM — I’m updating the forecast for Sunday. The latest ECMWF and Canadian RGEM show scattered showers on Sunday while the latest GFS keeps us dry with just high clouds. As I said below, a stalled front presents forecasting challenges. There’s too much uncertainty to hang one’s hat on either forecast Sometimes the best forecast is to indicate that we just don’t know for sure right now.

Posted Thursday 07/18/24 @ 5:32 PM — The front that went through Thursday night is slowly moving southward and drier air will filter in for Friday, eliminating the cloud cover. Sunny and pleasant should capture it for Friday.

The front boundary is expected to become stationary and persist for several days. Here’s the NAEFS depiction for Sunday afternoon—

12z NAEFS forecast for Sunday at 2 PM. I’ve drawn the approximate position of the stationary front. (Click on image for a larger view.)

Stalled frontal boundaries often present a forecasting challenge. Waves can develop along the front and a battle sets up between warm air to the south trying to move north while cooler, drier air tries to move south.

With potential wave development along the front, moisture can move over the boundary, causing clouds and the chance of showers.

This movement of moisture north of the front will cause cloudiness for Saturday. Light showers are predicted far northwestern areas of the forecast area, in Berks and Lehigh counties. Dry conditions expected in the immediate Delaware Valley area. Highs 86º -87º

For Sunday, the high pressure system to our north is expected to push a bit further south, helped by the wave that exits eastward. Sunday should be another very nice day, although somewhat warmer. Highs 89º-90º

As I said stalled fronts can be a challenge. Any unexpected change in the position of the front, or an unexpected development of a wave can ruin a forecast. We’ll see what happens.

BTW, this stalled front is currently expected to drift northward by Monday and next week may be very wet and unsettled. Stay tuned.

Thu 8:19 AM —Forecast Review— Here’s a summary of the rainfall last night. I always find it interesting to see how much it can vary from area to area.

MRMS rain-gauge measured rainfall combined with rain-gauge calibrated and interpolated radar-based summary of ACTUAL RAINFALL RECEIVED. Color shading is in inches. Line contour numbers are in mm. (25.4 mm= 1 inch) (Click on image for a larger view.)

Wednesday Thunderstorms

Posted Wednesday 07/17/24 @ 3:49 PM — A few strong scattered storms have developed and moved northeastward between 3 and 4 PM. The main activity will reach northwestern Montgomery and Bucks counties between 6:30 PM and 7:30 PM and move through the immediate Philadelphia area between 7:30 PM and 10 PM.

NexRad radar at 3:40 PM (Click on image for a larger view.)

The models are suggesting that weak low pressure will develop over us as the front moves through, enhancing rainfall intensity.

Posted Wednesday 07/17/24 @ 9:39 AM — A strong cold front will move through tonight with showers and some strong thunderstorms developing and moving through ahead of the front.

It appears that this will be a broad line of showers and storms and areas may receive several rounds of storms and rainfall.

Things are a little less clear about the timing. There’s the possibility of scattered storms breaking out well ahead of the main batch, as early as 3-4 PM. The main group of storms moves through between 5:30 and 10 PM.

Gusty winds and periods of heavy rain are likely.

06z HRDPS thunderstorm probability at 5 PM Most activity (but not all) still far northwest at this time. (Click on image for a larger view.)

Wed 9:25 AM —Forecast Review — With the exception of the NAM-NEST and HRDPS, yesterday’s models didn’t quite capture the line of storms that went through last evening.

Here’s the URMA summary of rainfall received—

Usually I use the MRMS for rainfall summaries. Today, I’m using the URMA model precipitation summary. Blue labeled contours are in inches. (Click on image for a larger view.)

Tuesday Evening

Posted Tuesday 07/16/24 @ 7:23 PM — A new line of storms, with a better wind trajectory to reach Philadelphia has formed. We’ll see if they hold together—

~ 7PM Radar with superimposed RAP model vertical velocity pressure (omega -orange contours). There’s downward motion ( denoted by the minus signs) between the storms and our area which may put a damper on their strength. We’ll see. (Click on image for a larger view.)

Posted Tuesday 07/16/24 @ 5:50 PM — The storms are diminishing in intensity and likely will move to our north, if they stay intact.

Posted Tuesday 07/16/24 @ 5:18 PM — Storms have developed pretty much as had been forecast by NAM-NEST and HRDPS, but not with the HRRR, GFS or ECMWF.

While the NAM-NEST does have them moving through Philadelphia around 6:30 PM- 8 PM, the upper air winds may take them just to our north, unless the line extends and builds further southward.

More interesting weather for Wednesday. Check back.

Tuesday Forecast Update

Posted Tuesday 07/16/24 @ 12:56 PM — A quick update. Some of this morning’s models have backed off on the chance of thunderstorms for this evening. Isolated to scattered thunderstorms still possible, north of the city. (The Canadian models are the most aggressive with a continued chance of storms, the ECMWF and GFS the least.)

Posted Tuesday 07/16/24 @ 8:39 AM — Latest Model Blend (NBM) forecast high temperatures for Tuesday—

Today’s 06z NBM high temperatures (actual, not heat index) (Click on image for a larger view.)

As for thunderstorms, the latest HREF has the greatest chance of thunderstorms from the city north and west—

HREF probability of radar > 40 db (Click on image for a larger view.)

Thunderstorms possible (isolated) as early as 4:30 PM. Scattered storms from the city west from 5-9 PM.

NBM (Model Blend) thunderstorm coverage isolated and scattered at 6 PM. (Click on image for a larger view.)

