Posted Friday 01/16/26 @ 9:47 AM —Currently, there is a cold front approaching from the Great Lakes area while an area of vorticity near Florida is expected to spawn a coastal low approaching to our southeast on Sunday.
As is often the case, the models vary in the track of the coastal low with a potential of [light] snow Sunday.
Current Radar with RAP model parameter overlay. Fine violet contours are potential vorticity. (Click on image for a larger view.)
First, on Saturday, we will likely see snow ahead of a cold front. Light intermittent snow begins in the morning and tapers off in the mid afternoon. Despite the forecast for temperatures to be above freezing, the precipitation will fall as snow or wet snow except in eastern NJ. Here’s the RRFS calculated snow accumulation—
06z RRFS calculated Snow accumulaton by 4 PM Saturday. (Click on image for a larger view.)
On Sunday, low pressure is expected to be spawned to our southeast and low pressure will track to the northeast. The current AI models show our area receiving snow, while the ECMWF, RRFS and to some extent, the latest GFS has the snow only affecting a small section of coastal NJ. On the otherhand, we have the AI models which all show some part of the Philadelphia area closest to I-95 corridor receiving some light snow, perhaps as much as an inch.
Here’s the latest 13z NBM with the mean snowfall for both Saturday and Sunday combined—
13z NBM shows some additional snow around Philadelphia on Sunday with most from Saturday. The accumulations near the shore are from Sunday’s coastal storm.(Click on image for a larger view.)
Updates later today with my weekly “Weekend Weather Forecast”.
Posted Thursday 01/15/26 @ 4:44 PM — A deep trough in the upper flow has resulted in an area very strong potential vorticity where the jet stream rounds the base of the trough—
Water Vapor Channel 9 -Thursday afternoon with superimposed RAP model parameters. Fine violet contours indicate potential vorticity. The base of the trough is outlined by the white square box. Areas of low pressure will develop in this area and move northeastward. The first on Saturday brings light snow. The second on Sunday brushes us with some more light snow. (Click on image for a larger view.)
As is seemingly always the case, there are differences in the model forecasts.
With the exception of the ECMWF, most models point towards light snow on Saturdaymorning. Temperatures will be at or just above freezing, so accumulations with melting will be difficult to pin down, but a coating to an inch is possible, especially areas north of the city. This will be a wet snow near the city and accumulations will likely be minimal.
18z GFS (just available) shows an area of precipitation moving through at 10 AM Note the position of the three “critical thickness” lines (red yellow, violet) which are east and south of our area, indicating what falls will be snow, even if the near ground temperatures are at or above freezing. (Click on image for a larger view.)
On Sunday, a more significant disturbance develops and moves northeast. Most models have this moving to our east, but the AI models consistently show the Philadelphia area gets brushed with light snow.
ECMWF-AI Forecast Sunday at 1 PM. Precipitation shield just makes it into Philadelphia and especially NJ. Notice the position of the three “critical thickness” lines (red yellow, violet) which are east and south of our area, indicating what falls will be snow, even if the near ground temperatures are at or above freezing. (Click on image for a larger view.)
Again, snowfall should be light with accumulations difficult to predict at this time. Stay tuned for updates.
FYI- The latest Canadian models show a heavier mix on Sunday.
Two Chances for Snow this Weekend
Posted Wednesday 01/14/26 @ 5:37 PM — The approaching cold front this evening is somewhat moisture-starved. Combined with warm temperatures, no snow is expected over night into Thursday.
The upper level deep trough will be a breeding ground for additional disturbances to form and to track up towards us this weekend. There’s considerable uncertainty regarding the tracks of these disturbances.
Least impressive is a disturbance that may bring light snow Saturday early afternoon. The Canadian GDPS with “AI Spectral Nudging” along with the latest RRFS are forecasting the possibility of light snow—
Today’s Canadian GDPS with “AI Spectral Nudging” forecast for 1 PM Saturday. (Click on image for a larger view.)
