Posted Friday 01/31/25 @ 9:32 AM — A slight shift in the timing of the heavier rain today— late afternoon through this evening, ending after midnight. Total rainfall in the city and surrounding areas is now forecast to be less, closer to 0.30″-0.40″. Heavier rainfall in South Jersey.
NBM12z Accumulated Rainfall for Friday into early Saturday. (Click on image for a larger view.)
The potential mixed precipitation storm I prematurely mentioned last evening has changed timing, from possibly next Saturday to possibly next Thursday. That’s a big change, suggesting the models, including the AI models, don’t yet have a good handle on things. Nonetheless, the setup ingredients appear to be there. Stay tuned.
Friday’s Rain Update
Posted Thursday 01/30/25 @ 6:01 PM — The rainy forecast for Friday hasn’t changed appreciably and rainfall totals are similar to the ECMWF forecast graphic posted yesterday. Rain begins about 3 AM Friday, earlier south and west and continues through about 9 PM Friday evening. It will be windy and gusty at times during the day, especially late afternoon when the heaviest rain is expected to fall.
The weekend looks as follows: Sunny on Saturday and cloudy on Sunday as a disturbance passes to our north.
No major storms until NEXT weekend, when things may get interesting, according to the AI models and the ECMWF—
The ECMWF forecast for Superbowl Weekend (NEXT Saturday) Very far off in the future, but the setup is for a mixed bag of very heavy precipitation here. (Click on image for a larger view.)
Friday’s Rain
Posted Wednesday 01/29/25 @ 7:42 PM — A storm bringing only rain to our area will move in several hours before daybreak on Friday. It will be rainy all day and the rain will taper off during the evening.
Continuing our ongoing trend towards lower than average rainfall, we’ll only see 0.50″ of rain in the immediate Philadelphia area according to the AI model. Areas not far to our south will receive over an inch, according to the latest models.
Staying with the AI model, here’s its forecast total rainfall for Friday—
Today’s 12z ECMWF-AIFS total accumulated rainfall by Saturday morning. (Click on image for a larger view.)
The NBM is forecasting heavier rainfall for Friday—
Today’s 18z NBM rainfall forecast (Click on image for a larger view.)
AI Model Looks Quite Good in Simple Experiment
Posted Wednesday 01/29/25 @ 5:47 PM — The temperature reached at Philadelphia airport today was 52º, 53º based on hourly records. ( If 53º was reached, it occurred off-hour and will be posted on the NWS site in a few hours.)
It’s very impressive that the AI model correctly forecast this range a week ago. Today’s NBM forecast was a still a degree so too cool.
Posted Wednesday 01/29/25 @ 9:39 AM —Over the past week, I’ve been conducting a simple observational experiment regarding today’s high temperature forecast. (Also see here.)
I chose to follow the AI model temperature forecast for today, because it was below freezing here at the time and the majority of the models, including my go-to model for temperatures (the NBM) were forecasting highs in the mid 40s for today, while the ECMWF-AIFS at that time was forecasting mid 50s! Quite a difference.
Incredibly, the regular model forecast have creeped upward towards the 50s and closer to the AI model while the ECMWF has shave off a few degrees to a high of near 53º.
Here’s the latest NBM —
Today’s NBM (12z) high temperature forecast for today. (Click on image for a larger view.)
Here’s the latest AI (ECMWF-AIFS) forecast for today—
Today’s ECMWF-AIFS high temperature forecast for today. (It should be noted that the ECMWF forecasts in 6 hour increments and the version I download does not include the day’s maximum temp. The only data is the actual temp at 1 PM as shown above. Since high temperatures typically occur around 3 PM, the actual max temp for today may be somewhat higher
What’s impressive is that the ECMWF-AIFS was forecasting this range of max temperatures almost a week ago! It took the computational models almost a week to approach its forecast.
Posted Tuesday 01/28/25 @ 10:53 AM — Radar shows some snow showers to our far northwest. Superimposed RAP model vertical motion (“omega”) shows strong downward motion, likely dissipating these snow showers greatly by the time they get to Philadelphia—
Radar shows some snow showers to our northwest. Superimposed RAP model vertical motion (“omega”- yellow dashed lines are negative) shows strong downward motion, likely dissipating these snow showers greatly by the time they get to Philadelphia (Click on image for a larger view.)
