Daily Archives: Sunday, December 21, 2025 6:06 pm

Philadelphia Weather Outlook

THIS WEEK’S WEATHER

#Philadelphia #weather #PAwx #PhillyWx

Another Forecast Update

Posted Friday 12/26/25 @ 4:27 PM — The latest radar and short range models increasingly suggest the snow will be the least important aspect of of this winter event. The earlier HRRR animation (below) along with the NAM-NEST model are consistently showing minimal snowfall. This will be increasingly a sleet, freezing rain and ice event.

Current Radar at 4:20 PM shows very light snow and freezing rain to our west. (Click on image for a larger view.)

The latest NAM-NEST captures my current view of this storm—

18z NAM-NEST shows minimal snow with tonight’s storm.

Since the NBM did so well with our last snowfall, I was inclined to go with it again. However, the NAM-NEST has consistently been predicting very low snowfall, with the storm producing more sleet and freezing rain.

Additionally, the latest NAM-NEST had reduced the total precipitation from 0.40″ water equivalent to closer to 0.20″ water equivalent.

Continuing with the NAM-NEST, here’s the expected precipitation type at 11 PM—

18z NAM-NEST precipitation type at 11 PM (Click on image for a larger view.)
Friday’s Snow Sleet Ice Update

Posted Friday 12/26/25 @ 12:13 PM — The latest HRRR shows a 2-5 PM onset. The HRRR is showing more freezing rain than sleet at the onset.

16z HRRR time sequence 3-5 PM. The way models work, this is not like radar. The actual precipitation shown is the for the previous hour, not the instantaneous hour. Which means that what you see may occur 1 hour earlier. Red is freezing rain Pink is sleet Purple is snow (Click on image for a larger view.)
Snow Sleet Ice Update

Posted Friday 12/26/25 @ 9:21 AM — The latest models continue with the trend from last night, predicting slightly more snow accumulation further south than previous models had shown.

Here’s the current forecast and uncertainties—

  • Snow begins between 3 and 5 PM, moving in from the northwest.
  • Most models show a changeover to sleet in most locations near the city and the immediate surrounding counties.
  • The NAM-NEST consistently forecasts very light accumulations of snow with an easterly wind bringing mild air into the region before midnight. The NAM-NEST shows most areas less than 1/2 inch or less!
  • The REFS (experimental) shows significantly more snow and colder temperatures.
  • Precipitation ends between midnight and 3 AM Saturday

Latest HRRR forecast:

12z HRRR Snow depth parameter (Click on image for a larger view.)

The latest REFS (experimental)—

06z REFS snow accumulatioin forecast (Click on image for a larger view.)

The latest NBM (13z) with updated precipitation data. Median Snowfall—

13Z NBM forecast snow accumulation (median) (Click on image for a larger view.)

To a large extent, the major issue with this storm will be icing and sleet, freezing rain in some areas. Predicting snow totals with compaction, ice on top, etc is just an exercise.

Friday Winter Weather Update

Updated Thursday 12/25/25 @ 11:18 PM — Tonight’s early models show an increase in the southern extent of the snow accumulations. Add about 1 inch to the map posted below. Updates tomorrow morning.

Posted Thursday 12/25/25 @ 8:43 PM — Additional models during the day have the following trends—

  • Total precipitation (total water equivalent) in our area has reduced to the 0.30″ to 0.40″
  • A warmer influx aloft will quickly change the majority of the precipitation to a wintry mix of mostly sleet mixed with snow in the north and freezing rain in the southwest of Chester County.
  • Precipitation begins light, between 3 and 5 PM
  • Precipitation ends for the most part before daybreak, but snow flurries are likely along with clouds on Saturday.

I’m using the latest NBM 00z, just available—

00z NBM PTYPE forecast for Friday at 4 PM (Click on image for a larger view.)

By 8 PM—

00z NBM Ptype forecast for Friday at 8 PM. (Click on image for a larger view.)

By 5 AM, the precipitation has mostly ended, except for flurries. Here’s the NBM median forecast total snow. THE ABOVE DOES NOT INCLUDE AN ADDITIONAL 0.25″+ WATER EQUIV SLEET AND ICE FROM FREEZING RAIN—

00Z NBM Median (not mean) snow totals. (Click on image for a larger view.)

