Mon 10:00 AM —Forecast Review — Last evening’s late posting showed major differences in forecasting between the HRRR & NAM-NEST model forecasts and the new, soon to be released (I hope), RRFS experimental model. (RRFS– Rapid Refresh Forecast System, an hourly updated model with the 00z06z12z and 18z runs forecasting out to 84 hours.)
The RRFS, at least in last night’s situation, demonstrated a superior forecast to the existing older models.
No specific release date has been made for the RRFS and its ensemble version, the REFS, but early 2026 is a good bet.
The NWS has already solicited comments regarding planned discontinuation of several older models including the SREF and the HRESW models. Likely to be included in the discontinued list will be the NAM.
The RRFS and the REFS have clearly been developed to replace the functionality of the older models. I’ve been using the RRFS experimental version as my principal model over many months now and the RRFS appears to be, overall, an improvement in forecasting.
Showers early Monday morning?
Posted Sunday 08/24/25 @ 11:20 PM — An interesting setup for early Monday morning. The 00z NAM-NEST and 00z HRRR show a broken line of showers and possibly thunderstorms moving through about 5AM -6 AM. The experimental 00z RRFS shows almost nothing. It will be interesting to see what happens.
Sunday Update
Posted Saturday 08/23/25 @ 9:13 PM — Sunday looks to be dry. All models show the line of showers that moves to our west will fall apart. A few sprinkles are possible from this same system before daybreak Monday.
Also on Sunday, a weak system in the western Atlantic will bring some clouds to NJ, both in the morning and again the mid afternoon Sunday. The Philadelphia area will see a very thick layer of cirrus clouds, but dimmed sunshine should be able to pass through.
18z RRFS model forecast for 3 PM Sunday. Thick layer of high clouds (tan) around the city. Dimmed sunshine. More cloudiness at the shore (mid level clouds- maroon) at the shore. (Click on image for a larger view.)
The coming week looks dry. No systems expected to bring any rain. So we’re in another dry spell.
Posted Saturday 08/23/25 @ 9:58 AM —The latest model guidance shows it being a bit more windy today (Saturday) than previously forecast. Otherwise, beautiful.
Here’s the wind meteogram for Blue Bell, PA—
06z RRFS forecast wind speed/gusts for grid point Blue Bell, PA Saturday through Monday. Gusty winds towards evening from the south, then southeast. (Click on image for a larger view.)
Originally Posted Fri 9:56 PM —The beautiful weather will continue through early Sunday. High pressure maintains its hold through Saturday, but a southeasterly flow from a newly formed weak coastal low and a front to our northwest that loses its upper air support will bring a bit more humidity and some cloudiness for Sunday afternoon and evening. (The shore stays sunny, according to the RRFS.)
18z RRFS clouds/precip at 3 PM Sunday. Approaching front falls apart due to wide, diffuse and split jet streak aloft. (Click on image for a larger view.)
Showers originally expected for Sunday afternoon will likely fall apart as they approach Philadelphia due to a diffuse, broken jet streak.
Saturday Forecast
Sunny and quite nice. High 84° Blue Bell, 83°Philadelphia (not a typo)
Sunday Forecast
Somewhat increased humidity, but dew points still in the low 60s. High thin clouds in the morning lower to mid level clouds in the afternoon. No showers expected until later in the evening. Most areas may receive little or no rain; any rain will be light.
NBM high temperatures: Blue Bell, PA 84º Philadelphia, PA 83º Uncertainty (based on standard deviation): ±1.4º
Posted Thursday 06/12/25 @ 5:49 PM — A cold front will slowly slip through tonight into Friday morning. Most models are keeping the rain to near zero in the immediate Philadelphia area on Friday.
The REFS captures this well—
Today’s 12z REFS total rainfall through Friday evening. The rain stays to our north and south. (Click on image for a larger view.
