Sun 4:54 PM —Forecast Review —Isolated thunderstorms formed as early as 1:30 PM and and have continued to re-form in pretty much the same areas this afternoon . The HREF forecast just below from this morning did pretty good. Most areas saw sunny skies.
Here’s the radar from 4:30 PM—
Radar composite 4:30 PM (Click on image for a larger view.)
Posted Sunday 07/21/24 @ 9:34 AM— While it’s sunny this morning, many models are showing more clouds with scattered to isolated showers and thundershowers developing early afternoon around the city and additional showers in NJ this afternoon. A line of showers may develop early this evening. It’s not the forecast I’m hearing on the radio this morning.
HREF precip accumulation with standard deviation (Click on image for a larger view.)Water Vapor Image 9:13 AM showing disturbance expected to trigger showers here this afternoon. (Click on image for a larger view.)
Posted Saturday 07/20/24 @ 10:45 PM —Tonight’s models are forecasting scattered showers in the Philadelphia area during the afternoon, as early as 1-2 PM. This is a significant change in forecast.
Sat 4:27 PM —Forecast Review —Today was a little cloudier than forecast. Of more interest is the large area of showers moving into South Jersey, that was only predicted by the ECMWF—
Radar image at 4:30 PM with superimposed RAP model jet level wind streams and Omega. While the showers to our north was predicted by most models, the large area of showers in Maryland was predicted best by today’ ECMWF model. This morning’s GFS showed some of this activity but not the full extent. Even the recent 18zHRRR model doesn’t show these showers!. (Click on image for a larger view.)
Posted Saturday 07/20/24 @ 4:40 PM —Based on the above radar and this morning’s ECMWF capturing it best, I’m going with the ECMWF forecast for Sunday. Sunday will have some clouds in the morning and even some light sprinkles in the Philadelphia area. Sunshine should return by noon, but there’s a chance of a few showers in South Jersey in the late afternoon.
Originally Posted Fri 7:56 PM —A stationary front well to our south will gradually move north as a warm front over the next several days. Moisture aloft, in a upper southwesterly flow, will result in showers to our north on Saturday as a weak wave moves up.
Following the weak wave of low pressure, sunnier skies are expected for Sunday.
Here’s the current setup on Friday evening—
Current satellite water vapor image with superimposed RAP model 500-1000 mb thickness lines (yellow), jet level wind streamlines (orange contour-arrows) potential vorticity (fine violet contours) and superimposed MRMS radar. (Click on image for a larger view.)
Saturday
Mostly sunny in the morning, then periods of clouds and sunshine through high clouds in the afternoon. Showers expected in Lehigh and Berks counties will likely stay to our north, although some models show a chance of a very isolated shower in the Philadelphia area.
NBM high temperatures: Blue Bell, PA 86º Philadelphia, PA 86º uncertainty (based on standard deviation): average ± 1.7º
Sunday
Mostly sunny and warmer. More humid. There’s still a chance of an isolated thunderstorm in the afternoon.
NBM high temperatures: Blue Bell, PA 89º Philadelphia, PA 90º uncertainty (based on standard deviation): ± 1.5º
Posted Friday 07/19/24 @ 9:47 AM — Last night’s models continue with a similar forecast for the weekend— an increase in humidity, temperatures and moisture aloft, a mix of clouds and sunshine. The only change for Saturday– a chance of an isolated shower closer to the city about 2 PM; most areas dry.
For Sunday, a further increase in temperatures and humidity. The ECMWF and RGEM continues with a chance of isolated showers near the city during the afternoon; most areas dry.
Check back later this afternoon for my regular Weekend Weather Forecast.
Posted Thursday 07/18/24 @ 7:26 PM — I’m updating the forecast for Sunday. The latest ECMWF and Canadian RGEM show scattered showers on Sunday while the latest GFS keeps us dry with just high clouds. As I said below, a stalled front presents forecasting challenges. There’s too much uncertainty to hang one’s hat on either forecast Sometimes the best forecast is to indicate that we just don’t know for sure right now.
Posted Thursday 07/18/24 @ 5:32 PM — The front that went through Thursday night is slowly moving southward and drier air will filter in for Friday, eliminating the cloud cover. Sunny and pleasant should capture it for Friday.
The front boundary is expected to become stationary and persist for several days. Here’s the NAEFS depiction for Sunday afternoon—
12z NAEFS forecast for Sunday at 2 PM. I’ve drawn the approximate position of the stationary front. (Click on image for a larger view.)
Stalled frontal boundaries often present a forecasting challenge. Waves can develop along the front and a battle sets up between warm air to the south trying to move north while cooler, drier air tries to move south.
With potential wave development along the front, moisture can move over the boundary, causing clouds and the chance of showers.
This movement of moisture north of the front will cause cloudiness for Saturday. Light showers are predicted far northwestern areas of the forecast area, in Berks and Lehigh counties. Dry conditions expected in the immediate Delaware Valley area. Highs 86º -87º
For Sunday, the high pressure system to our north is expected to push a bit further south, helped by the wave that exits eastward. Sunday should be another very nice day, although somewhat warmer. Highs 89º-90º
As I said stalled fronts can be a challenge. Any unexpected change in the position of the front, or an unexpected development of a wave can ruin a forecast. We’ll see what happens.