I’m using the median vs mean amounts because it more closely approximates the NAM-NEST and the Canadian RGEM.

Despite the low snow totals, traveling will be treacherous.

Updates tomorrow morning.

Weather Update

Posted Thursday 12/25/25 @ 11:42 AM — Merry Christmas and Happy Holidays!
First let’s look at today’s weather. There was more cloudiness this morning than predicted by the forecasts I heard last night. A disturbance passing to our south caused the cloudiness. Clearing should develop in the next hour or so.

Satellite with superimposed radar and RAP model Omega at 11:30 AM. Clearing on the way. (Click on image for a larger view.)

As for the Winter Weather for later Friday, the models are coming together for more of an sleet/snow storm here in Philadelphia and the immediate suburbs, while areas to our northeast, starting with Trenton and Doylestown may have significant snow accumulation. What’s saving us from a 4-8 inch snowstorm is warm air that is expected to move in about 5000 – 6000 feet above ground level. Ground level temperatures are still expected to remain below freezing.

Below, I’ve posted the latest NBM median snow depth at 1 AM. Don’t be fooled by the low numbers; a significant accumulation of sleet and ice will develop that is not included in the calculated values below.

12-25-25 15z NBM MEDIAN Snow accumulation by 1 AM Saturday. The NBM shows no further accumulation (Click on image for a larger view.)

A rapid changeover from snow to sleet will occur from south to north.—

NBM precipitation type at 10 PM (pink sleet. blue snow) (Click on image for a larger view.)

Precipitation begins 3-5 PM Friday. Ends Saturday before daybreak.

Friday Winter Storm Update

Posted Wednesday 12/24/25 @ 5:34 PM — A review of the new models available reveal the following trends—

  • The NBM mean and median snow accumulation have come closer together. The amount of snow forecast is lower than previously shown here due to a faster changeover to sleet and a mix in Philadelphia and the immediate suburbs.
  • Temperatures in the city and surrounding counties remain below freezing. Dangerous icy conditions are likely.
  • There’s significant differences between the European/Canadian models and the NOAA models regarding temperatures in the upper atmosphere which relate to whether what falls will be rain, freezing rain, snow or sleet.
  • The new GFS AI model and the experimental RRFS are colder and suggest more snow than forecast by the NBM.

Let’s go to some specifics. The GFS shows it’s sleet snow line with along the classic 700-850 mb critical thickness line—

18z GFS forecast for 9 PM Friday. Warm upper atmosphere (red line) and changeover along the 1540 thickness line (a measure of temperature about 6000 feet). (Click on image for a larger view.)

The experimental RRFS shows more snow initially with a colder forecast. I should add that the RRFS is showing a narrow band of precipitation that is unlike any other model. Perhaps that’s why it’s still experimental…or perhaps it knows something the other models don’t —

18z RRFS shows more snow and a critical thickness line that is further south, keeping our area in the snow region. (Click on image for a larger view.)

The NBM does the heavy lifting. Here’s its median forecast (see earlier discussion regarding mean vs. median)—

19z NBM median snowfall forecast. (Click on image for a larger view.)

What’s my current take on all of this? I’m actually leaning towards higher totals than shown above. How much higher to be determined.

Updates tomorrow. Happy Holidays and Merry Christmas!

Friday Winter Weather Update

Posted Wednesday 12/24/25 @ 11:01 AM — As promised, here’s the latest Model Blend (NBM 13z ) with updated precipitation (mean snowfall)—

13z NBM accumulated mean snowfall (Click on image for a larger view.)

As was the case earlier, the mean and the median are very different. This indicates that there’s uncertainty and the statistical distribution of all the models comprising the NBM are skewed and asymmetric.

While our previous snowfall about 10 days ago had a more similar mean and median, making the forecast highly certain, this system’s model’s means and medians are very different. (Likely a handful of high rollers are skewing the mean towards a higher value.)