Scattered light rain with periods of just clouds for Saturday as waves of low pressure move along this stalled frontal boundary. Sunday just looks to be cloudy with little in the way of showers. The timing of these waves will likely change. Stay tuned.
Thursday and Weekend Outlook
Posted Thursday 06/12/25 @ 9:11 AM — High temperatures for today will be near 90º in much of the area. Dew points near 65º will make the heat index (apparent temperature) about 1º higher than shown below.
NBM high temperatures expected 3-4 PM Thursday. (Click on image for a larger view.)
A cold front will slowly sink south of our area on Friday and will then become stationary. Some showers are expected from Allentown and northward with this front early Friday morning. While it will be cloudy here in our immediate area, no showers are current forecast here on Friday. (That may change.)
Some thinning of the clouds later Friday afternoon with some sun possible.
Weekend: A cooler easterly wind flow will develop behind this cold front with plenty of clouds.
Waves of low pressure will form along this front during the weekend. Periods of showers are expected, but timing these showers will be difficult.
The GFS has been consistently maintaining relatively low rainfall around the immediately Philadelphia area with much of the showers to our far northwest and to our south over the weekend. Total rainfall for the weekend only about 0.30″ (NBM) and 0.05″ (GFS).
06z GFS total rainfall forecast through 8 AM Monday. (Click on image for a larger view.)
But, it’s really too early to identify timing and exact locations of the heavier showers with any degree of certainty.
I’ll update later today.
Tuesday
Tue 7:09 PM —Forecast Review —The experiment with the short wave prediction from the experimental RRFS didn’t wasn’t a success. As late as 2 PM, the RRFS hourly was still showing showers to develop over Philadelphia from a short wave.
Both the 17z and 18z RRFS was showing significant dynamics (upward vertical velocity) at 5 PM, although the 18z had backed off a bit. Nonetheless, the forecast for rain didn’t verify. (Click on image for a larger view.)
At about the same time, the radar showed something trying to get going, but ultimately the forecast was a bust.
Radar at 5 PM showing something trying to develop in Philadelphia, but nothing really happened.. (Click on image for a larger view.)
These sorts of meteorological ‘experiments’ I thoroughly enjoy, even when the outcome is a bust. There’s always another day’s weather.
I should acknowledge that the RRFS is known to over-play this sort of convective rain forecast. Indeed, it’s one of the things they’ve been tinkering with to get the RRFS model ready for prime time release. The model is still considered experimental.
If and when they do release the model as operational, it will be likely known as RRFS-a. RRFS-b is already being explored, probably with a changed core from the current FV3 to a MPAS core. See my RRFS page for more info.
Posted Tuesday 06/10/25 @ 12:23 PM — Last night’s REFS correctly predicted the heavy rain that sprung up this morning over Philadelphia and northern sections. Last night’s (00z) HRRR really missed this forecast. I’m finding the experimental REFS increasingly useful and skillful.
Sticking with the REFS and the RRFS, the last two runs continue to show a break in the rain, but another short wave is expected to move through between 3 and 5 PM this afternoon with another round of showers, possibly some low level thundershowers. Let’s see if this happens.
REFS forecast at 4:59 PM shows short wave at 700 mb. Shading is relative humidity at 700 mb. (Click on image for a larger view.)
Tonight and Tuesday
Posted Monday 06/09/25 @ 6:03 PM — It was reasonably dry around here after 11 AM.
For the Phillies game this evening, it appears that the scattered showers originally expected this evening likely won’t materialize.
The rain will wait until after midnight to move in.
Rain ends Tuesday morning, by 10-11 AM in the city. About 0.35-0.40 inches of rain expected later tonight into Tuesday morning.
While the rain is expected to end Tuesdaymorning, several models show a ‘short wave’ moving through Tuesdayafternoon about 3-6 PM with showers and possibly some thunder. Wednesday looks good!
RRFS forecast for 4 PM. 500 mb height contours (blue) show the undulation due to the short wave moving through. (Click on image for a larger view.)
