Posted Saturday 09/07/24 @ 3:51 PM — Radar shows an additional area of light rain behind this anafront that may move in between 4 PM and 7 PM. This additional area is showing in today’s models that became available late this morning.
Sat 3:39 PM —Forecast Review — The showers moved in on schedule and many areas received minimal rainfall while some received a decent amount, but hardly a drought buster.
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.)
The model forecast below did pretty good at capturing the area around Philadelphia that got minimal rainfall.
Saturday Update
Posted Saturday 09/07/24 @ 9:45 AM — The approaching cold front (classified asa frontal type “anafront” due to the precipitation lagging the actual surface front) will pass through here between 1 PM (western Montco/Bucks/Chester) and 2-3 PM (Philadelphia) and 4-5 PM (NJ).
Here’s the Water vapor/radar image—
“Anafront” type cold front with the precipitation lagging the surface front (blue cold front) (Click on image for a larger view.)
Expected total rainfall continues to lessen for our immediate area, with areas receiving 0.00″ to 0.15″—
00zREFS experimental total rainfall forecast. (Click on image for a larger view.)
Originally Posted Fri 5:34 PM —A cold front will move through our area between noon and 3PM Saturday. What had looked earlier this week to be a decent rain-maker has diminished to a chance of a few showers with precipitation totals of about 0.20″ or less. Our dry pattern continues and appears to be worsening.
Those of you who followed the discussion this past week about rain “skipping over” Philadelphia will recognize the rainfall pattern of the 06z experimental REFS forecast for Saturday—
06z REFS total rainfall The NBM model forecast is below. (Click on image for a larger view.)
The high pressure system that follows for Sunday will further suppress any possibility of rainfall far south of our area for much of the upcoming week.
Current satellite water vapor shows the current setup—
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. Cold front will bring light showers on Saturday. The moisture and rain is suppressed to the southern US. (Click on image for a larger view.)
Originally Posted Fri @ 5:34 PM —
Saturday Forecast
Cloudy. Showers move in to western Montco/Bucks/Chester counties about noon and through the city by 2 PM into NJ. Strong wind gusts with the frontal passage.
Total Rainfall (Model Blend)
NBM high temperatures: Blue Bell, PA 76º Philadelphia, PA 78º High Uncertainty (based on standard deviation): ± 2.3º
Sunday Forecast
Sunny and breezy.
NBM high temperatures: Blue Bell, PA 71º Philadelphia, PA 73º Low Uncertainty (based on standard deviation): ± 0.7º
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.
Here’s the model blend (NBM)—
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.
