Updated Saturday 12/13/25 @ 5:43 PM —I’ve used the NBM for most of this snow event, so continuing with the latest 19z NBM run (the NBM updates its precipitation forecasts at 01, 07, 13, and 19z), it’s still showing higher snow totals despite temperatures not falling below freezing near the city until 3-4 AM. Here’s the latest forecast—

The near ground temperatures somewhat challenge my confidence in these numbers, but they are consistent with the total water equivalent forecast. Additionally, the mean amounts shown are close to the median amounts predicted,

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Updated Saturday 12/13/25 @ 3:26 PM — While we have the potential for over 5 inches of snow, this afternoon’s models have temperatures near the surface now remaining above freezing until 3AM. This will seriously cut into snow accumulation totals. So we’re back to the 3” to 4.5” range from this morning’s forecast.

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Updated Saturday 12/13/25 @ 11:08 AM —The latest models are showing snowfall in the Philadelphia area in the 4-6 in range.

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Updated Saturday 12/13/25 @ 9:18 AM —The latest models have the current forecast on track. Light snow moves in between 10 PM and midnight and tapers off early Sunday morning. I’ve been posting the NBM forecasts, so we’ll switch to the REFS which is in the same range—

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Low pressure over the Great Lakes will bring a cold front through our area early Sunday morning. The timing of the front during the period of the lowest temperatures of the day along with the upper levels of the atmosphere through18,000 feet being below freezing ensures that any precipitation will fall mostly as snow.

Low pressure expected to form along this front will enhance the precipitation rate, resulting in increased snow totals, now expected to occur in NJ.

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#Philadelphia #weather #PAwx #PhillyWx

Sunday Forecast

Posted Sunday 12/07/25 @ 10:31 AM — The forecast posted earlier for Sunday remains on-track. Mid level clouds move in later this morning.

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Saturday Forecast Update

Posted Saturday 12/06/25 @ 10:09 AM — As described yesterday, the light frozen precipitation forecast was “unusually low confidence”. The model forecasts for freezing rain turned out to be light snow in the immediate area and it has continued a few hours later than expected.

Basically, a band of slightly warmer moist air was brought into an area of weak upward motion, causing precipitation. My usual graphics use low level vertical motion (Omega) between two different pressure levels (typically 850mb -700 mb) which didn’t really show what was going on.

There’s another less common way of viewing atmosphere dynamics— instead of vertical movement between pressure levels (heights), it views atmospheric dynamics as movement along equal energy levels (as in Temperature) and equal Potential Temperature. Viewing the current situation along these lines, the area of vertical motion causing the light snow is clearly visible as the drawn yellow Omega contours—

The current bright spots and breaks of sun are forecast to fade behind clouds again for the rest of early and afternoon. Some clearing late afternoon.

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Originally Posted Fri 7:49 PM —The forecast for tonight is an unusually low confidence forecast. A weak flow of warmer moist air near the surface may become trapped below cold air aloft. Some drizzle, freezing drizzle or even light snow is possible during the night time hours into early morning. All the models show a low probability of one of the above , but it’s unclear to me what’s driving this. The models are on to something because current radar and water vapor images show some slow moving light precipitation developing upstream from us—

Temperature rise during the night time hours. Any precipitation will be a function of the vertical thermal profile based on location.

Since the NBM was so good at forecasting our lack of any precipitation here, here’s the NBM with tonight’s forecast.

Here’s the REFS, which got today’s forecast really wrong here. See the caption for important info ——

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#Philadelphia #weather #PAwx #PhillyWx

Friday Forecast Update- A New Wrinkle

Posted Friday 12/05/25 @ 9:05 AM — Current Radar and Water Vapor imagery shows the snow falling to our south. Unless there is strengthening of the southern system expanding it northward, it looks unlikely that the precipitation shield will over-spread the area as forecast by last night’s REFS. Yesterday’s NBM forecast looks to be the ‘winner’. Flurries, at most, can be expected near the city.

Current Radar Water Vapor—

Here’s the latest 12z NBM, just available—

We’ll stay with the NBM with this system. As the storm moves off the coast, some moisture and warm air will be thrown back into our area. Some freezing rain/drizzle is expected after midnight.

I’ll update later this afternoon with this new wrinkle in the forecast.