Monday Forecast Update
Posted Monday 06/09/25 @ 9:22 AM — The latest ECMWF continues with light scattered showers through the day.
However, the latest RRFS, REFS and Canadian HRDPS forecasts no rain after 11 AM through the afternoon and based on current radar, I’m leaning in that direction. So low clouds with some breaks possible mid to late afternoon in western suburbs. Some widely scattered showers are possible as early as 6 PM. Light to moderate rain moves in tonight.
Latest Radar (~9 AM) shows exiting showers. Just an area to watch is is moderately strong upward 300mb vertical motion area indicated by (?) (Click on image for a larger view.)
Originally Posted Sun 4:49 PM —Some of the uncertainty with this weekend’s forecasts will spill into Monday.
The showers that did move in this afternoon (forecast best by the European AI-Single model as well as the regular ECMWF, and disappointingly forecast by the HRRR, RRFS, REFS, GFS and NAM-NEST) will continue to develop this evening and into Monday morning.
Radar GIF 3 PM Sunday More rain than forecast by several models. (Click on image for a larger view.)
The models are suggesting that the showers/rain taper late morning Monday with most (but not all) areas dry Monday afternoon. Rain redevelops Monday evening and night.
Here’s the latest ECMWF forecast for Monday afternoon, which shows the next element to affect our weather– another upper level low with a flow of cooler air.
ECMWF forecast for Monday at 4 PM. Upper level low in Canada, a dip in the jet stream with cooler air and the beginnings of low pressure expected to develop over the Ohio Vallye moving towards us for Tuesday. Rain expected Tuesday, possibly moderate rain. (Click on image for a larger view.)
Moderate rain on Tuesday, then things improve. Unfortunately, there’s a hint that we’ll see rain part of next weekend.
Sun 5:22 PM —Forecast Review —There’s a lot more activity after 5 PM today than predicted by my ” RRFS forecast experiment” —
Radar at 5:20 PM From https://weathertap.com (Click on image for a larger view.)
We’ll see if the next main area of storms comes in between 8 and 10PM.
Watching the Phillies – Watching the Weather
Posted Sun, 08/18/24 @ 3:13 PM— It’s the bottom of the 5th inning and the storms have formed in generally the same location as predicted by the RRFS experiment. Here’s the current radar—
Current Composite Radar with RAP model 850 Omega (yellow contours) and 700 mb wind streams. Remarkably similar to the RRFS forecast. (Click on image for larger view.)
The storms are moving towards the stadium and should be there within the hour, although they may sideswipe that part of the city.
Forecast Experiment
Posted Sunday 08/18/24 @ 11:43 AM — I’m going to do a forecast experiment based exclusively on the experimental RRFS.
This morning’s 12z experimental RRFS just became available (forecast through 19 hours). It has little activity until 3:30-5 PM. Then storms develop and move in from the southwest in just a few spots—
RRFS 5 PM 1 hour accumulated forecast (Click on image for a larger view.)
By 8-10 PM the main area of storms move in from the west—
12z RRFS forecast for 10 PM 1 hour accumulated rain (Click on image for a larger view.)
Interesting Weather Day – Sunday
Posted Sunday 08/18/24 @ 10:30 AM — An upper trough with increasing jet stream wind flow and a favorable position for jet streak lift (“right entrance region”) along with high CAPE values (>2000 joules/kg) and moderately high PWAT (1.8″ water) will create conditions for showers and thunderstorms this afternoon with heavy rainfall.
While a scattered shower and thunderstorm is possible late morning and early afternoon, things begin to get going between 3 and 6 PM. Many of these storms will develop in-place instead of the usual line of moving storms. Low shear values and low helicity suggests severe storms or tornadoes notlikely, although still a low possibility extreme South Jersey and Delmarva area.
It’s unclear how this might affect today’s afternoon Phillies game; they may be able to finish the game with current timing of things.
The more sun we get before 3 PM, the more intense the thunderstorms will be.