Posted Tuesday 01/28/25 @ 9:17 AM — When I went to bed last night, the forecast for Philadelphia on our news radio station called for “‘”snow showers, possibly reducing visibility” and clouds. Did I miss something with my update yesterday? What were they talking about?
We’ll have a mix of sun and clouds today. Several weak disturbances are in the pipeline for us over the next few days. Snow/rain showers are a slight possibility.
WV image with superimposed RAP model parameters. Disturbances 1-3 are in the pipeline to affect us. The upper level low (4) will bring rain on Friday. (Click on image for a larger view.)
There’s a chance that the storm on Friday may start as a bit of freezing rain or sleet here before a change to all rain. Not a certainty.
Posted Monday 01/27/25 @ 5:55 PM — A slight warmup for Tuesday and Wednesday. The ECMWF-AI model has us in the low 50s on Wednesday. The model blend (NBM), my go-to model for temperatures, now has a forecast high of 48º in Philadelphia with a rather large standard deviation of ± 2.8º.
A series of weak fronts and upper air disturbances come through over the next few days, specifically before daybreak Tuesday and Tuesday night. A rain maker is on the way for Friday, although the heaviest rain may move to our north and south.
The front and the system bringing rain on Friday are visible on water vapor imagery—
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. The closed low in the southwestern US will move into our area on Friday. (Click on image for a larger view.)
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Originally Posted Mon 10:45 AM —A split jet flow and a still somewhat chilly northwesterly flow will keep temperatures slightly above average through Wednesday, becoming somewhat colder Thursday, then warming again Friday with rain.
Today’s 06zNAEFS jet stream wind speed forecast for Wednesday morning. The upper closed Low (L) will eject towards us bringing rain by Friday. (Click on image for a larger view.)
Regular readers here know that over the past week, I’ve been watching the AI version of the ECMWF which has consistently forecast high temperatures on Wednesday to be in the low 50s! Every other model has us in the mid to upper 40s. So this is a little experiment to see if the AI model has any special insights into high temperatures—
Today’s 06z ECMWF-AIFS forecast temperatures on Wednesday. Much higher, and consistently so. then the regular models. (Click on image for a larger view.)
It appears we may have some rain on Friday into early Saturday. This feature has been in the forecast for several past days, with each model run moving it earlier. (It was originally forecast for this rain to move in Saturday into Sunday.)
Today’s 06z NAEFS forecast for Friday at 1 PM. The white contour is 32º, the red, yellow and magenta lines are critical thickness lines, indicating the average temperature of different lower levels of the atmosphere. With all of these lines to our north, we will have rain, not snow. (Click on image for a larger view.)
The weekend looks to be dry.
No significant snow storms are on the horizon for us. For that matter, we’re still running a rain/snow deficit and the dry conditions of last Fall have not shown signs of ameliorating to this point.
An upper level trough with cold high pressure that has been with us seemingly forever, will lift off to our northeast on Saturday. A disturbance passing to our north will bring just cloudiness Saturday night and during the morning on Sunday. Clearing Sunday afternoon.
WV image 5 PM today shows several disturbances that will pass to our north. The subtropical jet stream remains to our south, but a slight ridge tries to build in late Sunday into Monday. (Click on image for a larger view.)
The latest GFS shows the disturbances that will move to our north—
Today’s 18z GFS has the clipper-type disturbance moving to our north late Saturday. (Click on image for a larger view.)
By Sunday afternoon , the weak disturbance has moved to our northeast along with a weak trough. High pressure builds in from the west—
GFS forecast for Sunday 4 PM. (Click on image for a larger view.)
Saturday Forecast
Mostly sunny and continued cold. Increasing cloudiness in the afternoon. Temperatures still remain near or below freezing.
