Posted Thursday 09/12/24 @ 8:13 PM — Despite this morning’s forecast for coastal redevelopment of Francine, today’s models are all different with no clear forecast. Perhaps the most consistent forecast is for a poorly defined area of rain to try to move northward as part of an inverted trough. The heavy rain may not make it into Philadelphia. (Wow, what’s new.)
The artificial intelligence ECMWF model has a storm off our coast next Tuesday, but it’s an outlier right now. So no clear forecast.
Posted Thursday 09/12/24 @ 8:15 AM —Recent model runs now show the secondary low remnants of Francine off the North Carolina coast re-forming back to a possible hurricane next Tuesday and moving northward, affecting our area next Wednesday or Thursday.
06zGFS forecast for next Tuesday afternoon. (Click on image for larger view.)
An interesting potential development. Stay tuned.
Blocking High Pressure
Posted Wednesday 09/11/24 @ 5:18 PM — The models have been in remarkably good agreement regarding the path and fate of hurricane Francine’s energy over the next several days. Blocking high pressure will prevent northward movement of the moisture remnants and been a forecast for secondary low pressure to develop off of the southeast coast. This, too, will be blocked from reaching our area.
ECMWF AI model forecast for Saturday night. (Click on image for larger view.)
Some moisture will attempt to move northward aloft on Thursday and Friday in the form of high cirrus clouds, but even that moisture will be pushed southward by Saturday.
Posted Monday 09/09/24 @ 5:02 PM — A mid and upper level high pressure system, part of a strong upper ridge, will keep us in sunny skies and gradually warmer temperatures. The upper air configuration shows the current blocked setup—
NAEFS Jet stream level winds (200 mb) Forecast for Friday. (Click on image for a larger view.)
While hurricane Francine is likely to make landfall in Louisiana late Wednesday, the upper ridge over our area will block the storm from affecting us directly.
ECMWF forecast for Hurricane Francine (Click on image for a larger view.)
Some high clouds may move into South Jersey and Delaware of Friday, but they’re forecast to dissipate by Saturday.
Tropical System Enters the Picture this Week
Originally Posted Sun @ 10:44 AM — —Most of our week will be dominated by large, sprawling high pressure that warms up to above seasonable levels, into the low 80s, by Wednesday. Another beautiful weather week ahead for us.
A likely tropical system or hurricane is expected to develop in the western Gulf of Mexico and move towards Louisiana this week. Most models show this developing into a tropical storm or hurricane, likely directly impacting Louisiana with rain with the system extending into western Tennessee The path of the tropical moisture associated with this storm beyond this that point is uncertain.
Here’s the current water vapor image—
Sunday morning 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 tropical system looks to develop into a tropical storm or a hurricane and make landfall
Here’s the latest ECMWF AI (machine learning- artificial intelligence) model forecast for Wednesday—
06z ECMWF-AI model forecast for Wednesday at 2 PM EST (Click on image for a larger view.)
The moisture from this storm may never make it into our area, being blocked by high pressure (GFS forecast) or may move up from the south to give us rain sometime next weekend (ECMWF).
Posted Thursday 09/05/24 @ 5:53 PM — Little change in the forecast for Friday and Saturday.
Friday will have clouds with some light drizzle or sprinkle early in the day as a warm front approaches.
Saturday will be cloudy with light sprinkles possible in the morning, showers move through with a cold front early Saturday afternoon, clearing early evening.
We need some rain and the total rainfall looks disappointing, with many areas coming with 0.20 inches or less of rain. The trend has been towards lower rainfall totals.
18z NBM rainfall totals forecast by 8 PM Saturday. (Click on image for a larger view.)
Posted Wednesday 09/04/24 @ 8:57 PM — Latest models showing minimal rainfall for Saturday and a faster frontal passage
Posted Wednesday 09/04/24 @ 6:00 PM — One more day of incredible weather on Thursday.
Friday‘s weather is looking much less outstanding. Earlier this week, it looked like some clouds would move in with an easterly flow. Several model runs now show some light showers and possibly some drizzle by early afternoon on Friday with the easterly flow. This is especially true at the NJ Shore.
The rainfall expected on Saturday will not be a drought buster and may be a disappointment. A front moves through during the daytime Saturday with showers and even some thundershowers. Showers may start as early as mid to late morning and continue through the early evening.
