WEEKEND WEATHER FORECAST

#Philadelphia #weather #PAwx

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—

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.)

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)


THIS WEEK’S WEATHER

#Philadelphia #weather #PAwx

Rain Update

Posted Friday 07/12/24 @ 3:40 PM

So where’s the heavy rain? Not where forecast. So far, the heaviest band set up over eastern NJ—

MRMS rain-gauge measured rainfall combined with rain-gauge calibrated and interpolated radar-based summary of ACTUAL RAINFALL received by 3 PM. Color shading is in inches. Line contour numbers are in mm. (25.4 mm= 1 inch)      (Click on image for a larger view.)

The 2 PM (18z) HRRR still shows areas of heavier rain to fall closer to the city over the next 24 hours, by 2 PM Saturday—

18z HRRR rainfall forecast 2PM Friday through 2 PM Saturday. (Click on image for a larger view.)

We’ll see if it happens!

Posted Friday 07/12/24 @ 11:40 AM

The latest HREF model (along with the experimental REFS model) are predicting truly unbelievable rainfall totals over the next 24-30 hours. I’m not sure that I buy it. Probably overdone. The latest HRRR shows rainfall totals in the 1.5″=3.5″ range and is likely more accurate.

Here’s the very latest HREF—

Today’s 12z HREF model. The two amounts shown are based on the two different statistical methods that the model uses to combine the data from its constituent models (HRRR, HIRESW, HIRESW-FV3 etc) (Click on image for a larger view.)

Posted Friday 07/12/24 @ 8:58 AM — The first round of showers has moved up over us and it was the first of many rounds of showers and even some thunderstorms that will move through from the southeast for the next 30+ hours.

Radar with RAP model 500 mb wind streams at 8:42 AM. (Click on image for a larger view.)

The models are still forecasting 1-2 inches of rainfall with locally higher amounts through late morning Saturday.

Latest NBM 12z total rainfall. (Click on image for a larger view.)

Clouds may break for sun here in Philadelphia and westward on Saturday, but showers and clouds may linger at the Jersey shore into late afternoon Saturday.


Showers and Thunderstorms move in Friday Morning

Posted Thursday 07/11/24 @ 9:14 PM — The stalled front off of the coast will retrograde westward as a warm front Friday through Saturday.

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). I’ve drawn in the position of the stationary front that will return as a warm front. (Click on image for a larger view.)

High moisture flow will fuel showers and thunderstorms, beginning early Friday morning and lasting into Saturday morning. There’s some indication that considerable clouds and spotty showers may linger into Saturday afternoon, especially east of the city and along the NJ coast.

Rainfall totals will be over 1″ in many areas, with some areas receiving much more.

The latest NBM (00z) total rainfall through 8AM Saturday morning. Additional rain expected after 8AM. (Click on image for a larger view.)

I’ll try to nail down the details with a Friday morning update.


Thu 8:20 AM —Forecast Review —As always, the model precip forecasts gave a feel for range of the rainfall totals, but was far off regarding the placement and axis of the heaviest rain. Accurately captured was the forecast decrease in intensity eastward into New Jersey. Not captured was the heavy band in of rain in Delaware.

Here’s the rainfall totals from last night and compare here

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

Hot and less humid today. Showers and thunderstorms arrive Friday morning as a stalled front retrogrades westward as a warm front. Showers linger into Saturday morning and Saturday looks to be unsettled with a spotty shower possible through the daytime hours, especially at the shore. Sunday looks to be hot again, perhaps with isolated pop-up thunderstorms

Much Needed Rainfall

Posted Wednesday 07/10/24 @ 4:55 PM — The first period of some much needed rainfall will be this evening, as early as 10 PM in western suburbs and continuing into Philadelphia by midnight. All models have the rain diminishing in intensity as it moves into Philadelphia and then NJ.

This first batch of rain won’t be all that heavy; many areas will receive only 0.10-0.50 inches with a few locally higher amounts in western suburbs.

This afternoon’s HRRR total rainfall this evening. (Click on image for a larger view.)

For Thursday, mostly sunny, through high cirrus clouds. Highs upper 80s just short of 90º

Friday looks unsettled. Clouds and scattered showers moving in from the southeast during the morning. Heavier rainfall at night. Highs in the low to mid 80s.

The rain may linger early Saturday morning, but sunshine by afternoon.


Heavy Rainfall Looking More Certain

Posted Wednesday 07/10/24 @ 8:19 AM — I’m glad I waited the extra 12 hours for forecast clarification. The major models have all moved towards heavier rainfall in the Delaware Valley with the usual significant differences.

Tonight: The initial front moves through about 2 AM Thursday morning with a line of showers and thunderstorms. The front stalls close to the Philadelphia area. The exact location will determine the axis of the heavy rainfall. It appears that the front will linger until Saturday morning with on again, off again showers. Some will be heavy, especially later Friday.

Here’s the latest GFS model—

GFS total rainfall tonight through Saturday morning (Click on image for a larger view.)

Here’s the current ECMWF

Current ECMWF total rainfall forecast through Saturday morning (Click on image for a larger view.)

So there are unknowns with this forecast: the exact position of the stalled front with the exact axis of heavy rainfall and the exact timing of the heavier bands of rain.


More Humidity and Maybe Some Rain

Posted Tuesday 07/09/24 @ 7:50 PM

A stalled frontal boundary is expected to be a focal point for some much-needed rainfall late Friday. Unfortunately, there’s a trend with this afternoon’s models for the heaviest rainfall to fall in a narrow band, possibly to our south and east. There’s also some considerable model differences with several showing surface low pressure developing in the southeastern US, while the GFS and ECMWF don’t show this low pressure development.

Too early to say what will happen. I’ll update tomorrow morning with some graphics.


Posted Monday 07/08/24 @ 5:48 PM —The trend for the Tuesday and especially Wednesday is a significant increase in humidity and discomfort, with high temperatures remaining in the 93º-95º range. With the increase in humidity (dew points in the 70s) there will be an increased chance of isolated thunderstorms .

Many areas in our area need rain and the excessive heat is parching lawns and desiccating trees and other flora. Rainfall potential towards the end of the week is increasing and, with the exception of the latest GFS model, the ECMWF, Canadian Global and Germain ICON models are showing rainfall to be exceeding 1 inch in the Delaware Valley by Saturday morning.

The increased potential for rainfall Thursday, Friday and very early Saturday morning is the result of a stalled frontal boundary and a southeasterly moist flow off of the Atlantic. (The moisture from Beryl will likely move past us to our north.) The wettest day, if it plays out, will be Friday

The latest ECMWF shows rain moving in from the southeast on Friday (Forecast map is for 11 AM) (Click on image for a larger view.)

Previously Posted Sun 6:12 PM —A southwesterly flow of very hot air will remain in place through at least Wednesday. Thursday and Friday, the remains of Beryl with interact with a stalled frontal boundary in our area. The exact situation may be too complex to model in advance, so we’ll have to wait and see.

NAEFS model statistical “mode” version with overlayed GEFS model “bias-corrected” precipitation forecast for Wednesday morning. The remains of Beryl may move to our north over the top of the heat dome (orange 576 contour) (Click on image for a larger view.)

The US Navy’s COAMPS model’s track of Beryl—

Some much needed rain may be in store for Thursday through Friday, but current forecasts aren’t predicting all that much in the Philadelphia area. We’ll have to wait and see the position of the stalled front and the trajectory of the tropical moisture from Beryl.

For tomorrow, Monday, and Tuesday, continued hot, with temperatures in the mid to even upper 90s.

The NAEFS keeps us in the 90s through early next week!


Weather… and Other Things 'Up in the Air'