NBM high temperatures: Blue Bell, PA 31º Philadelphia, PA 32º Low uncertainty (based on standard deviation): ± 1.4º
Sunday Forecast
Cloudy in the morning. Winds increase along with gusts towards noon. Decreasing clouds in the afternoon with some sun likely during the Eagles game. Wind decrease towards late afternoon.
NBM high temperatures: Blue Bell, PA 39º Philadelphia, PA 41º Low uncertainty (based on standard deviation): ± 1.2º
Just out of curiosity, the ECMWF AI forecast is still forecasting 50º + temperatures for Wednesday while the operational models have us just in the mid 40s The NBM shows the standard deviation for the high temp on Wednesday to be an incredibly high: ± 7º. I’m curious how it plays out.
BTW, much more active weather is likely beginning next weekend. Punxsutawney Phil will have his hands (paws) full. Stay tuned.
Sun 7:52 PM —Forecast Review —The NBM appears to have done well in predicting the water equivalent precipitation; about 0.38- 0.45″of liquid water fell as either graupel or snow. Snow totals are usually posted by the NWS here.
A surprise was the amount of graupel that fell early on, decreasing the snow accumulation depth.
The back end of the heavier precip is just reaching the immediate Philadelphia area from the southwest about 8 PM. Light snow and snow flurries are possible for several hours more.
RADAR ~ 8PM (Click on image for a larger view.)
Yet Another Snow Update
Posted Sunday 01/19/25 @ 11:21 AM — Perhaps I should follow my own advise and not update the forecast as I posted at 8:41 AM this morning? Precipitation did start as originally forecast between 10 AM and 11 AM.
The big issue will be temperature. It won’t be a question of snow vs rain, but rather snow falling verses accumulatingsnow that doesn’t melt and compact.
The NAM and NAM-NEST still show only small accumulations near the city—
This mornings’ NAM (12z ) Snow depth forecast Most of this occurs after 4 PM (Click on image for a larger view.)
The NBM still maintains a significant snowfall, with accumulations near the city occurring after 3 PM and continuing heavy until 9 to 11 PM.
This morning’s 13z NBM with updated precip parameters. Most of this accumulation occurs after 4 PM from the city east. (Click on image for a larger view.)
With the latest NBM above supporting last night’s forecast, I’m inclined to stay with it and its more substantial snowfall.
Sunday Snow Update- Significant Change
Posted Sunday 01/19/25 @ 8:41 AM —— There’s always a tendency to update the forecast with “the latest models”. Over the years, I’ve found that to be a mistake. Numerous weather journal articles point to the observation that often, the model run from the day before is more accurate than the most recent model run just a few hours old
This recognized phenomenon is called “model spin up error” and manifests itself in the first 3 to 6 hours of a model run. Very simply, it takes at least 3 to 6 hours for most models to reach equilibrium of the model state and during this spin up period, the model is not all that accurate.
That said, let me point out some observations from this morning—
The observed temperatures this morning are higher than the forecast temperatures. Below (left) are the temperatures forecast by the NBM and on the (right) are actual temperatures registered on the RTMA at 6:45AM—
Forecast NBMActual RTMA Temps observed
These higher temperatures may cut into the higher snow totals.
2. The 06z (1AM EST) models have snow moving in closer to 12- 1 PM instead of the 10-12 AM time frame.
3. I’m trying to assess model forecast errors as they occur. Below is the RTMA (near actual) precipitation compared to the GFS forecast. Notice the GFS is too far south with it’s precip—
GFS precip (3 hour) (color) with current RTMA precip. (White Contours) GFS is too far south. (Click on image for a larger view.)
Here’ s the same comparing actual RTMA precip with NAM model precip. A closer match!
NAM 06z Forecast (color) with actual RTMA precip (contours) at 7 AM. A better match!
So, I hate to disappoint snow lovers, but based on the warmer temperatures and the closer match on precipitation of the NAM, I’m changing the moderate snow forecast to a much lighter snow forecast from the NAM—
06z NAM snow accumulation forecast (Click on image for a larger view.)
Sunday Snow Update
Posted Saturday 01/18/25 @ 10:01 PM — I wanted to do one more update based on the tonight’s early models. As mentioned earlier this evening, the precipitation from in the city and westward will likely be all snow.