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Friday’s “Minimal Snow” Update

Posted Thursday 12/04/25 @ 7:24 PM — The latest ECMWF-AI, REFS, and Canadian AI models are showing a bit more snow, but it still comes down to only a dusting to a coating— much less than 3/4″. Here’s the REFS model forecast—

Interestingly, the REFS forecast has changed the timing from the earlier posted NBM, with light snow moving up from the south about 10 AM and taping off significantly during the early afternoon. Not much additional after 2 PM.

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Friday’s “Minimal Snow” Outlook Update

Posted Thursday 12/04/25 @ 11:32 AM — A quick update on the “minimal snow” forecast by some of the models for Friday afternoon.

This “snow” was always a low probability forecast, especially when predicted accumulation is in the coating to dusting range.

The Model Blend (NBM) statistically combines the forecasts of 30-40 different models, statistically weighting each constituent model’s forecast through a comparison of how each performed over the prior 6 hours compared to the actual conditions (as captured in the 6 hour delayed URMA model). It also weights each model based on historical accuracy.

The latest Model Blend (NBM) shows almost no snow in its statistical outputs.

Here’s the mean (average) snow predicted—

Here’s the 75 percentile accumulation total. The 75 percentile means that 75 percent of the 30-40 models are forecasting at or below this amount—

It should be noted that the low resolution ECMWF-AI still is forecasting a coating, especially for South Jersey.

This amount of snow for us doesn’t really merit the time put into it, but I’m staying in practice for later season snow events.

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Thursday Update

Posted Thursday 12/04/25 @ 9:02 AM — As forecast by the REFS yesterday, a line of very light snow flurries/snow showers is expected to move through about 10 AM. It’s currently visible on radar, although much is not reaching the ground. See caption for more info—

The rest of the forecast remains on track for Thursday, as does the snow outlook for Friday.

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Thursday: Cold to Colder

Posted Wednesday 12/03/25 @ 5:17 PM — This late autumn season is much different than recent previous years. It wasn’t too many years ago that I still had some hardy flowers on our deck hanging on for dear life in early December and I can remember recent years where temperatures repeatedly reached the mid 60s until the last week of December.

Thursday will likely dispel any hopes of similar warm ups in the near term. A strong cold front approaches with an increase in clouds during the morning.

The current air mass is dry and there isn’t much moisture to wring out in the form of showers with this front. A latest REFS ensemble member shows some light snow showers moving through Thursday morning about 10AM, just ahead of the front with some dark clouds.

The actual front moves through early afternoon with another reinforcing shot of cold air towards evening as winds pick up. Partly cloudy skies the rest of Thursday afternoon.

Friday will be cold. High temperatures on Friday may not get above 32º with wind chill temps in the mid 20s at best.

Over the past week, I’ve been watching a storm for later Friday that most, but not all, models have the snow associated with it suppressed to our south. The few models that show light snow reaching our area only crank out about 0.5″ of snow at most- a dusting or a coating. I’m watching this system and will continue to update.

After reminiscing about warmer Decembers earlier, I’m reminded that some very cold Thanksgivings and early Decembers gave way to very mild Februaries . Current cold conditions don’t necessarily portend a very cold winter.

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Wednesday through Friday

Posted Tuesday 12/02/25 @ 7:20 PM — Skies clear for Wednesday. There should be plenty of sunshine and while instability cloudiness sometimes develops with an exiting storm, the models don’t show this for Wednesday.

Thursday, a strong cold front moves through mid day with just clouds at this time. Winds pick up quite a bit towards evening as temperatures plummet.

Very cold Friday, with highs in the low to mid 30s! For Friday, another coastal system is expected to move up the coast, but slide just to our south. Most models have the precipiation of this storm totally missing our area, but the ECMWF-AI model has consistently showed some light snow falling by Friday evening.

This afternoon’s German ICON model has joined the ECMWF-AI with a forecast of a dusting to a 1/2 inch coating by Saturday morning. But our GFS model and the Canadian GDPS-AI version have the precipitation passing us to our south, as does the current RRFS.

I’ll keep an eye on it.

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Tuesday Mix-> Rain Update

Posted Tuesday 12/02/25 @ 8:24 AM — The precipitation began as early as 6:45 AM -7 AM in our area as forecast and there was/is some ice pellets mixed in closer to/in the city.

Here’s the current MRMS “hydrometeor” scan showing various forms of precipitation —

For most of the area, this changes to all rain. The latest models show the rain ending from west to east as early as 2:30 PM to 4 PM.

