Philadelphia Area Weather Forecasts

WEEKEND WEATHER FORECAST

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Sun 8:28 PM —Forecast Review —The snowfall rate did blossom during the late afternoon and early evening. In my neck of the woods, we had 1.8″ this morning through 11 AM and an additional 2.7″ through 8 PM. Interestingly, all of the models underestimated the snow depth accumulation.

The NWS someewhat official snow totals can be found here. The MRMS model records water equivalent through radar estimation and rain gauges. Here’s the MRMS precipitation water equivalent summary—

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

Areas which had all snow over the past day may be able to multiply the water equivalent in inches by 10 for a rough measure of snow depth. Most of us near the city and especially in NJ had a mix for part of the storm and the simple 10:1 doesn’t apply.

For those who didn’t get enough snow, the HGEFS (AI) is showing another storm for next weekend.

Additional Snow Update

Posted Sunday 01/18/26 @ 2:20 PM — The lull in the snow was expected. Expect additional snow to pick up in intensity between 3 PM and 7 PM, tapering off from west to east about 8-9 PM.

Using the latest 17z RRFS data, here’s my calculation of the expected additional snow accumulation from 12 PM through 10 PM—

17z RRFS additional snow accumulation from 12 PM through 10 PM Sunday. I am not using the built-in model algorithms. This is my own calculation which may run a bit high. We’ll see. (Click on image for a larger view.)

For those of us still shoveling snow, temperatures are expected to fall below freezing around between 6 and 7 PM in the city, earlier west of the city.

Snow Update

Posted Sunday 01/18/26 @ 10:27 AM — As of 10 AM, most of the heavier snow has moved off to the northeast.

Radar and MRMS about 10:15 AM with RAP model 700 mb wind streams (green contours) and max Omega (yellow contours). (Click on image for a larger view.)

While there is little upstream from us at this time, the models are forecasting strengthening of the low pressure system later this afternoon off the coast with a widening of its precipitation shield back into our area mid to late afternoon along with wrap-around snow. Heaviest snow about 6 PM. An additional 1.5- 2.5 inches is possible in many areas.

Sunday Forecast Update

Posted Saturday 01/17/26 @ 9:37 PM — The latest models show about 0.20″ water equivalent, falling as snow in our region beginning before daybreak and ending sometime late Sunday afternoon or evening. Here’s the current water vapor image showing the disturbance that will affect us—

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. . (Click on image for a larger view.)

The latest available ECMWF-AI model from this afternoon shows this amount of additional snow—

18z ECMWF-AI snow addition snow accumulation by Sunday night. (Click on image for a larger view.)

The latest HRRR is similar to the above ECMWF-AI

The latest NBM just available (01z) with updated precipitation totals—

Tonight’s 01z NBM mean snow totals. (Click on image for a larger view.)

Be advised that the NBM median and mean totals are different with the median showing less snow. With the median snow less than the mean suggests the median may be the best choice.

01z NBM median snowfall (Click on image for a larger view.)

I guess we’ll see!

Sunday Forecast Update

Posted Saturday 01/17/26 @ 5:45 PM — Right now, the latest models show the developing coastal low to have its westward precipitation shield give us another inch or two of snow on Sunday. Looking back, the ECMWF-AI was the most accurate today. Unfortunately, their servers are overloaded and not providing any data right now. The latest NBM has large differences between its mean and median snowfall forecasts, indicating high uncertainty.

Here’s the latest RRFS just available which captures a middle ground —

Today’s 18z RRFS additional calculated snowfall by 11 PM Sunday night. (Click on image for a larger view.)

Most of the models have the snow ending during the afternoon Sunday, as the arctic front moves through.

More models of interest become available between 7 PM and 10 PM. I’ll try to update again before 10 PM.

Sat 11:35 AM —Forecast Review — I violated my own rule of forecast updates. “Never update the forecast to a newer model run when precipitation is active or imminent.” The newer model hasn’t had enough “spin-up” time to be accurate. (Models recreate the atmosphere and require spin up time to simulate what’s happening, often several hours.)

That said, at least in my neck of the woods, there is some accumulation, similar to the 11z model but not to the 12z model. An area of high vertical motion (“Omega”) has set up over us, accounting for the increase in snow intensity.

Yellow contours are RAP model vertical motion (Omega) The value of 15 over our area is high. (Click on image for a larger view.)

Here are the current temperatures at 11:15 AM—

RTMA temperatures at 11:15 AM Areas at 32º and north will have the most accumulation (Click on image for a larger view.)

Had I stayed with my snow forecast from last night, I’d be looking pretty good right now.

Sunday’s Interesting Forecast Update

Posted Saturday 01/17/26 @ 9:51 AM — Saturday forecast’s precipitation onset timing and location was uncertain and turned out incorrect. Let’s take a stab at Sunday’s forecast. As mentioned throughout this weekend forecast, uncertainty rules, with the AI models having showed more snow for Sunday than the numerical models.