In this case, the rule of thumb is to lean towards the median. Here’s the median forecast. (Half the models forecast more than this value, half the models forecast less.) It better captures the rain/sleet/snow transition and the ‘warmer’ models in a skewed distribution—

13z NBM median snow accumulation. (Half the models have more than this value, half the models have less.) (Click on image for a larger view.)

I expect the mean and the median come together as we get closer to Friday.

In all cases, the snow starts 3-5 PM Friday afternoon with a [possible] changeover to a mix south of the city. It ends Saturday morning from south to northeast.

I’ll update again later this afternoon.

Friday Winter Weather Update

Posted Wednesday 12/24/25 @ 8:03 AM — The latest model forecasts are less in agreement than yesterday’s models. Here are the trends—

  • The storm will split into two lows, diminishing the total water equivalent falling. Many models have backed off from 0.70+” to 0.30″ water equivalent. That reduces potential snowfall.
  • The temperature profile from the latest HGEFS (AI) is significantly colder, suggesting whatever precipitation falls will fall more as snow.
  • The current Model Blend (NBM) has large differences between the mean snowfall and the median snowfall, suggesting large differences.

Here’s the latest HGEFS (AI) forecast for 1 AM Saturday. The HGEFS does not explicitly predict snow, but PTYPE as snow can be inferred from its other temperature parameters—

06z HGEFS shows two lows, with 32º temperature contours at critical atmospheric levels for snow all below freezing, reducing the chance of a mix north of these lines and suggesting more snow and less ice. (Click on image for a larger view.)

Here’s the latest NBM (Model Blend) with mean snow totals—

07z NBM (Model Blend) with mean snow totals. Note that several models do NOT show the precipitation maximum in NYC. The blue contours are 1″ increments. Additionally, the NBM mean and median predictions are very different, indicating uncertainty. (Click on image for a larger view.)

The next NBM run with updated precipitation data is the 13z run, available later this morning. I expect it to be different from the above. I’ll update by noon. Check back.

More on Friday’s Potential Storm

Posted Tuesday 12/23/25 @ 7:57 PM — The models have good consensus regarding a moderate precipitation event for later Friday afternoon into Friday night. As much as 0.40″ to 0.60″of water will fall, as snow sleet or freezing rain/rain.

An all-snow event would give us 4-7″ of snow, BUT temperatures in the atmosphere between 5000 and 10,000 feet will rise above freezing during the storm, reducing accumulations but making for a potentially high ice condition.

Here’s the latest RRFS model for the precipitation type expected—

Today’s RRFS forecast “PTYPE” at 1 AM Saturday morning. This will likely change as the week progresses, but it doesn’t look good, as ice can be worse than snow. (Click on image for a larger view.)

Stay tuned for updates.

“Interesting” Weather Setting up for the Weekend

Posted Tuesday 12/23/25 @ 4:08 PM — A strong jet stream and subtropical jet stream flow of very moist Pacific air has been affecting areas in California. The energy and moisture are poised to move in for the weekend and will cause the development of several areas of low pressure affecting the Mid-Atlantic region over the coming days.

Water Vapor image at 4 PM with superimposed RAP model pressure (black). 500-1000 mb thickness (yellow) and potential vorticity contours (violet) Dark Blue areas are high level moisture. The white arrows capture the current model track for these areas of energy and moisture. (Click on image for a larger view.)

Over the last several years, the possibility of snow in our area seemed elusive. Not so much right now with the current pattern.

As shown with the water vapor image above, we’re set up for disturbances going over the upper air ridge in the center of the country and then descending upon us in an area that’s an upper air trough. We’ll likely be on the cold side of the storms that develop from this pattern.

Last week, the NWS-NOAA made their AI versions of the GFS and GEFS operational. Of particular interest is a third model now known as the HGEFS, a Hybrid model that combines the new AIGFS and the AIGEFS.

The HGEFS consists of 62 models, more specifically 30 “perturbations” of the GEFS and 30 “perturbations” of the AI (Machine Learning) GEFS plus 2 ‘control models’.

The 30 “perturbation” models of each are creating by complex mathematical vector breeding of the main “control” models. These models statically compensate for unknowns in the data input, errors from the mathematical blending of the grids, and rounding/truncating errors from equations that are infinite series.

