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.
Posted Friday 03/28/25 @ 8:38 AM — Over the past several days, there have been significant changes in the model forecasts for this weekend. Here’s a brief summary—
The warm front will become stationary far north of our area. This area will be the focus of most of the rain for this weekend. Much of the rain will stay to our north. What had looked to be a wet latter half of Sunday, now appears to be much dryer.
Rain from the system will affect our area on Monday and Monday evening instead of Sunday evening into Monday.
Current water vapor image—
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. The red line north of the red warm front will be the area that receives the most rain with this system. (Click on image for a larger view.)
As for today, Friday, most models still have the showers associated with the advancing warm front to pass us to the northwest. The ECMWF-AI, the HRRR and the NAM-NEST keep a forecast of very light showers Friday evening, anytime between 6 PM and midnight.
I’ll post my regular “Weekend Weather Forecast” late this afternoon with an update.
Thursday and Friday Outlook
Posted Thursday 03/27/25 @ 10:04 AM — I’m finally hearing the words “near 80 degrees” for Saturday in the TV/Radio forecasts. The ECMWF-AI model has been predicting these temperatures since Tuesday.
For today, Thursday, it will be sunny and continued windy and somewhat chilly.
To get to 80 degrees, a warm front will need to pass through our area Friday night.
ECMWF-AI-Single forecast for 2 PM Friday.
Earlier this week, the models only showed showers to our far northwest with this warm frontal passage. The latest models are showing some showers breaking out as early as Friday afternoon ahead of the front.
Here’s the latest HRRR which is forecasting showers and thunderstorms as early as 4 PM Friday afternoon—
Latest HRRR (12z) forecast for 6 PM Friday evening. (Click on image for a larger view.)
Thursday and Weekend Outlook
Posted Wednesday 03/26/25 @ 5:15 PM — Showers formed, mostly to the north of the Philadelphia area today, and radar showed precipitaton over Philadelphia which evaporated before making to the ground. Significant shower activity broke out at the Jersey Shore this afternoon from the upper air disturbances discussed in this morning’s update.
Thursday is still showing cooler temperatures,(53º-55º) but plenty of sunshine. Still somewhat windy and gusty.
A warm front moves through Friday night and the ECMWF-AI-Single and Canadian RGEM models are still showing 80º+ for our area on Saturday.
ECMWF-AI forecast for Saturday at 2 PM. The Canadian RGEM shows even higher temps, although it tends to run warm. (Click on image for a larger view.)
Rainy weather moves in late Sunday afternoon and continues through Monday. the Phillies opener may be affected.
ECMWF-AI Forecast for Sunday at 2 PM. (Click on image for a larger view.)
Wednesday Forecast Update
Posted Wednesday 03/26/25 @ 9:48 AM — Several models (ECMWF, Canadian RGEM, German ICON) are showing some fast moving instability showers moving through here early afternoon. There will be some sunshine in the [late] afternoon, according to the HRRR, but not as much as forecast yesterday. So clouds and some sprinkles is a better bet for late morning and early afternoon.
Current Water Vapor shows showers likely to pass to our north, but positive upward motion just to our west may produce showers here between noon and 1 PM and in NJ—
Current Water Vapor with RADAR and RAP Model low level Omega (upward motion) X denotes centers of upward motion leading to potential showers. (Click on image for a larger view.)
Wednesday through Saturday
Posted Tuesday 03/25/25 @ 6:04 PM — The fast moving low pressure system expected to form this evening will remain to our south. Rain may extend north into South Jersey, but the Philadelphia area will remain dry. Colder weather for Wednesday and Thursday with high temperatures around 50º with sunny skies. A warm up Friday. A big surprise for Saturday- high temperatures may approach 80º!
ECMWF-AI Temperature forecast for 2 PM Saturday! Contours are in 1º increments. (Click on image for a larger view.)
A rainy period develops Sunday into next Monday.
Winds Return for Tuesday
Posted Monday 03/24/25 @ 5:50 PM — Windy conditions, especially Tuesday morning, with gusts 25-30 mph. Highs return to about 57º-58º after a cold morning. Some clouds move in late afternoon.
Mon 5:47 PM —Forecast Review — The rain ended about 4 PM and some sun is breaking out as I write this. Total rainfall, was somewhat higher than forecast in areas east of the city.
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) It appears that there are some radar artifacts in this MRMS summary. (Click on image for a larger view.)
Posted Monday 03/24/25 @ 8:58 AM — Based on radar and the latest HRRR, the rain will be ending about 2 PM-3 PM around Philadelphia and about 5 PM to 6 PM at the Jersey Shore. (The Canadian RGEM has the rain ending about 3-4 PM here with some sun possibly by 6 PM. )
Radar at 8:50 AM with superimposed RAP mode low level Omega (vertical velocity pressure). Some enhancement of the rain this morning around Philadelphia is expected with positive Omega. The entire line is moving to the east with the storms moving to the northeast. (Click on image for a larger view.)
As for that potential ‘experiment’ with the ECMWF-AI model, its forecast for some rain here is now late Tuesday night, with light rain ending for the daytime Wednesday. The GFS has moved closer to the ECMWF-AI with some rain in NJ late Tuesday night.
Originally Posted Sun @ 4:25 PM — —Low pressure and an associated cold front will move through on Monday. Rainfall looks to be in the 0.45″ range, but several models have as much as 0.75″ of rain falling over our area. Rain starts before daybreak Monday and continues through at least 4 PM, when any remaining precipitation will become light and widely scattered.
Overall, this coming week looks to be colder than recent weeks with a persistent dip in the jet stream over the Central and Northeastern US.
The current satellite water vapor image with superimposed RAP model parameters and MRMS radar shows the setup that will be persistent—
Current (Sunday afternoon) 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. A persistent dip in the jet (white arrows) will be with us for most of the week. (Click on image for a larger view.)
Of interest is the energy in the Pacific Northwest diving southward into the upper trough. The latest ECMWF-AI model shows another storm developing Wednesday which lends itself to another “AI model Experiment“.
Basically this AI model shows much more development of this Wednesday low pressure system than the either the GFS or ECMWF—
Wednesday forecast from the latest ECMWF-AI-Single model. Much greater storm development than either the GFS or ECMWF (Click on image for a larger view.)
Yet another system associated with a warm front approaches Friday or Saturday. Temperatures look to moderate by next weekend.