Last night, the numerical models have moved towards the the AI model forecasts with a closer track and more snow in the Philadelphia area. ‘

Here’s the new hybrid AI ensemble model, the HGEFS—

06z HGEFS (AI hybrid) forecast for 1 PM Sunday. Low pressure track further westward with westward precipitation shield over our area. (Click on image for a larger view.)

The latest 13z NBM captures the shift in its mean snow totals by Sunday evening—

01-17-26 13z NBM forecast snow accumulation through Sunday evening. (Includes Saturday) (Click on image for a larger view.)

There’s still uncertainty in the forward speed of the system, which may reduce totals from the above.

Updates later this afternoon.

Saturday Forecast Update

Posted Saturday 01/17/26 @ 9:34 AM — The 12z RRFS has become available (with new upper air data). Snowfall totals are much lower than the previously posted update based on the 11z RRFS.

Current Radar—

Current Radar with MRMS and RAP model parameters. ~ 9:30 AM (Click on image for a larger view.)

12z RRFS Accumulated Snow forecast at 1 PM (Click on image for a larger view.)

Updated Saturday 01/17/26 @ 9:02 AM — The band of snow that had been expected to move through 12-2 PM has set up earlier over our area. (That’s where the uncertainty in this forecast is manifesting itself.) The latest available hourly RRFS has this continuing until about 1-2PM with accumulations in the 1.5” range. (Can we believe it?)

Saturday Forecast Update

Posted Friday 01/16/26 @ 9:52 PM — Tonight’s early models show a slight change. Some snow or mixed precipitation possible around daybreak. The main area of wet snow enters western Chester/Montco/Bucks about 10 AM but doesn’t reach the city until 12-2 PM. So a later start near the city.

Based on the latest RRFS (just available), here’s what I think will be the snow accumulation for Saturday, likely the upper range considering how wet some of the snow will be—

00z RRFS snow accumulation (calculated total). Likely the upper range. The grey regions between 0.5 and 1″ are lower amounts. (Click on image for a larger view.)

Some clarification for Sunday’s forecast: Little additional snow in Pennsylvania. Snow will fall in central and southeastern NJ.

Models guiding this forecast: , NBM, RRFS , ECMWF-AI, HRRR, NAM-NEST

Updated Friday 01/16/26 @ 7:22 PM — A review of additional models suggests that only the current AI models show a small additional snowfall near Philadelphia on Sunday. The regular numerical weather prediction models are leaning towards an eastward track with little westward precipitation shield. It will be interesting to see how it plays out.

Originally Posted Fri 5:46 PM —A cold front will approach and move through Saturday preceded by light snow. For Sunday, a storm developing off the coast will brush our area as it moves northeastward. There is still uncertainty regarding the exact track and more so, the westward size and extent of the precipitation shield.

There is uncertainty with this forecast. The current situation captures the uncertainty, as radar currently shows precipitation aloft to our west not predicted by the models.

Rule number 1 of forecasting- when the initial conditions aren’t captured well by the models, there is uncertainty with the rest of their forecast.

Current at 4:30 PM Friday—

Radar plus MRMS shows precipitation this afternoon not forecast by the models. By the way, the air is too dry for this to reach the ground. (Click on image for a larger view.)
18z GFS forecast for 1 PM Saturday. (Click on image for a larger view.)

Saturday Forecast

Some snow before daybreak and a band of light to moderate snow will move through about 10 AM as a cold front moves through. About 0.20″ of precipitation water equivalent. Despite temperatures being above freezing near the ground, I expect it to fall as snow and wet snow. If near ground temperatures were colder, we’d be talking about 2-3″ of snow. But with melting and compaction, snow totals will be closer to the 1/2″ to 1.5″ range.

Based on review, I’m going with the NBM 75 percentile forecast, higher than the mean and median totals.

NBM 75 Percentile snow.

NBM high temperatures: Blue Bell, PA 40 º Philadelphia, PA 41º
Higher than average uncertainty (based on standard deviation): ± 2.4º

Sunday Forecast

Sunday’s forecast is the most uncertain. I’m leaning towards the AIGFS and the HGEFS (AI) models which have the precipitation shield fairly westward. I’m not prepared to put out exact numbers, but an additional 3″ is possible near the I-95 corridor and more into NJ if the AI models are correct. I’m waiting for Saturday to make a final forecast.

It should be noted that the standard numerical weather models have the Sunday storm mostly missing our area, except for the Jersey shore. The latest RRFS also has minimal additional snow for us on Sunday.

The ECMWF-ENS model shows little uncertainty in the track away from us. The new HGEFS shows more uncertainty—

HGEFS MSL pressure standard deviation. Uncertainty on the west side of the storm. (Click on image for a larger view.)

So it’s a question of the AI verses the regular numerical weather prediction forecast models. Check back on Saturday.

NBM high temperatures: Blue Bell, PA 34º Philadelphia, PA 36º
Higher than Average Uncertainty (based on standard deviation): ±2.4º