Posted Tuesday 01/20/26 @ 8:05 AM — A major long-duration winter snow storm is increasingly likely beginning Sunday. The ECMWF-AI and HGEFS (AI) models along with the GFS are all forecasting a major storm to move just south of us beginning late Saturday (most likely beginning early Sunday) and continuing possibly through Monday! The ECMWF is the slowest and strongest with this storm.
Today’s 06z ECMWF-AI model forecast for 7 PM Sunday. Our entire area will be north of the RED 500-1000mb thickness line, indicating all snow and temperatures are expected to be below freezing. (Click on image for a larger view.)
As is always the case, there are differences in the forecast northern extent of the system and in the speed. The GFS is much faster, the ECMWF is much slower, giving higher snow totals.
I’ll leave the heavy lifting at this point to the the model blend (NBM model) which, at last count, combines and statistically weights about 40 different models based on their performance over the previous six hours.
Here’s the current NBM snow forecast by Monday morning—
07z NBM forecast mean snow totals 7 AM Monday. White contours 1 inch increments. The storm may last longer than this! (Click on image for a larger view.)
The latest available ECMWF shows higher snow totals with the storm lasting into Tuesday. I’m not sure I believe it at this point in time.
This storm is still well in the future, weather-wise, and we’ve all seen these storm forecasts become big busts. And it might be sacrilegious to post potential snowfall this early in the week. But hey.. that’s why you visit this site. No hype, just the facts as I see it.
Stay tuned!
Cold! & Possible Snow Again this Weekend
Posted Monday 01/19/26 @ 4:51 PM — It’s winter in Philadelphia and it does get cold, and cold it will be. Earlier I posted the forecast low temperatures Wednesday morning.
The wind chill temperatures for tomorrow, Tuesday look impressively cold. (I often don’t “do wind chills” because I think it’s an opportunity to hype and conflate low temperatures on TV.)
That said, Tuesday morning at 8 AM looks to be very windy and cold with wind chills below zero!
Todays’ 18zRRFS calculated wind chill temperatures (“apparent temperatures”) at 8 AM Tuesday. Black Contours are 2º increments (Click on image for a larger view.)
Of interest continues to be a disturbance forecast to move just south of us, then develops off the coast late next Saturday through Sunday.
The HGEFS (AI Hybrid) and the ECMWF-AI are forecasting this storm. The latest NAEFS and the AIGFS currently show less snow with a more southern track. I’m keeping an eye on it.
Originally Posted Mon 9:50 AM —The cold weather that has arrived today and that will be reinforced with another front later today has been well-advertised. By sunrise Wednesday morning, we’ll have reached our lowest temperatures—
NBM Mean Low temperatures with standard deviation (uncertainty) at 7 AM Wednesday. The large standard deviation is typical for low temperatures, and in this case, the actual lows likely be below the mean temp. ( light blue contours are 2º increments) (Click on image for a larger view.)
Of interest this week is our next possible snow storm for next weekend, currently predicted by both the ECMWF-AI and the HGEF (AI) models. This storm is far in the future (at least as weather forecasting goes), but indications are that it will drop a minimum of 4+ inches and likely more if the current AI model forecasts are correct. The moisture source will be the Gulf of Mexico. There’s also a possibility of mixed precipitation at some point in the storm.
Here’s current ECMWF-AI forecast for next Saturday—
Today’s 06z ECMWF-AI forecast for 7 PM next Saturday. The precipitation (green shading) will be all snow at the start, as indicated by our being north of the red, yellow and magenta critical thickness lines. (Click on image for a larger view.)
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 RRFSadditional 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 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.)
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.
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
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.