Here’s the current setup—
Current conditions Sunday morning. Water Vapor with RAP model 700 mb windstreams and 700 heights. Superimposed composite Radar. (Click on image for a larger view.)
Total Rainfall predicted by 8 AM Monday —
12z NAM-NEST(Click on image for a larger view.)12z, HRRR (Click on image for a larger view.)06z experimental RRFS (Click on image for a larger view.)
Which of the above are correct? None of them. Model predicted heavy rainfall is simply a guide of what rainfall potential is in an area.
Here’s the rainfall totals estimate we’ve received so far this weekend—
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.)
I’ll be keeping an eye on things for any changes. Stay tuned.
Posted Saturday 08/17/24 @ 10:36 AM — Those inconsistent model forecasts I spoke of yesterday are coming home to roost. The showers have moved in mid morning, per my original forecast, before I updated it last night to a later onset of the showers. Here’s the current radar/water vapor—
Water Vapor/Composite Radar with superimposed RAP model vertical velocity pressure (Omega- yellow contours) and horizontal moisture convergence (blue contours) and 700 mb wind streamlines. These showers are moving up from the southwest. (Click on image for a larger view.)
Yesterday’s 12z models (at least some of them) accurately captured this, but later model runs did not. Of interest is the experimental 12z RRFS model which was superb with the forecast, but like the other models, it backed away with later model runs. Here’s yesterday’s 12z RRFS—
08-16-24 12z RRFS forecast was incredibly good with this forecast. (Click on image for a larger view.)
For weather nerds out there, I’ve been keeping a separate web page with changes and observations about the RRFS model. Just two days ago, they changed the physics package in the model for predicting precipitation.
It appears that additional showers are likely today, with some breaks in the action.
Update Fri 8/16 8:32 PM — A quick update. I’ve reviewed some additional models that have become available since posting this. Instead of mid morning, showers will move in between 11 AM and 2 PM Saturday. The Jersey shore may not see any rain during the daytime Saturday.
Originally Posted Fri 4:48 PM —While Ernesto is predicted to be about 600 miles east of the Jersey Shore this weekend, the presence of a tropical system often seems to create inconsistent model forecasts for areas seemingly unaffected directly by the storm.
Such is the case this weekend, with Saturday‘s forecast already having changed from yesterday.
An approaching warm front had not been expected to affect the immediate PHL area with showers until the evening. That’s no longer the case. Today’s models have an area of showers and possibly thundershowers well ahead of the warm front moving in during the morning hours Saturday.
Here’s the current (Friday) position of the warm front —
Friday afternoon RAP model 500 mb wind streams with superimposed MRMS radar. I’ve drawn the warm front. Showers ahead of this f ront will move in mid morning Saturday. (Click on image for a larger view.)
On Sunday, the low pressure system in the Great Lakes area will slowly move east, as weak disturbances develop and move over us from the south on Sunday.
GFS 700mb level (10,000 feet) forecast for Sunday 2 PM. Waves of low pressure will cause showers and thunderstorms to develop throughout the day Sunday into Sunday night. (Click on image for a larger view.)
Expect further changes in this weekend’s forecast. Check back for updates.
Saturday Forecast
Cloudy with showers, maybe a rumble of thunder mid morning. Then a mix of clouds, bright spots, even some sun, with scattered showers.
NBM high temperatures: Blue Bell 83º Philadelphia 84º Uncertainty (based on standard deviation): much higher than average ± 3.5º
Originally Posted Fri @ 4:48 PM — —
Sunday Forecast
Cloudy with showers and thundershowers/thunderstorms throughout the day. An occasional bright spot and even some sun can’t be ruled out. Chances of a heavy thunderstorm increases late in the afternoon and evening. Very humid with dew points in the 70s.
NBM high temperatures: Blue Bell 81º Philadelphia 83º Uncertainty (based on standard deviation): above average ± 2.5º