More impressive, but with higher track uncertainty is a low pressure system expected to form and move up towards us on Sunday—
Today’s 18z AIGFS forecast for 1 PM Sunday. It should be noted that the greatest uncertainty is a more westward track. (Click on image for a larger view.)
This Sunday storm is forecast by the AIGFS, the HGEFS and the GFS, but in all cases, we’re on the western fringe of the precipitation shield. The ECMWF and the Canadian GDPS-AI keep the storm further east, mostly missing our area. The greatest uncertainty shown by the ensemble models is west of the mean low.
We’ve already seen that the forecast for tonight’s snow has fallen apart. We’ll see how things evolve in the coming days for the next potential possibilities of winter weather. Stay tuned.
Wednesday Thursday Update
Posted Wednesday 01/14/26 @ 11:02 AM — A quick update. The models have backed off on the precipitation considerably in our area. Very little precipitation falling as light scattered rain showers is expected about 1-3 AM Thursday morning. No accumulations expected in the Philadelphia area with ground temperatures above freezing. Winds pick up before daybreak tomorrow. Quite gusty.
Thursday’s Non-Event
Posted Tuesday 01/13/26 @ 8:38 PM — Today’s models continue the trend for a ‘non-event’ on Thursday. While there is still some uncertainty in the east-west position of the low pressure development, all models have it occurring north of our area, keeping precipitation amounts to less than 0.18″ water equivalent.
Much of the precipitation will occur as light rain ahead of a cold front Wednesdaynight. A changeover to some scattered rain-snow mix and to snow showers will occur early Thursday morning, about 5 AM, and will end about 9 AM. With ground temperatures above freezing, little to no accumulation is expected with less than 0.25″ snow falling. Winds increase Thursday morning with the front.
Here’s the Model Blend (NBM) mean snowfall forecast, with most areas getting possibly a wet coating at best—
00z NBM mean snowfall forecast by Thursday morning. Maybe a scattered coating, maybe nothing. (Click on image for a larger view.)
Moving back to tonight, there’s a chance of some widely scattered light sprinkles or rain showers tonight (Tuesday). Wednesday is looking cloudy with light showers in far western sections (Lancaster/Berks counties).
Thursday looks to be windy and cold.
Thursday’s Potential “Storm” Downgrade
Posted Monday 01/12/26 @ 5:28 PM — Today’s models have backed off from low pressure development and the latest model trends explain the high uncertainty seen with yesterday’s ensembles.
The system that will affect us Wednesday night into Thursday morning appears to now mainly to be associated with strong cold front with a rain-snow mix changing to snow before ending. (Low pressure that had been expected to develop near us Thursday appears to happen north of our area according to the latest Canadian and US models. The latest ECMWF available still has weak low pressure development, but there’s a model multi-hour time lag for their free “open data”. Their afternoon data won’t be available until 7:45 PM.)
Here’s the latest GFS model—
Today’s 18z GFS shows the deep dip in the jet stream and a poorly defined Low pressure system far north of our area. (The blueL 541 is the upper level low, which always hangs back from the surface low.) (Click on image for a larger view.)
The latest Model Blend (NBM) still shows low to minor snow accumulations, similar to those I posted this morning. Even with rain changing to snow, actual accumulations will be challenging with melting on warmer ground and with mixing.
Backing up a bit, the clouds that affected parts of our area today (Monday) were associated with warmer moisture moving aloft, sort of a pseudo warm front. Tuesday will be milder than Monday with temperatures from 48º to 50º.
There may be some light showers around midnight Tuesday night into Wednesday. Then mostly cloudy.
Thursday’s Potential Snow looking to be Minor
Posted Monday 01/12/26 @ 9:07 AM —Thursday’s storm continues to be uncertain but is trending towards low to minor accumulations.
The latest HGEFS (Hybrid AI) ensemble model shows the storm to develop somewhat north and east of our area, causing us to miss any potentially heavy snow.