Depending upon the closeness of a low pressure system in the Atlantic, an unknown amount of moisture may be available for showers here on Saturday.
The ECMWF and ICON models have the low further away and the total precipitation here may only be about 0.2-0.3 inches. The GFS and Canadian GDPS have the Atlantic low closer to us, with more moisture available here. Amounts may range from 0.4″ to 0.6″.
The model blend (NBM) is showing the average, about 0.3″ rainfall. Not a drought buster.
12z GFS forecast for 1 PM Saturday. Front moving through our area. Low pressure off the North Carolina coast. Exact position of this low will determine how much moisture becomes available for rain in our area. (Click on image for a larger view.)
Forecast Update
Posted Tuesday 09/03/24 @ 6:07 PM — Wednesday and Thursday continues to promise beautiful weather here in Philadelphia with gradually increasing temperatures approaching or exceeding 80º —
18z NBM meteogram forecast for KLOM Blue Bell, PA Temperatures approach or exceed 80º. (Click on image for a larger view.)
Clouds increase on Friday, but rain is currently expected to wait until Saturday, as an approaching cold front and moisture from a system in the expected to develop near the North Carolina coast that may take on some tropical characteristics.
Here’s the latest ECMWF- AI (machine learning) model forecast for 2 PM Saturday—
12z ECMWF AI machine learning model for 2 PM Saturday. This is my first posting of an AI-model forecast. (Click on image for a larger view.)
At this time, total rainfall in the Philadelphia area on Saturday will range from 0.40 to 0.50 inches. which is already trending lower from yesterday. The higher resolution models will provide better clarity with the areal coverage of the rainfall, but Saturday is currently beyond the forecast range of these models. Some thunderstorms are also expected on Saturday ahead of a cold front passage.
More Diminished Rainfall for Philadelphia
Posted Tuesday 09/03/24 @ 8:10 AM — I’m continuing with the thread about the current trend of rainfall diminishing as it approaches Philadelphia.
Our next chance of rain will be Saturday, with clouds on the increase Friday, possibly with some light showers east on Friday. Here’s the latest ECMWF and GFS forecast rainfall for Saturday, ending Saturday night.—
GFS total rainfall by Saturday midnight (Click on image for a larger view.)ECMWF total rainfall by 2 AM Sunday morning. Frankly, I would be surprised if so much rain falls into Philadelphia. (Click on image for a larger view.)
These numbers and areal coverage will certainly change by Saturday. The observation that the rainfall drops off towards Philadelphia isn’t by chance; there is a true mesoscale physical process at work, captured in the models, in the current weather pattern. This process is likely related to the points made in the boxed comment below.
Regarding our dry conditions, prompted by a reader’s comments —
I’ve been doing much reading behind the scenes to try to understand why the rain is dissipating in the immediate PHL area. I’ve seen this behavior over the years, during what I call “drought-like” times. Last time was the 2022.
One thing seems to be a anticyclonic curvature of the upper air winds and the development sort of a high pressure wall. Certainly true with the 576 thickness makes a curvature just east of Philadelphia.
The other thing is that Philadelphia is located just east of what they call the ‘fall line’ – the terrain altitude drops 200-300 feet from parts of upper Montgomery and Chester counties to the river. That drop in altitude seems to counteract any upward vertical motion needed for the rain to occur. By the time disturbance gets to the Delaware river, the terrain altitude is flat, so there is no other loss of vertical motion and whatever weak ‘forcing’ upward of air for rain can resume without being negated. So it skips over Philadelphia.
There is something called a local heat dome, with the “urban heat island” so warm that normal convection is inhibited right over the city.
Finally, there is a moisture feedback loop. Once soil moisture is depleted, you get into a dry begets dry situation that results in a dry feedback loop.
Any and all of these theories could explain what we’re seeing.
Originally Posted Mon @ 10:10 AM — —High pressure builds in and moves due eastward, giving us fair weather and comfortable temperatures/humidity Monday through Wednesday and possibly Thursday—
NAEFS forecast for Wednesday at 11 AM High pressure (1) will be giving us sunny skies, but with an easterly cool flow of air. Low pressure (area 2 and and 3) will develop and bring rain for possibly as early as Friday and likely Saturday. There are differences in the GFS and ECMWF regarding the rain and the area of eventual low pressure development. (Click on image for a larger view.)
There is also potential tropical development in the Gulf of Mexico later this week, but it’s unclear if it will actually occur.