Posted Monday 07/15/24 @ 4:43 PM — A small upper level wave will move through late afternoon Tuesday. With high temperatures and humidity and with instability also high, thunderstorms will be triggered. Currently the time frame is from 7 PM through 11 PM Tuesday evening.

Some of these storms may be severe, especially northwest of the city.

The storms may weaken as they move through Philadelphia, as the wave will already be to our east and upper air “heights” will begin to increase again. Here’s the NAM-NEST model showing the wave, visible in the jet-stream level wind streams —

NAM-NEST model forecast 8 PM Tuesday showing jet stream level (250 mb) winds streams. (Click on image for a larger view.)

Posted Monday 07/15/24 @ 10:50 AM —Monday will be hot, with humidity and temperatures creating an “apparent temperature”, also referred to as a “heat index” or “feels like” temperatures of over 100º in many areas—

Latest Model Blend (NBM) 12z forecast apparent temperature (Click on image for a larger view.)

There’s a chance of isolated thunderstorms northwest of our area.

Originally Posted Sun 8:41 PM —Our current heat wave will continue through Wednesday. High temperatures will be in the mid 90s on Monday and approach or exceed 100º on Tuesday. With some increased cloudiness from an approaching cold front, Wednesday will ‘only’ be in the mid 90s.

A cold front is expected to move through late Wednesday. Showers and thunderstorms will accompany the front—

NAEFS model statistical “mode” version with overlayed GEFS model “bias-corrected” precipitation forecast for Wednesday. A cold front approaches late Wednesday or Wednesday night. (Click on image for a larger view.)

Philadelphia Area Weather Forecasts

WEEKEND WEATHER FORECAST

Friday, July 12, 2024 5:50 pm GFS

#Philadelphia #weather #PAwx

Sun 6:37 PM —Forecast Review —Thunderstorms developed an hour or two earlier than forecast and somewhat further west than forecast. Here’s the MRMS rainfall totals —

Posted Sunday 07/14/24 @ 11:35 AM —

Some of this morning’s models are showing an increased possibility of isolated severe thunderstorms. Not all areas will have a thunderstorm. The time window has increased from 3 PM to 9 PM.

Posted Sunday 07/14/24 @ 8:02 AM —

Several ingredients are coming together Sunday afternoon that will result in thunderstorms in our area, especially from Philadelphia and eastward. An upper trough, and thermal instability with high moisture will allow thunderstorms to develop from Lehigh county in the early afternoon to the Philadelphia area eastward into central NJ between 4 an 6 PM. Some scattered activity is possible through 8 to 9 PM.

HREF probability of thunderstorms. **The entire area could see isolated storms as early as 1-2 PM and as late as 8 PM, but the highest probability of scattered storms will be in the red area at about 4-6 PM.** (Click on image for a larger view.)

While heavy rain and gusty winds can occur with most thunderstorms, these storms can be strong, but not necessarily ‘severe’.

Posted Saturday 07/13/24 @ 9:50 PM —

A weak upper air trough will move through late Sunday afternoon, triggering scattered thunderstorms in the Philadelphia area. Peak probability is 4 PM-6 PM. A few may be strong. Look for an update Sunday morning.

Sat 8:42 AM —Forecast Review —It’s hard to remember a recent past forecast that was as much of a bust as the forecast for last night and this morning.

Incredibly, every model, including those run at 00z (8PM EDT) and become available hours later (often after 11:30 PM EDT) continued to show heavy rain for the early morning hours in the immediate Philadelphia area.

Here’s the rainfall that fell over the past 12 hours, from 8 PM to 8 AM—

Clearly, the precipitation actually received doesn’t really approach any of the model forecasts posted yesterday.

So what could cause so many different models to be so wrong? This precipitation originated from a moist flow around high pressure in the Atlantic. There are relatively few direct measurements of pressure, moisture and temperature, especially vertically into the middle and upper atmosphere, in that region.

While new satellite techniques are used to infer and indirectly measure this some of this missing data, clearly there were errors and missing information. So the input conditions, especially in the middle and upper atmosphere were incomplete causing the model “initialization” to be wrong.

While numeric weather prediction has come an incredibly long way in the past 50 years, it simply can’t compensate for wrong or missing initial conditions. Can machine learning and AI, take it to a new level? I’m a little skeptical, especially with missing input information. I guess we’ll see.

Previously Posted Fri 5:50 PM —As posted earlier today, the heavy rain expected in the Philadelphia area did not materialize as forecast, but the heavy rain developed near the eastern shore of NJ (see here) with some extremely heavy downpours.

The latest model blend (NBM ) still shows significant rain for the immediate Delaware Valley, mostly occurring late Friday night into the morning hours on Saturday—

18z NBM total rainfall through Saturday (Click on image for a larger view.)

With the Friday forecast such a bust, I’d say this is a low confidence forecast. Nonetheless, the NBM is supported by the latest HREF which just became available.

On the other hand, the latest Canadian RGEM, also just available, shows a heavy axis shifted eastward-

18z Canadian RGEM model total precipitation (Click on image for a larger view.)

I’m leaning towards the RGEM, but who knows? 🙂

Saturday

Rain through the morning, ending from west to east, noon through 2 PM in the Delaware Valley, and as late as 5 PM at the shore. Some sunshine is likely here in the afternoon. Very humid!

NBM high temperatures: Blue Bell, PA 87º Philadelphia, PA 88º
uncertainty (based on standard deviation): ± 2.3º (higher than average)

Sunday

Mostly sunny, hot and humid. Heat indices near 100º

NBM high temperatures: Blue Bell, PA 93º Philadelphia, PA 94º
Uncertainty (based on standard deviation): ± 1.9 (average uncertainty)

Philadelphia Weekend Weather & Storm Forecasts