Snow moves in from southwest to northeast between 10 AM and noon. Despite temperatures being slightly above freezing (33-34º) into the early afternoon, the precipitation will fall as snow because temperatures aloft are below freezing. I can’t rule out a slight mix around noontime, but if it occurs, it will be brief. The snow tapers off between 8 PM and 10 PM.
The models have fairly settled on a range of 0.38 inches water to as much as 0.53 inches water with the latest HRRR.
The big question is not be how much falls, but how much accumulates. The ECMWF “skin” temperatures drop below freezing early in the afternoon, which should allow accumulation despite temperatures being 33-34º early on. If it weren’t for temperature based melting and snow compaction, areas in the city would be seeing 6 inches.
I’m going with tonight’s HRRR forecast which captures the trend towards higher snow-water, mostly all snow, and colder temperatures—
Tonight’s 00z HRRR snow depth forecast by 10 PM Sunday. (Click on image for a larger view.)
The latest model blend (NBM) has similar total, yet tends to lag with changes in the forecast—
Tonight’s 01z NBM snow accumulation forecast (Click on image for a larger view.)
Sunday Snow Update
Posted Saturday 01/18/25 @ 5:17 PM — Each model run seems to vacillate between higher precipitation amounts and lower; between a delayed transition from rain to snow in the city to an earlier changeover.
The latest ECMWF has less total precip but an earlier changeover to snow.
The latest RGEM, compared to this morning captures the changes—
Today’s 18z RGEM More rain/mix less snow. (Click on image for a larger view.)
The models have, for now, consolidated on about 0.35-0.40 inches of water, falling as a mix, rain or snow. When the changeover occurs will determine the final snowfall.
My own sense is that most of the precipitation will fall as snow, but may not accumulate initially due to melting with the temperatures at the surface above 32º. I base that on review of critical thickness temperatures.
Here’s the latest model blend (NBM) which I think captures the current forecast well—
19z NBM Accumulated snow. Areas east of the city may be less than show. (Click on image for a larger view.)
The latest GFS, just available, has a similar forecast as the above.
If I were to modify the above, I would add 1″ to the above in areas west of the city to account for more snowfall.
I may update again this evening if I can. Otherwise check back tomorrow morning.
Sunday Snow Update
Posted Saturday 01/18/25 @ 2:18 PM — You may have noticed that we had more rain today (Saturday) than predicted last night. After reviewing the models, the only model that got this right was the Canadian RGEM; the GFS, NAM and HRRR were way off.
Does that mean the RGEM has a better handle on the snow for Sunday? The latest RGEM from this morning has trended colder with increased precipitation. Here’s the latest RGEM snow depth forecast—
This morning’s RGEM has trended colder and wetter. Its snow forecast in western suburbs may be low. (Click on image for a larger view.)
A note for those of you who are visiting this site from “X“. Due to its highly unbalanced political environment, I’ve considerably cut back auto-posting there. To receive all forecast postings, I recommend following me on mastodon.social or bluesky.social
Sunday Snow Update
Posted Saturday 01/18/25 @ 11:35 AM — A quick update. Based on the latest GFS model which just became available, it appears that a major storm may affect us on Sunday. I’m bumping up potential snow accumulations from the city and westward to the 5″ to 8″ range.
Here’s the latest GFS surface map—
Today’s 12z GFS shows a major coastal storm with temperatures in the upper atmosphere (colored lines) all cold enough for mostly snow at 5 PM. An increase in expected precipitation leads to snow totals of 5-8 inches from the city westward. (Click on image for a larger view.)
Posted Saturday 01/18/25 @ 9:42 AM — Some important changes noted in the latest models—
Light snow or flurries begins earlier, as early as 8-9 AM in areas west and southwest of the city.
Total Precipitation has increased, from 0.20-0.30 water equivalent last night to 0.4-0.7″ water equivalent. Both a track closer to the coast and an increase in temperatures aloft may prolong a mix of snow/rain in the city but increase snow totals just to our west.
Heaviest snow between 5 PM and 7 PM. Ends about 9 PM.