At the same time, there continues to be areas of uncertainty based on the forecast spread of the 62 model variations that make up the HGEFS—
Today’s 06z HGEFS AI model. L is the mean position of the surface low, based on 62 model variations. The ? indicated areas of high spread (uncertainty). (Click on image for a larger view.)
The areas of uncertainty away from the mean position indicate possible areas of additional low pressure development, unknowns in speed and depth of development, unknowns in track.
The latest NBM (12z) just available shows this for the current mean snowfall accumulation by Friday morning. (The NBM (Model Blend) is based on a statistical treatment and weighting of about 40 models based on their performance predicting the weather 6 hours earlier.) —
12z NBM mean snow accumulation by Friday morning. (Click on image for a larger view.)
My impression is that you won’t need the snow blower for this snowfall. Yet there’s plenty of time for the forecast to become more certain. Stay tuned.
Possible Storm Thursday
Originally Posted Sun 9:13 PM —High pressure has built in on Sunday with the return of cold temperatures, as the jet flow is in a trough configuration over our region.
This dip will be short-lived, as it is expected to flatten on Tuesday with relatively milder air returning for Tuesday and Wednesday.
HGEFS (AI) model shows a flattening of the jet stream, as reflected in the RED 500-1000mb thickness contour. The blue arrow points to the next reload of cold air that will push the jet flow into a significant trough by Thursday.
By Thursday, cold air has reloaded from Canada and the jet flow is pushed into a very deep trough—
HGEFS (AI hybrid ensemble model) shows a deep trough with low pressure directly over us by Thursday morning. Notice that the white and yellow contours are north of our area, indicating temperatures near ground to about 2000 ft are above freezing to the south of these lines. (Click on image for a larger view.)
There’s much uncertainty with the evolution, position, path and expected intensity of this low at this time. This is reflected by the latest ECMWF- Ensemble model showing standard deviation (uncertainty) —
12z ECMWF-ENS surface pressure with standard deviation (Click on image for a larger view.)
This amount of uncertainty is not unusual in this time frame, but since the high standard deviation is not where the mean low pressure location is, it suggests that there are issues in modeling the position and evolution of this storm.
At the current time, this does NOT look to be a significant storm. Light rain, wet snow and graupel Wednesday night changing to light snow during the day is my current bet. 1/2 – 2 inches possible. Again much uncertainty. Stay tuned for updates.
Posted Friday 01/09/26 @ 10:06 AM — Just as there was a dramatic change in the with Saturday’s high temperature forecast, a comparison of this morning’s models with the real time RTMA data suggests that the models are having some issues modeling today’s rain moving in.
Current (10AM) radar and Clear Channel (14) satellite image. The radar echos are closer than forecast by several models. (Click on image for a larger view.)
The actual observations (the RTMA) suggests that high pressure is blocking advance of the rain, while the advanced location of the line of showers west of our area suggests something quite different. The only thing I can say— there’s uncertainty with the amount of rain and its onset this afternoon.
It should be noted that the latest ECMWF is forecasting more rainfall this afternoon than was being forecast yesterday. Still going with 3-5 PM onset This continues to be an interesting system. Stay tuned.
Models guiding this forecast: HRRR, RTMA RRFS ECMWF
One More Thing
Posted Thursday 01/08/26 @ 7:54 PM — For days now, I (and everyone else) has been talking about temperatures reaching and exceeding 60º on Saturday. The forecast has changed significantly over the past 18-24 hours and the NBM (which is my go-to model for temperatures) has been slow to capture the trend.
The latest models from this afternoon (18z) are forecasting highs only in the mid 40s for Saturday due to an easterly flow instead of a previously forecast warmer southerly flow. The actual high temperatures will occur before noon on Saturday, dropping during the afternoon.
18z RRFS temperatures at 2 PM Fine black lines are 1º increments (Click on image for a larger view.)