BTW, this stalled front is currently expected to drift northward by Monday and next week may be very wet and unsettled. Stay tuned.
Thu 8:19 AM —Forecast Review— Here’s a summary of the rainfall last night. I always find it interesting to see how much it can vary from area to area.
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.)
Wednesday Thunderstorms
Posted Wednesday 07/17/24 @ 3:49 PM — A few strong scattered storms have developed and moved northeastward between 3 and 4 PM. The main activity will reach northwestern Montgomery and Bucks counties between 6:30 PM and 7:30 PM and move through the immediate Philadelphia area between 7:30 PM and 10 PM.
NexRad radar at 3:40 PM (Click on image for a larger view.)
The models are suggesting that weak low pressure will develop over us as the front moves through, enhancing rainfall intensity.
Posted Wednesday 07/17/24 @ 9:39 AM — A strong cold front will move through tonight with showers and some strong thunderstorms developing and moving through ahead of the front.
It appears that this will be a broad line of showers and storms and areas may receive several rounds of storms and rainfall.
Things are a little less clear about the timing. There’s the possibility of scattered storms breaking out well ahead of the main batch, as early as 3-4 PM. The main group of storms moves through between 5:30 and 10 PM.
Gusty winds and periods of heavy rain are likely.
06zHRDPS thunderstorm probability at 5 PM Most activity (but not all) still far northwest at this time. (Click on image for a larger view.)
Wed 9:25 AM —Forecast Review — With the exception of the NAM-NEST and HRDPS, yesterday’s models didn’t quite capture the line of storms that went through last evening.
Usually I use the MRMS for rainfall summaries. Today, I’m using the URMA model precipitation summary. Blue labeled contours are in inches. (Click on image for a larger view.)
Tuesday Evening
Posted Tuesday 07/16/24 @ 7:23 PM — A new line of storms, with a better wind trajectory to reach Philadelphia has formed. We’ll see if they hold together—
~ 7PM Radar with superimposed RAP model vertical velocity pressure (omega -orange contours). There’s downward motion ( denoted by the minus signs) between the storms and our area which may put a damper on their strength. We’ll see. (Click on image for a larger view.)
Posted Tuesday 07/16/24 @ 5:50 PM — The storms are diminishing in intensity and likely will move to our north, if they stay intact.
Posted Tuesday 07/16/24 @ 5:18 PM — Storms have developed pretty much as had been forecast by NAM-NEST and HRDPS, but not with the HRRR, GFS or ECMWF.
While the NAM-NEST does have them moving through Philadelphia around 6:30 PM- 8 PM, the upper air winds may take them just to our north, unless the line extends and builds further southward.
More interesting weather for Wednesday. Check back.
Tuesday Forecast Update
Posted Tuesday 07/16/24 @ 12:56 PM — A quick update. Some of this morning’s models have backed off on the chance of thunderstorms for this evening. Isolated to scattered thunderstorms still possible, north of the city. (The Canadian models are the most aggressive with a continued chance of storms, the ECMWF and GFS the least.)
Posted Tuesday 07/16/24 @ 8:39 AM — Latest Model Blend (NBM) forecast high temperatures for Tuesday—
Today’s 06z NBM high temperatures (actual, not heat index) (Click on image for a larger view.)
As for thunderstorms, the latest HREF has the greatest chance of thunderstorms from the city north and west—
HREF probability of radar > 40 db (Click on image for a larger view.)
Thunderstorms possible (isolated) as early as 4:30 PM. Scattered storms from the city west from 5-9 PM.
NBM (Model Blend) thunderstorm coverage isolated and scattered at 6 PM. (Click on image for a larger view.)
Posted Monday 07/15/24 @ 4:43 PM — A small upper level wave will move through late afternoon Tuesday. With high temperatures and humidity and with instability also high, thunderstorms will be triggered. Currently the time frame is from 7 PM through 11 PM Tuesday evening.
Some of these storms may be severe, especially northwest of the city.
The storms may weaken as they move through Philadelphia, as the wave will already be to our east and upper air “heights” will begin to increase again. Here’s the NAM-NEST model showing the wave, visible in the jet-stream level wind streams —
NAM-NEST model forecast 8 PM Tuesday showing jet stream level (250 mb) winds streams. (Click on image for a larger view.)
Posted Monday 07/15/24 @ 10:50 AM —Monday will be hot, with humidity and temperatures creating an “apparent temperature”, also referred to as a “heat index” or “feels like” temperatures of over 100º in many areas—
Latest Model Blend (NBM) 12z forecast apparent temperature (Click on image for a larger view.)
There’s a chance of isolated thunderstorms northwest of our area.
Originally Posted Sun 8:41 PM —Our current heat wave will continue through Wednesday. High temperatures will be in the mid 90s on Monday and approach or exceed 100º on Tuesday. With some increased cloudiness from an approaching cold front, Wednesday will ‘only’ be in the mid 90s.
A cold front is expected to move through late Wednesday. Showers and thunderstorms will accompany the front—
NAEFS model statistical “mode” version with overlayed GEFS model “bias-corrected” precipitation forecast for Wednesday. A cold front approaches late Wednesday or Wednesday night. (Click on image for a larger view.)
