Monthly Archives: March 2025

Philadelphia Weather Outlook

THIS WEEK’S WEATHER

#Philadelphia #weather #PAwx

Strong Thunderstorms Possible Late Tonight

Posted Thursday 04/03/25 @ 4:32 PM — The warm front moved north of our area today, Thursday, allowing very mild and humid air to move into our region. The same air mass boundary will return tonight as a cold front.

The air over us is unstable and there is will be enough CAPE, vertical wind shear and helicity to aid in the formation of some strong, possibly severe thunderstorms in the 2 AM -6 AM time frame. Some heavy rain is possible. Here’s the latest HRRR forecast—

18z HRRR simulated radar forecast for 5 AM Friday morning. Some very heavy thunderstorms in red. (Click on image for a larger view.)

Rain will continue through Friday morning, ending northwest to southeast during the early afternoon.

The same boundary will move north again as warm front on Saturday, with showers, thunderstorms possible late morning into Saturday afternoon.

Interesting Forecast

Posted Wednesday 04/02/25 @ 5:14 PM — The upcoming forecast period into the weekend will be interesting. The AI models are forecasting considerable rain in our area, while the regular numerical models are forecasting the axis of rain to be to our north. The current water vapor image illustrates this difference—

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. There are two possible tracks for the expected stream of disturbances moving around the southeastern high pressure ridge. (Click on image for a larger view.)

The latest HRRR has moved towards some rain over our area Thursday associated with a warm front moving through later tonight. Most of the rain will be ending around 12 noon – 1 PM from the city and westward.

HRRR (18z) accumulated rain through 1PM Thursday (Click on image for a larger view.)

There may be breaks of sun mid afternoon, but overall it will mostly cloudy. A weak cold front moves through Thursday evening with more showers into Friday morning. Then there’s uncertainty. The AI model shows plenty of showers for Friday Saturday and Sunday. Will need to see if/how the models are coming together!

Wed 10:24 AM —Forecast Review — I just wanted to post the rain totals from Monday’s storms. Total (much needed) rainfall exceeded the model predictions.

MRMS rain-gauge measured rainfall combined with rain-gauge calibrated and interpolated radar-based summary of ACTUAL RAINFALL RECEIVED Monday March 31 into early Tuesday 4/1 . Color shading is in inches. Line contour numbers are in mm. (25.4 mm= 1 inch)      (Click on image for a larger view.)

For part of this month of April, this blog’s weather updates will be less frequent than usual.

Monday Evening Storm Update

Posted Monday 03/31/25 @ 4:42 PM — No significant change from previous forecast update. Here’s the near-current water vapor/radar—

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. Area 1 shows enhanced upward vertical motion (positive Omega) , indicating intensification to occur. (Click on image for a larger view.)

Here’s the latest HRRR forecast at 8 PM—

HRRR shows storms coalesce into a line with heavier cells at 8 PM (Click on image for a larger view.)
Monday Evening Possible Severe Weather

Posted Monday 03/31/25 @ 9:05 AM — A strong cold front approaches today and is expected to move through the Philadelphia area about 10 PM. Ahead of the front, unstable conditions will allow for the possibility of severe thunderstorms ahead of the front. The most likely timing is between 6 PM and 9 PM with a peak at 7-8 PM according to the latest HRRR.

As for the Phillies opening game at 3 PM, most models keep any storms to the western suburbs, but the latest HRRR does show some possible activity closer to the city at 5 PM—

11z HRRR simulated radar and clouds forecast at 5 PM (Click on image for a larger view.)

As mentioned, there’s the potential for severe weather, especially this evening. The HRRR shows areas of highly elevated severe weather parameters (vertical shear, helicity and upward helicity) approaching the city about 7 PM—

It should be noted that a high shear and helicity forecast at 8 PM shows possible low tornadic potential—

High helicity and vertical shear at 8 PM (1) shows low potential of tornadic activity. (Click on image for a larger view.)

As always, the model placement for the potential severe weather is rarely exactly where it will occur. (It’s only a model.) But the potential and general timing is what’s important.

