Daily Archives: Monday, January 19, 2026 9:50 am

Philadelphia Weather Outlook

THIS WEEK’S WEATHER

#Philadelphia #weather #PAwx #PhillyWx

Sunday Storm Update

Posted Wednesday 01/21/26 @ 4:46 PM — There have been some significant changes in the model forecasts today and it’s unclear how much these changes will play into the sensible weather (read: how much snow vs mixed precipitation ).

Most important- the system is now further north and more coastal in nature.

Several models, including the AI models, are now showing two low pressure centers. Having two centers may bring some warm air into our area for part of the storm, creating a mixed precipitation event (sleet-freezing rain) at times and reducing total snowfall. The most extreme is the current available ECMWF where the inland low’s circulation is significantly developed.

01-21-26 12z ECMWF forecast for 4 PM Sunday. Two distinct low centers. Most important is the location of the red and violet critical thickness lines. They are north of us suggesting the strong possibility of mixed precipitation . (Click on image for a larger view.)

The Canadian GDPS with “AI Spectral Nudging” is even more pronounced with the inland low, also bringing warm air aloft for some mixed precipitation—

The German ICON and our NOAA GFS model show the inland low to be much weaker, keeping the precipitation all snow in the city. The ECMWF-AI and AIGFS models are in the same camp. The latest NBM with a precipitation update is on board with this and shows the potential for major snowfall amounts.

So for now, we’re going with an all snow event in the city, with a likely mix in southern NJ (sleet/freezing rain). Light snow starts as early as Saturday evening.

Here’s the 19z NBM total (mean) snowfall—

01-21-26 19z NBM mean snowfall forecast by Monday evening. (Click on image for a larger view.)

Don’t take the following graphic too seriously, but here’s the NBM 75 percentile snowfall—

01-21-26 19z NBM 75 percentile snow accumulation. PERCENTILE SNOW AMOUNTS ARE NON-INTUITIVE. IT MEANS THAT 75% OF THE MODELS ARE AT OR BELOW THIS AMOUNT. IT DOES NOT MEAN THAT 75% OF THE MODELS ARE PREDICTING THIS AMOUNT. Even stranger, it means that 25% of the models are predicting MORE! (Click on image for a larger view.)

So with two centers and warm air, this is becoming a forecasting challenge. Stay tuned.

Winter Storm Update

Posted Wednesday 01/21/26 @ 8:32 AM —I’ve reviewed the latest models which continue to forecast a major snowstorm for our area this weekend.

Here are the trends—

  • The dip in the jet has sharpened slightly, moving the storm further north and closer to us.
  • The snow may begin late evening Saturday and last into Monday morning. There may still be some light snow ending noon on Monday.
  • Much higher snow accumulation totals are now forecast.

Here’s the latest ECMWF-AI, which explicitly calculates snow depth. I believe these numbers are too low—

01-21-26 06z ECMWF-AI snow depth parameter by Tuesday. I currently believe this is too low. (Click on image for a larger view.)

Closer to the likely snow totals is the “Model Blend” (NBM model). (It’s called the National Blend of Models based on its original design, but it now includes model data from the European ECMWF, Canadian CMC and even the Australian Weather Service. Over 40 models are statistically combined and weighted based on their historical performance and more importantly, the accuracy in their forecast six hours prior. It currently does not include the AI models.)

Here’s the NBM model through early Tuesday morning. Based on current trends, I believe this is closer to the truth—

01-21-26 07z NBM snow accumulation totals (calculated) through Tuesday morning. Fine white contours are 1″ increments and labeled. (Click on image for a larger view.)

Updates later this afternoon. Stay tuned.

About the Weekend Storm…

Posted Tuesday 01/20/26 @ 8:14 PM —First, let me say that this potential storm is over 110 hours in the future. When it comes to this time frame, there aren’t all that many models that extend their forecasts out that far. Most of the higher resolution models only have their forecasts extend out to 48-84 hours, depending on the specific model.

So at this point in time, we’re dealing with the lower resolution global models, global Ensemble models and newer AI global models along with a couple of hybrids. Specific accumulation forecasts really require weather events to fall into 60 hours or less in the future. But at this point, we can look at the track, speed, intensification and general thermal profile of a system and these Global “synoptic” models can provide these general trends.

Earlier today, these synoptic scale models were forecasting incredible amounts of snow, especially areas just south of the Philadelphia region.

Things have changed somewhat since this morning (and they’re very likely to change again.)

Here are the trends—

  • The GFS has joined the ECMWF with a later onset of snow. Snow now looks to start about 8 AM Sunday morning instead of Saturday evening.
  • The HGEFS ( new Hybrid AI) still shows a Saturday evening start.
  • The storm looks to move a little less to the north, reducing our accumulation. We’re now in the 4-5 inch range. Not ready to hang my hat on this value, as some intensification at the coast compensate.
  • It no longer looks like the storm stalls and moves up the coast.
  • The GFS has the storm ending before daybreak Monday, instead of stalling off the coast as some AI models had forecast.
  • The HGEFS and the ECMWF is slower with the storm’s exit, somewhat later into Monday morning.
  • The ECMWF-AI still shows more snow than the GFS, in the 5-8 inch range.

This afternoon’s AIGFS forecast at 7 PM Sunday—

01-20-26 18z AIGFS model forecast 7 PM Sunday (Click on image for a larger view.)

The latest ECMW-AI snow depth forecast—

01-20-26 18z ECMWF-AI snow accumulation by Monday 7 AM (Click on image for a larger view.)

Tonight’s models should help move towards some clarification. Stay tuned.

Major Storm Possible Beginning Sunday

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’ 18z RRFS 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.)

Stay tuned.