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Late Forecast Update

Posted Monday 12/01/25 @ 9:00 PM — Temperatures around the city have dropped below freezing (expected). The latest 00z models (HRRR NBM) are still painting a similar forecast.

But, inspection of the vertical thermal profile of the latest HRRR suggests that the early period, 7AM-9AM, may have more icy conditions or sleet closer to the city than the model precipitation type has been showing. (The models are still saying rain near the city.)

Some ground surface temperatures may be still cold enough for freezing and temperatures aloft at critical levels are still below freezing in our area at 9 AM. Snow isn’t likely, but ice pellets or some rain ice mix is. Here’s what I’m talking about—

Just a possibility. Sometimes the model precipitation types are wrong.

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Rainy Tuesday Update

Posted Monday 12/01/25 @ 4:48 PM — No significant changes in the forecast posted this morning. Here are some additional points—

• Rain or Snow (depending upon location, see animated gif below) begins between 6 AM and 7 AM
• Changes to all rain by late morning in most of our forecast area.
• Little to no final accumulation in Upper Montco, Bucks, and Chester counties— areas where it had snowed.
• Rain ends from southwest to northeast around 3-5 PM
• Some sun breaks out in western areas before sunset.
• Winds increase and become quite gusty towards evening.

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Tuesday Forecast Update

Posted Monday 12/01/25 @ 9:49 AM — Last night’s models along with the early morning models continue with these trends for Tuesday’s storm. Here are the current trends—

• Precipitation begins as light rain or light snow (north west) as early as 6 AM. Virtually no snow in NJ, in Philadelphia and immediate locales north and west.
• Snow north and west likely changing to rain.
• Far northwestern suburbs may have a period of freezing rain.
• Transition to rain by late morning.
• Heaviest rainfall in NJ.
• No changeover back to snow expected at the end early Tuesday evening.
• Windy early Tuesday evening.

At 8 AM most of the area will be rain—

Total Snowfall Accumulation (Model Blend, also supported by RRFS forecast)—

Total Precipitation (water and snow-water equivalent)

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Stormy Tuesday

Updated Sunday 11:15 PM — The trend with tonight’s models is for little to no accumulation except in north and west Montco Bucks and Chester counties. It appears there will be no changeover from rain to snow towards the end of the storm. The heaviest precipitation (rain) will be in NJ. Updates tomorrow.

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Originally Posted Sun 5:10 PM — This week will see the approach of a winter-like coastal storm at a time when we are still in astronomical Autumn.

Following a sunny but unseasonably cold Monday (high low 40s), clouds will increase around sunset Monday. Low pressure developing near the Gulf of Mexico (America) Mexico will lift northeastward towards our area.

Precipitation may start as light snow near or north and west of the city, but is expected to quickly change to rain. Northwest of the city, (Berks county, Lehigh county) the precipitation may mix with rain or stay mostly snow.

Staying with the AI model and zooming in at 7 PM, the atmosphere (based on the thickness lines) has chilled down to support some snow just north and west of the city just as the precip is raining. Note the the 32º freezing line is still far northwest, so not much of any snow is expected to accumulate on wet ground. But there’s some uncertainty.

So mostly rain, heavy at times, after some light snow in the morning, possibly changing back to light snow with light accumulations possible early Tuesday evening.

There’s fairly good model agreement with the track of this storm and the been good continuity between model runs.

In winter with an overall colder atmosphere, the would be a major snow storm for us. However, it’s early in the season and perhaps this year will be like last year, where there had been “not enough cold air” for major snowfalls here.

Total precipitation or snow-water equivalent will be fairly high, 1-1.3″ water.

Things to watch—

• If the precipitation comes in early, much before daybreak (as some models are showing), we may have more snow accumulation before the changeover to rain.
• High precipitation rates early in the storm may cause dynamic cooling, bringing temperatures near the surface closer to freezing.
• Cold air may rush in after the main low passes, either drying things out or causing a burst of snow.

Food for thought

If the immediate PHL area will have any snow accumulation, it will occur towards the end of the storm. This is low confidence, but still a possibility.

Here’s the Model Blend forecast snowfall —

Here’s the latest GFS forecast, just available—

The latest RRFS and REFS not yet available, but the 12z RRFS was leaning towards much lower snowfall totals

An interesting early winter storm. Stay tuned for updates.

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