Stay tuned. I’ll update later this afternoon.

Philadelphia Area Weather Forecasts

WEEKEND WEATHER FORECAST

#Philadelphia #weather #PAwx

URMA vs RTMA Temperatures

Posted Sunday 03/30/25 @ 9:58 AM — The high temperatures graphic I posted yesterday was based on the RTMA, updated every 15 minutes. Usually, there aren’t huge differences in the RTMA and what’s considered the “true final word” on actual conditions, the URMA. The true “final word”, the URMA, is available 6 hours later.

Here’s the URMA for 3 PM yesterday, and it shows the ACTUAL recorded [near high] temperatures. Yesterday, there were large differences in the “official temperature” recorded at the airport (KPHL) appearing in the URMA and the provisional temperatures appearing in the RTMA—

Official FINAL URMA temperatures (contours are 1º increments) recorded at 3 PM. (The 4 PM URMA was similar, since the high temperature actually occurred around 3:30 to 3:45 PM. ) There’s a 2.5º difference! It supports my contention that Blue Bell- Wings Field (KLOM) is more representative of actual temperatures around Philadelphia than Philadelphia Airport (KPHL) located near sea level adjacent to a large cold river. (Click on image for a larger view.)
Sunday Forecast Update

Posted Saturday 03/29/25 @ 8:53 PM — The warm front that moved through last night will return as a back door cold front. An easterly wind flow will result in high temperatures almost 20 degrees colder on Sunday compared to Saturday (highs in the low to mid 60s on Sunday).

Today’s temperatures did reach the 80s. Blue Bell reached 81.9º at Wings Field. Here’s the RTMA temperatures at 4 PM—

RTMA Temperatures at 4 PM Saturday. (Click on image for a larger view.)

As described above Sunday will be much cooler. Temperatures will have a sharp gradient—

ECMWF-AI Temps at 2 PM (Click on image for a larger view.)

There’s also wide range in the forecast cloud cover. The GFS forecasts significant cloudiness for Sunday.

The NAM-NEST forecasts clouds in the early morning, breaking for sunshine. The ECMWF-AI is similar to the optimistic NAM-NEST regarding cloudiness.

Originally Posted Fri @ 5:00 PM — —This weekend’s weather will be tricky to forecast accurately. There have already been some surprises in the short term (e.g. a line of showers in Chester County early this afternoon, not forecast by any model.) Warm fronts this time of year are notorious for difficult predictions. Let’s take a shot at it.

A warm front will move north tonight. Ahead of the front, some significant showers have developed to our far west. Current RAP model data shows areas of enhancement (positive Omega) and an area of negative Omega just the west of Philadelphia —

RADAR at 5:30 PM with superimposed RAP model Omega (yellow contours) and 700 mb (10,000) wind streams. The upward arrows are positive Omega, enhancing rainfall. The downward arrow is negative Omega, which works against rainfall. (Click on image for a larger view.).

Earlier today most of the models had none of this rain reaching us. The latest HRRR does have some showers moving through this evening with the warm front. Here’s the HRRR forecast for 1 AM tonight—

Today’s 18z HRRR simulated radar forecast for 1 AM Saturday morning. (Click on image for a larger view.)

Once the rain moves through, skies clear slowly on Saturday and temperatures rise to summer-like temperatures. Here’s the latest ECMWF-AI Temperature forecast for 2 PM. Actual highs may be greater, since high temperatures are reached at 3:30 PM or so. (The ECMWF-AI only does forecasts every 6 hours; 2 PM is the nearest forecast time.)

ECMWF-AI Temperature Forecast for Saturday at 2PM

Skies should be a mix of clouds and sun on Saturday.

Sunday should be more cloudy, but still with breaks of sun and sun through mid level clouds. An easterly wind flow will bring temperatures down the the low 70s but there’s a high uncertainty with a standard deviation of ± 3º . As for rain, most models have the rain holding off until the evening hours, but this is where it gets tricky. Showers moving in sooner is definitely a possibility.

Check back for updates tomorrow.