Posted Saturday 10/10/26 @ 10:05 AM — The latest models continue with a similar forecast with some changes—
Rain begins in our area as early as 2-4 AM Sunday morning.
There will be several rounds of heavy rain during the event.
Embedded thunderstorms are possible, especially after midnight Sunday into early Monday morning. A low probability of severe storms, mostly at Jersey shore.
The latest NBM shows little to no chance of tornadoes.
Rainfall in the 1″-2″ range.
Wind gusts about 25-35 mph, somewhat higher along the Jersey shore.
Rain continues into early Monday morning (7AM-9 AM). Considerable cloudiness lingers into much of Monday.
12z NBM model forecast total rainfall by Monday morning. (Click on image for a larger view.)
Updates late this afternoon.
Originally Posted Fri 6:21 PM — This weekend’s forecast is focused on the effects of the remnants of Hurricane Isaias, as it has been forecast for several days to loop into our area on Sunday.
Several changes have occurred with this storm and its forecast.
Isaias is now a marginal Category 3 that rapidly decreases to tropical storm intensity and lower within the first 24 hours.
The ECMWF-AI in land track has flattened a bit today, taking the storm more to our south. However, the precipitation shield of the remnants of the storm is well north of the actual track.
Cloudiness will move in sooner than forecast yesterday, now considerable cloudiness on Saturday afternoon.
Some of the severe weather forecasts for our area from the new RRFS have decreased considerably with each model run.
ECMWF-AI forecast track. Select “allow animations” in your browser to view this .gif.
The rain and showers from this storm will likely continue into early Monday morning.
Here’s the latest total rainfall forecast from the RRFS—
18z RRFS total rainfall forecast for our area by Monday morning. These forecasts are a guide. The banding of heavy rain rarely conforms to the model forecast. (Click on image for a larger view.)
Saturday
Sunshine through thin high cirrus clouds in the morning with cloudiness increasing during the afternoon to mostly cloudy. Highs (Model Blend): Blue Bell- 71 º Philadelphia 73º ±1.9º Average standard deviation and uncertainty.
Sunday
Cloudy with rain and showers for much of the day, starting before daybreak. There may be some rumbles of thunder during the afternoon and early evening. Highs (Model Blend): Blue Bell- 65 º Philadelphia 67º ±3.1º Above average sd and uncertainty. Windy and gusty to about 30 mph.
Posted Friday 10/09/26 @ 9:29 AM — The ECMWF-AI forecast track of Hurricane Isaias and its remnants remains unchanged. The ECMWF-AI continues to forecast the rain to start in our area around 2 AM-3 AM Sunday morning. Heavier rain is expected during the afternoon and evening hours. Some showers are forecast to continue into early Monday morning.
Satellite water vapor image with RAP model parameter overlay at 9:15 AM Friday. (Click on image for a larger view.)
The RRFS has backed off from the extreme frontogenic forcing it was showing yesterday, but the latest RRFS has increased helicity values to be in the 1000+ level early evening Sunday. This suggests that low level tornadic activity is possible in our area. (I have not seen forecast helicity values in our area this high since Hurricane Ida.)
06z RRFS forecast Helicity values at 8 PM Sunday. These are very high levels. (Click on image for a larger view.)
Like the extreme frontogenic forcing predicted last night, future model runs may back off from the predicted extreme helicity values shown above.
There is still a wide spread in forecast rainfall, with several models showing the immediate Delaware Valley in a rainfall minimum while others forecast 1-1.5″ of rain.
Updates with my regular “Weekend Weather Forecast” to be posted later today.
Posted Thursday 10/08/26 @ 6:05 PM — No major changes in the model forecasts for our area. Sunny skies (through increasingly thick cirrus clouds) on Saturday will give way rain on Sunday, beginning as early as 2 AM Sunday, but a number of models have the rain starting Sunday around daybreak, 5 AM- 8 AM.
The moisture structure of the storm’s remnants is impossible to determine at this early point in time, but the total rainfall will be in the range of 0.60″ to 1.5″ in the immediate PHL area. Those numbers will likely change.
Of interest… the RRFS shows some very strong dynamics between 5 PM and 8 PM Sunday evening: Helicity values in the 850 range which is very high, and vertical shear values in the 30s, suggesting possible low level tornadic activity and some strong localized wind gusts.
Coinciding with this high helicity is extremely high ‘frontogenic forcing’ that will result in strong upward motion.
18z RRFS forecast 850mb theta-E frontogenic forcing at 7 PM. Very strong upward motion despite low CAPE values. (Click on image for a larger view.)
CAPE values are very low, so I’m not sure how this will come together. But the potential exists for a period of wild weather. I’ll be keeping an eye on this. Stay tuned.
Posted Thursday 10/08/26 @ 8:48 AM — The tropical storm is now Hurricane Isaias and continues to generally follow the forecast track of the ECMWF-AI; other models such as the GFS have joined in with similar forecast tracks.
Here’s the Friday 8 PM forecast, about to make landfall—
06z ECMWF-AI forecast for Friday 8 PM (Click on image for a larger view.)
By Sunday at 2 PM the rain shield is well ahead of the surface low and upper air low—
06z ECMWF-AI forecast for Sunday at 2 PM. The rain shield is well ahead of the surface low (white L) in WV and the upper low (blue L) in Kentucky. (Click on image for a larger view.)
