#Philadelphia #weather #PAwx #PhillyWx
Friday & The Weekend Outlook
Posted Thursday 07/30/26 @ 8:01 PM —The scattered showers forecast for this evening are moving through and should dissipate later tonight. Friday looks to be mostly sunny, although some periods of cloudiness will develop mid to late afternoon Friday, especially in NJ. Scattered showers are possible again, mostly at the Jersey shore.
For the weekend, Saturday will be mostly sunny through high thin cirrus clouds. Towards the late afternoon, an increase in cloudiness is expected as a warm front moves towards us.
For Sunday. there’s some uncertainty regarding cloud cover and the northern movement of the very warm humid air behind the warm front. Most models keep the rain to our far west (Lancaster and Berks counties) and some have some have a few showers near Cape May county in NJ. It will be noticeably warmer and certainly more humid.
We’ll have to wait another day or so for the Sunday forecast to clarify.
Thursday Weather Update
Posted Thursday 07/30/26 @ 11:50 AM — The upper air low continues to rotate close enough to cause some cloudiness and showers. (Some areas saw late day showers yesterday; many areas did not.) Here’s the current water vapor image—
This morning’s RRFS shows considerable shower coverage this evening about 6 PM. (Some isolated showers may develop earlier this afternoon as well.)
Folks, my posts have been less frequent over recent days. A recent MacOS update decided that my model post-processing scripts (python-based) were malware and both quarantined and inactivated them., Apparently my python certificate had also expired. Diagnosing the issues and repairing them has taken the better part of almost 3 days. During that time, my model downloads were reduced significantly. I’m now back with 4 computers working again.
Wednesday Update
Posted Wednesday 07/29/26 @ 9:56 AM — The system has exited our area, but there is an upper low with cold air aloft in New York State may cause instability showers and thunderstorms to develop later today. The latest RRFS has scattered showers and thunderstorms north of our area during the afternoon and some of this activity may develop in the immediate Philadelphia area around 5 -8 PM—
Here’s the rainfall totals, MRMS-based for Tuesday—
Storm Review
Posted Tuesday 07/28/26 @ 8:35 PM — Today’s heavy rain has begun to enter the Philadelphia area. Very little with today’s weather has gone as forecast yesterday or even this morning. Much of the activity went to our north and west much of the day. Here’s the current radar—
I don’t see much in the way of very severe weather. Nonetheless, the NWS has issued a tornado watch for some of the area. Additionally, much of the immediate Philadelphia area hasn’t received anywhere near the heavy rain that had been forecast.
The back end of the rain is already visible on radar and it should be ending around midnight. As for the forecast for Wednesday, it appears now that there won’t be as much in the way of shower activity as had been forecast. Wednesday may be a decent day weather-wise with just a few scattered storms.
Comparison Table- Severe Weather
Posted Tuesday 07/28/26 @ 11:58 AM — With the sun breaking out through clouds (and smoke) additional energy for storms is available.
| Severity Parameter (HRRR) | Sept 1 , 2021 Tornado Outbreak (example of highly severe) | Recent 4-1-23 Tornadoes (Example of severe) | Recent 08-07-23 18z HRRR Bow Echo | 08-10-23 Today’s 06z HRRR & Impact | Today 07-28-26 HRRR |
| CAPE Joules/kg | 3500-4200 ⚑⚑⚑⚑ | 2100 ⚑⚑ | 2600 ⚑⚑ | 1200 ⚐ ⚐ | 2800 ⚑⚑ |
| Helicity m^2/s^2 | 1350 ⚑⚑⚑⚑⚑ | 655 ⚑⚑ | 726 ⚑⚑ | 560 ⚑ | 583 ⚑ Bucks county |
| Vertical Shear 1/sec | 40-46 ⚑⚑⚑⚑ | 40-45 ⚑⚑⚑ | 30 ⚑ | 32 ⚑⚑ | 21 ⚑ |
| Precipitable Water | 2.7″ ⚑⚑⚑⚑ | 0.83” ↓ | 2.1″ ⚑⚑ | 2.3″ ⚑⚑⚑ | 2.3″ ⚑⚑⚑ Delaware County S Jersey |
| Lifted Index º K | minus 6º ⚑⚑ | minus 9.3º ⚑⚑⚑ | minus 9.9º ⚑⚑⚑⚑ | Minus 4.4º ⚐ | Minus 6.7 ⚑⚑ |
| HRRR Hail inches | 1.9 ⚑⚑ | 1.4 ⚑ | 1.8 ⚑⚑ | 1.6 ⚑ | 1.8″ ⚑⚑ |
| Peak Wind Gusts mph | 40-50 ⚑⚑⚑ | 40-50 ⚑⚑⚑ | 65mph ⚑⚑⚑⚑ | 32mph ⚐ | 36 mph ⚑ |
| Storm Motion Shear Vector Alignment | Aligned ⚑⚑⚑ | Almost aligned ⚑⚑ | Aligned ⚑⚑⚑ | Somewhat aligned ⚐ | Variable ⚐ |
| 250 mb Jet Stream Wind mph | 63 ⚑ | 135 ⚑⚑⚑⚑ | 74 ⚑⚑ | 95-104 ⚑⚑⚑ | 42 ⚐ |
| CWASP (NBM Model) | 70 ⚑⚑⚑⚑ Chester Co | ||||
| SWEAT Index (RRFS) | 302 ⚑⚑ | ||||
| K Index (RDPS) | 40.2 ⚑⚑⚑⚑ Philadelphia |
↓ indicates works against Severity ⇩ Significant, but less impact
Heavy Rain Update
Posted Tuesday 07/28/26 @ 11:32 AM —Many of the 12z models are in. There have been changes in the forecast with a later start and the current appearance of the precipitation on radar.