Models guiding this forecast: GFS, NBM, RRFS , REFS, ECMWF-AI,
Rain moves in Late Afternoon Friday
Posted Thursday 01/08/26 @ 5:38 PM — The models have trended towards minimal/no rain before 4 PM for Philadelphia and its immediate western suburbs on Friday, somewhat earlier far western sections. Much of the day will cloudy, but a few bright intervals during the day. The Model Blend (NBM) is showing very light rain after 4 PM with totals less than 0.04″ but greater in western sections. Here’s the REFS forecast accumulation—
12z REFS forecast accumulated rainfall by10PM (Click on image for a larger view.)
Considerable rain for Saturday for most of the day. Clearing and sunny skies for Sunday.
Thursday through Sunday Outlook
Posted Wednesday 01/07/26 @ 11:30 AM — The forecast for above average temperatures through Saturday has been well-advertised. Here’s the NBM high temperature meteogram for Philadelphia—
12z NBM forecast Temperatures/Dew Points for grid point Philadelphia Center City. (Click on image for a larger view.)
Also as forecast, rain will move in on Friday, either early afternoon or mid to late afternoon.
There are differences between the models regarding the speed and development of this Friday system: The European and Canadian models are faster with rain moving in on Friday afternoon and much faster to move it out on Sunday.
The comparison below of the AIGFS and the ECMW-AI forecasts at 1 PM Sunday show significant differences:
Comparison of the rain coverage forecast for the six hours preceding 1 PM Sunday. AIGFS (slower) ECMWF-AI (faster). The Canadian GDPS-AI is somewhere in the middle of the two above.
If the AIGFS (and Canadian GDPS-AI) is correct, the Eagles will have to contend with some rain on Sunday. If the ECMWF-AI is correct, it will be mostly dry during the game.
Tuesday Update
Posted Tuesday 01/06/26 @ 9:34 AM —A low pressure system passing to our north this evening will bring a warm front through today followed by a cold front tomorrow. Despite the clouds this morning, it looks like sunshine through high clouds this afternoon.
Wednesday looks to be mostly sunny and quite windy. High 51º-52º
Thursday highs will be in the same 50-52º range during the daytime, but temperatures warm up further later Thursdayevening.
There’s a range of model forecasts for the rain expected Friday through Saturday. The GFS and the AIGFS is slower with the rain onset on Friday and slower for it to exit on Sunday. There’s also a question of weather the rain will be substantial or less than 1/2″ total.
Todays’s 06z GFS forecast for 3 PM Friday. Rain doesn’t move in to the city until mid to late afternoon. (Click on image for a larger view.)
Models guiding this forecast: REFS, NBM, GFS. AIGFS
Originally Posted Sun 8:37 PM —This week’s main ‘theme’ will be an increase in warmth, expected to begin in earnest on Wednesday.
18z NBM (model blend) meteogram for temperature/dew point for KLOM Blue Bell. High temperatures on a gradual but consistent climb to near 50º on Wednesday.(Click on image for a larger view.)
Several systems are expected to pass to our north with any precipitation also staying north, extending as far south as Berks county on Monday.
For the Philadelphia region and the Delaware Valley, considerable cloudiness is expected on Monday as the low passes to our north and warm moist air is brought in aloft. Highs remain in the 30s.
Another system passes to our north on Wednesday, that brings a weak cold front through. The chill down from this front will be short-lived.
A slowly moving cold front approaching from the west with waves of low pressure moving along it will pass to our northwest on Friday; most of the precipitation (rain) will pass to our northwest. Yet another wave of low pressure moves up the same path on Saturday.
18z ECMWF-AI forecast for Friday early afternoon. The area of rain moves to our northwest, just brushing our area. (Click on image for a larger view.)
While the general features of this weather outlook are consistent across models, it would not take much of change in intensity or track to greatly change our area’s forecast and weather. Stay tuned.
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.