The storm’s moisture is forecast to move into our area on Sunday about 2 AM in the morning as rain; it will be an extratropical low by the time it reaches us late Sunday.
Our weekend will feature sun through high clouds (with a likely lower cloud mix) on Saturday and rain on Sunday.
06z RRFS forecast cloud cover on Saturday. Mostly high thin cirrus but some lower clouds likely to mix in at times. Blue=low clouds, maroon=mid-level clouds, tan = high cirrus clouds (Click on image for a larger view.)
Total rainfall amounts are estimated to be in the 1-2 inch range (1.4″) but that will likely change with new model runs after the storm goes on land.
Tropical Storm Isaias
Posted Wednesday 10/07/26 @ 5:29 PM — This has been a good week to take a break from forecasting, as high pressure has kept delightful weather over us.
As mentioned this past Sunday, several models were hinting at the formation of a tropical system in the Gulf of Mexico. The ECMWF-AI has an excellent track record forecasting tropical systems and has been the most consistent with its forecast of this system.
Here’s the latest ECMWF-AI forecast which takes the tropical system (Tropical Storm Isaias) up to the the Alabama coast Friday night and then has it move its remaining moisture and energy over our area later Sunday. —
12z ECMWF-AI forecast for Tropical storm (or possibly a hurricane) Isaias with the position of the extratropical low Friday through 8 PM Sunday night. Blue is high pressure and Pink is low pressure. (Click on image for a larger view.)
While I have confidence in the ECMWF-AI model, tropical systems and hurricane tracks are always forecast with many models, some specific for tropical systems, with their respective paths laid out as a “speghetti diagrams”—
Despite the large spread of possible tracks, my money is on the ECMWF-AI which brings some substantial rain to our area on Sunday into Sunday night. I’ll be keeping an eye on it.
Posted Monday 10/05/26 @ 8:38 AM — Our generally quiet weather week continues on-track. Of interest is a tropical system (tropical storm or Hurricane) that will develop in the Gulf of Mexico according to a consistent forecast by the ECMWF-AI with other models joining in with a similar forecast yesterday.
ECMWF-AI forecast for Friday at 2 PM. The white arrows show the current track of the storm which will likely be just a moisture-laden extra-tropical low by the time it moves near us. The current track is likely to change. (Click on image for a larger view.)
While too soon to take this literally, the remnants are forecast to reach us around Sunday.
Originally Posted Sun 5:06 PM — This week should feature some very nice fall weather with temperatures on the cool side early in the week. An elongated area of high pressure will be over us for much of this week.
For a change of pace, I’ll use the Canadian Global AI model to illustrate some of this week’s weather with the caption below providing the details—
Today’s 12z Canadian GDPS “with AI spectral nudging” shows all three seasons across the US. Elongated high pressure will provide very nice Autumnal weather. Summer-like heat affecting California and the southwest. (note the infamous orange 576 thickness line indicating the edge of a heat dome. In the Great Lakes area, the weather map shows some other thickness lines we have not seen for a while, specifically the 540 (red) 1000-500 mb thickness line. In winter, the 540 line indicates the approximate dividing line between rain vs. snow. Finally, in the Gulf of Mexico, there are signs of a possible tropical system developing later in the week. (Click on image for a larger view.)
For us, generally fair crisp weather. A likely dry cold front is expected to pass through on Thursday.
Mon 10:45 PM Update — More new model data. Wind gusts may exceed 60mph in Philadelphia and immediate suburbs.
Mon 10:15 PM Update — Tonight’s short range models just becoming available continue with the forecast of a generalized 3-5 inches of rain. Heaviest amounts in the western suburbs. The precipitation maps posted below are still valid. The storm will be approaching sooner and moving faster. The precipitation rate peaks around 11 AM -2 PM with over an inch of rain per hour falling. Wind gusts are higher, especially in NJ.
Tropical Storm/Hurricane Isaias continues to move up the coastline. Very heavy rain continues to be the main threat from this storm. The heaviest rain may fall over a period of just a few hours.
The trend has been for the heaviest rain to fall slightly further west than yesterday’s model runs. Some models are predicting amounts as high as 5-7 inches of rain in some spots! The heaviest rain will fall between 11AM and 3 PM Tuesday.
The peak winds will occur around 1-2 PM. Gusts around 35-50 MPH are forecast by the latest GFS in the PHL area.
GFS wind gusts forecast 1 PM EDT Tuesday (Click on image for a larger view.)
Here are the latest models with their rainfall totals –
GFS model accumulated rain forecast (Click on image for a larger view.)
The latest National Blend of Models (NBM version 4.0 ), often correct for QPF values, has even greater amounts—
NBM Accumulated Rain forecast. (Click on image for a larger view.)
The latest NAM NEST model has the heaviest rain more westward—
NAM NEST Precip forecast (Click on image for a larger view.)
The Canadian Global (GDPS) is also a bit westward with higher amounts—
Canadian (CMC- GDPS) Rain Forecast (Different color coding) (Click on image for a larger view.)
The German ICON model has a similar precip forecast—
German ICON model (Click on image for a larger view.)
So a generalized very heavy rain event is expected
Weather… and Other Things 'Up in the Air'
Jet Streak illustration; The Right entrance region and the Left Entrance region have ascending flow that enhances precipitation.