- The HRRR differs somewhat from the RRFS with the HRRR having most of the rain and storms expected this afternoon storms move north of the immediate Philadelphia area. It delays the heavy rains near the city until about 10 PM. The RRFS has scattered activity near the city late afternoon and evening, again now with the heavy activity mid to late evening about 9 PM – 1 AM Wednesday.
- Severe weather (tornadoes, hail,) is still not expected with these storms but wind gusts and heavy rain still in the picture.
- None of the models capture the current radar at all. This suggests that this is a low confidence forecast in terms of timing.
I’ll be posting the severe weather comparison table over the next hour.
Heavy Rain
Posted Tuesday 07/28/26 @ 9:11 AM — The morning models (12z) haven’t been completed yet. But based on the 06z models (2 AM EDT), here’s the trends I’m seeing.
- Heavy rain will be the primary concern with this system. Precipitable water (PWAT) is ‘only’ in the 2 inch range (last severe system we had had PWATs close to 2.5″. However, the expected longer duration of the system and development of a surface low (and upper air low) is expected to cause strong dynamics any precipitation accumulate and be significant.
- The storms move in between 12 PM and 2 PM. Heaviest activity is between 4 PM and 10 PM .
- Severe parameters are not overly impressive. I don’t expect tornadic activity with this system. Wind gusts may reach 30 mph in S. NJ.
- First look suggests heaviest rain from just west of Philadelphia into NJ.
I’m short on time right now. I’ll update with a severity table and new model data before noon today.
Tuesday Weather
Posted Monday 07/27/26 @ 7:57 PM — We had quite the rain-maker develop this afternoon. In addition to the triggered storms (see below), many other cells developed in-place and moved very little causing significant rainfall and flooding. Here’s the MRMS summary of rainfall through 5 PM; incredible totals considering the rain lasted not much more than 2-3 hours—
For tonight (Monday), some additional widely scattered storms are still possible. The real action begins on Tuesday morning (10-11 AM) where showers and thunderstorms will develop and move in from the northwest. —
Rain continues through Tuesday night and tapers early Wednesday. But an upper low associated with this system lingers and additional showers and storms are expected Wednesday afternoon.
As much as an additional 1-3 inches of rain is expected by Wednesday evening. I’ll update on expected severity parameters tomorrow morning.
A Quick Update
Posted Monday 07/27/26 @ 1:33 PM — As mentioned predicting these popups is an exercise in utility. But I’m trying. Current RRFS shows and area of thermal potential energy (theta-e) and an area of moisture convergence at the same level—
The latest water vapor image with hourly RAP model data shows thunderstorms moving into an area that the RAP model feels is favorable for storm strengthening—
Monday Forecast Update
Posted Monday 07/27/26 @ 10:33 AM —This morning’s 12z HRRR just became available. Today’s weather will have some isolated to scattered showers and thunderstorms. The triggers for today’s weather will be weak upper air disturbances with some help from thermal/dew point gradients. Severity parameters are in the low to moderate range with shear values less than 15 and helicity less than 200.
The triggers for today’s storms are visible on water vapor imagery—
At 5 PM, the RRFS shows scattered storms with the triggers—
That translates into moderate to low level storms where they pop up. That said, expect the NWS to issue some severe thunderstorm alerts.
Attempting to forecast today’s storms location is an exercise in futility, but that doesn’t keep me from trying. Based on this morning’s RRFS and the latest HRRR, storms will first form around 12-3 PM in NJ, along the coastal areas. Storms closer to the immediate Philadelphia area will likely form closer to 4-7 PM.
As shown in the water vapor image, there’s a strong impulse far northwest that will give us heavy storms on Tuesday. Stay tuned.
Originally Posted Sun 8:08 PM —With the weather so perfect over the past few days, I’ve taken a break from forecasting. With the coming week, we have some active weather expected from Monday through part of Wednesday.
Warm humid air is moving northward towards us as a disturbance riding over the heat dome will descend towards us —
With the gradient in temperature moving in, showers and thunderstorms will develop during the afternoon and evening on Monday. The RRFS has most of the activity to our north by late afternoon, but several models are forecasting showers and thunderstorms as early as 2PM in our immediate area.
The ensemble model, the REFS, captures this, perhaps better than the RRFS—
The Canadian GDPS with “AI spectral nudging” captures the forecast for Tuesday into Wednesday. A surface low forms over Pennsylvania with a nearby upper level low. Periods of rain and thunderstorms likely on Tuesday with additional showers developing again Wednesday.
The models have been predicting 1-3 inches of rain from this multi-day system. Exact placement can’t be predicted in advance and the total rainfall may change from the forecast.