All 62 comprise this model group or what’s called an Ensemble model. (That’s the “E” in GEFS). The HGEFS promises to be an advance in longer range forecasting. Details can be obtained here from NOAA.

So enough with the technical stuff. Here’s what the latest HGEFS shows for late Friday into Saturday—

Todays’ 12z HGEFS with 1000-500 mb thicknes (red) 850mb (5000 ft) 32º line (violet) and 925mb ( 2000 ft) 32º line (yellow) . The white line is the mean 32º near ground temperature and the fine white lines show standard deviation of uncertainty in the mean 32º line. Notice that Philadelphia is below freezing at several levels as the storm approaches at 7 PM. (Click on image for a larger view.)

This storm has the potential for more than 4 inches of snow, according to the HGEFS and the regular GFS. Stay tuned.

Early Wet Mix Near the City

Posted Monday 12/22/25 @ 5:54 PM — The latest models continue with a similar forecast as posted yesterday. The immediate Philadelphia area and south will be above freezing when the precipitation starts. However, a layer of cold air about 4000 feet aloft will cause any snow to turn to a mix of wet snow and sleet and rain near the city and the immediate suburbs.

The GFS captures the general picture. The white (32º) line near the surface will be north of the city at 7 AM. However. the magenta line running through Honey Brook (below) shows that north of the line, precipitation will fall as a frozen mix of snow and sleet mixed with rain.

Today’s 18z GFS showing light precipitation has started at 6 AM and accumulated somewhat by 7 AM. The near ground freezing line and critical thickness are shown. (Click on image for a larger view.)

By noon, most of the precipitation has ended having changed to rain in most areas by 9 AM to 10 AM. Here’s the latest NBM with updated snow accumulations—

19z NBM mean snow accumulations. Even the 0.5″ snow line may be overstated. (Click on image for a larger view.)

This forecast is complicated by the following factors:

  • Ground temperatures will not be near freezing
  • Accumulations mostly on grassy surfaces near the city.
  • Total accumulation is light ranging from 0.09″ water equivalent to 0.15″ water equivalent (AI models)
  • Frozen wet mix accumulations are difficult to get correct and are dynamic with packing, compression and melting; once the changeover to light rain occurs, accumulations will significantly reduce.

The snow and snow/sleet mix starts just before daybreak and ends around noontime.

Originally Posted Sun 6:06 PM —High pressure moves off to our south on Monday as a disturbance to our west moves to our north on Tuesday.

GFS forecast for 11 AM Monday. Clouds move in from the west. (Click on image for a larger view.)

Precipitation associated with this disturbance will bring some light snow mixed with rain or sleet before daybreak Tuesday.

Today’s 18z RRFS forecast for 6 AM Tuesday. White line is the 32º line at the surface. North of the magenta (700-850 mb) and/or yellow (850-1000 mb) lines, the atmosphere is cold enough for sleet and/or freezing rain. We will be north of the yellow thickness line, Much of what falls will be melting, but may freeze as it falls through this lowest freezing layer. (Click on image for a larger view.)

Temperatures will be above freezing for much of the immediate PHL area. Total water equivalent is less than 0.15″ and many models show less than 0.07″ water equivalent.

The Model Blend (the NBM) did well with the last snowfall, so I’m leaning on it for this ‘snowfall’, if we can even call it that.

19z NBM forecast snow accumulation (mean accumulation) (Click on image for a larger view.)

The NBM provides percentiles for snow accumulation. The above graphic is the mean calculated on 30-40 models.

Below is the 75 percentile accumulation, meaning 75% of the 30-40 models forecast the amount shown or less. (It does NOT mean that 75% of the models forecast this amount.) Thinking about this, it also means that only 25% of the models forecast this amount or more.

19z NBM 75 percentile snow totals. Likely too high. (Click on image for a larger view.)

While the last snowfall actually was correctly captured by the 75 percentile forecast, this is system is very different. Even the lower ‘mean’ forecast may be too high.

Either way, calling this a ‘snowfall’ in our area is stretching it.