Here’s the latest NBM precipitation type map at 3 PM—
13z NBM surface temperatures and Precipitation Type (snow-sleet-rain) at 3 PM (Click on image for a larger view.)
Some other trends—
The Canadian RGEM maintains a very wet storm from the city eastward.
There’s much uncertainty demonstrated with the ECMWF model, as captured by the ECMWF ensemble. The large standard deviation to the northwest suggests a possible closer track to the coast—
ECMWF ensemble shows the mean (average) location of the main low (L) but large standard deviation (color) shows much uncertainty in the northwest suggesting a possible closer track. (Click on image for a larger view.)
With this storm, I’m going to continue to leave the heavy-lifting of sorting through the various models to the model blend (NBM) algorithms. Here’s one version of likely snow totals—
13z NBM snow totals using my own algorithm. (Click on image for a larger view.)
I’m just sharing below my own algorithm for snow totals, based on NBM snow-water ratio and total precipitation. Areas to the east of the city are calculated too high. That said, the 6″ totals are possible west of the city—
My own snow total algorithm based on NBM data. Just sharing as a TEST.. I wouldn’t be surprised if we see these totals west of the city. Don’t take too seriously. (Click on image for a larger view.)
Updates later today between 5 PM and 6 PM.
Sunday Snow Update
Posted Friday 01/17/25 @ 9:51 PM — A quick update. This evening’s early models have come in along with the 01z model blend (NBM). There is increasing consensus that the overall precipitation will be on the order of 0.20 to 0.30 inches water which translates into about 2-3.5 inches of snow. This includes this afternoon’s ECMWF model.
Of interest (at least to me) is the NAM, which historically slightly over-predicts snowfall here compared with the GFS. With Sunday’s storm, it is consistently forecasting lower snowfall totals than the GFS. Something not to be ignored.
Here’s the latest NAM precipitation type (snow-sleet-rain) forecast at 4 PM Sunday—
Tonight’s 00z NAM PTYPE and temperature at 4 PM Sunday. Light scattered snow in most of the region.
The 01z model blend (NBM) also became available with its updated precipitation. Here’s it’s current forecast—
Tonight’s 01z NBM total snowfall forecast (Click on image for a larger view.)
The new GFS won’t be available until after 11:05 PM but I go off-duty at 10:30PM. I’ll update tomorrow morning.
Previously Posted Fri @ 5:44 PM — —A warm front will move through Saturday with plenty of cloudiness for much of the day. Some widely scattered sprinkles/showers are possible in areas mostly east into NJ, but many areas will just be cloudy. There may be some clearing around sunset.
Saturday High Temps: 41 Blue Bell, 44 Philadelphia.
A cold front moves through overnight Saturday but slows and becomes nearly stationary as low pressure develops either near the coast or inland. Precipitation moves in from the southwest as snow in northwestern sections and possibly a mix nearer the city.
There’s still a wide range of forecasts forSunday. What’s consistent is that some form of precipitation will start late morning. During the Eagles game, a mix of snow/sleet to start will change to all snow towards the end of the game. Temperatures fall during the game and it becomes increasingly windy.
Let me use several models to illustrate the unknowns. The ECMWF currently has a more off-shore track for the low pressure system. It’s forecast is colder, with most areas seeing snow, but precipitation amounts are low.
ECMWF forecast snowfall based on all-snow and an 11:1 snow water ratio. Areas east into NJ may be more rain and sleet than shown above. (Click on image for a larger view.)
The GFS continues with a rather hefty snowfall. It’s colder with more snowfall—
Latest GFS (18z) built in snow algorithm forecast snow depth(Click on image for a larger view.)
The NAM has a more scattered snowfall with a mix of sleet and rain reducing accumulations east of the city. It should be noted that the NAM forecast is supported by the Canadian RGEM —
Leaving the heavy lifting to the model blend, here’s the latest NBM snowfall—
NBM snow accumulation forecast (Click on image for a larger view.)
So there is a wide range of snow forecasts at this time. Regular visitors here know my mantra, “never ignore the NAM”. We’ll see if it becomes the leading forecast. I’ll update tomorrow morning.
