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Philadelphia Area Weather Forecasts

WEEKEND WEATHER FORECAST

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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 accumulating snow 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—

  1. 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 NBM
Actual 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.)

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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 for Sunday. 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.

Philadelphia Weather Outlook

THIS WEEK’S WEATHER

#Philadelphia #weather #PAwx

Sunday Snow Update

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Posted Friday 01/17/25 @ 9:49 AM — This update is a bit delayed to allow me to review some of this morning’s models. Here are the general trends—

  • The NAM has finally joined the other models in forecasting some snow, but it’s snow totals are still low compared to the other models: in the 1-2″ range.
  • The GFS continues to forecast a moderate snowfall (see graphic below)
  • The ECMWF continues to forecast snowfall less than the GFS
  • The Canadian continues to forecast a western track, heavier on precipitation with more rain than snow.

Here’s the latest GFS snow accumulation based on its built-in snow algorithm—

06z GFS forecast snow accumulation by late Sunday evening (Click on image for a larger view.)

I’m going to leave the heavy-lifting to the model blend (NBM). Here’s its forecast based on all-snow with an 11:1 snow ratio—

13z NBM snow forecast based on all-snow and an 11:1 snow water ratio (Click on image for a larger view.)

The storm looks to be a fast mover, with snow starting as early as mid to late Sunday morning. The Eagles (and more so the Chargers) will have an additional challenge.

Updates later this afternoon after the latest GFS becomes available (after 5:20 PM) with my regular “Weekend Weather Forecast”

Forecast Update and Sunday Outlook

Posted Thursday 01/16/25 @ 6:04 PM — As mentioned below, the latest GFS has become available and it’s forecasting a significant snow storm for us on Sunday. There’s still much uncertainty with this storm but I wanted to share with you the latest GFS forecast snow accumulation—

Today’s 18z GFS snow accumulation forecast for Sunday’s afternoon and night storm (Click on image for a larger view.)

To be balanced, here’s the latest NAM which has the storm totally missing us—

Today’s 18z NAM forecast for 10 PM Sunday. The storm moves off to our east missing us. (Click on image for a larger view.)

Posted Thursday 01/16/25 @ 5:17 PM — The light snow we’ve been having started a bit earlier than forecast and the coverage is a more widespread than forecast yesterday. The snow should be ending between 10 PM and midnight.

Current Radar/MRMS—

~5 PM Radar/MRMS with superimposed RAP model 700 mb wind streams and vertical motion (Omega) (Click on image for a larger view.)

Milder temperatures on Friday will give way to some very light showers on Saturday.

A potential snowfall is still in the cards for Sunday. There’s still a wide range of forecast tracks with the Canadian model showing the most inland (warmer- more rain, less snow) and the ECMWF showing a colder, easterly track with little rain, mostly snow but light accumulations.

Of interest is the NAM which has the storm moving easterly, away from us. giving us neither rain nor snow!

If the model consensus of the model blend is correct, the light snow may start as early as the late morning on Sunday.

Here’s the latest model blend forecast snow accumulation by Monday morning.

NBM (model blend) snow accumulation based on a 12:1 snow water ratio. (Click on image for a larger view.)

The latest GFS just became available as I write this and it is forecasting amounts similar to the above model graphic, although if the GFS is correct, we might see as much as 3-5 inches of snow.

Stay tuned.

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

Snow Showers this Afternoon – Some Snow likely Sunday

Posted Thursday 01/16/25 @ 10:06 AM — Today’s forecast of snow showers moving in remains intact. Current radar shows considerable light snow, but most is not reaching the ground at this time—

MRMS Radar this morning. Most of the snow is not reaching the ground due to evaporation in very dry air near the surface. Current forecasts for a coating to a fraction of an inch remain intact for today. The only accumulation currently is west of Harrisburg. (Click on image for a larger view.)

Current snowfall expected today is similar to the graphic posted below. The light accumulation is not expected to begin until 3 PM or later.

As for Sunday, this looks to be “interesting”, as several models have switched places forecasting snow for us. The usual “track” of the storm is “what’s up in the air”.

The latest ECMWF had been most consistent forecasting snow, but now has backed away from that forecast with a more easterly track. The GFS which had not forecasting snow, is now forecasting some snow for us.

The Canadian models now show an inland storm with plenty of precip, not all in the form of snow. Likely an outlier.

The German ICON is somewhere between the ECMWF and GFS, forecasting some snow. (“Some snow” is 0.5-1″.)

Here’s the latest model blend (NBM) cumulative snowfall forecast, likely too high—

This morning’s 19z Snow accumulation forecast for Sunday. Likely too high. (Click on image for a larger view.)

So there’s still too much uncertainty with this forecast. Hopefully by this evening’s update, things get clarified.

Snow Showers Thursday Evening – Some Snow Sunday

Posted Wednesday 01/15/25 @ 4:47 PM — Today’s models continue with a forecast of light snow/flurries for late Thursday afternoon. Accumulations will be minimal (under 1/3 inch) and spotty. Here’s the model blend forecast for snow coverage Thursday evening—

Today’s 19z NBM forecast accumulation for snow. Many areas will see no accumulation, others maybe a coating (Click on image for a larger view.)

As for Sunday, it appears that light snow may be falling during the Eagles game Sunday afternoon. The latest ECMWF and GFS AI models have similar forecasts and support the deterministic ECMWF forecast. The German ICON model has joined in with a similar snow forecast.

The model that is not on-board is our own GFS model, which consistently shows no snow for us.

It’s too far in future (more than 96 hours) to really talk about details, and it’s beyond the range of many models. But the best guess is for light snow to start as early as 1 PM and continue through the afternoon and evening. By the time the Eagles win, there may be as much as 0.5″ of snow on the ground.

Here’s the latest model blend (NBM) total snowfall forecast—

Today’s 19z NBM forecast for Monday morning showing snow cover. This is very preliminary and there are some trends suggesting it may be higher. (Click on image for a larger view.)
Wed 5:13 PM —Commentary — Despite one or two heavy rains in December and several light snowfalls, I don’t think the pattern of dryness and drought we experienced in the Fall has abated.

Total precipitation, rain and snow-water equivalent, continues to be low. The heaviest precipitation continues to move off to our northwest and to our south. Just something to keep in mind. There’s still time for things to change before Spring.

Posted Wednesday 01/15/25 @ 7:53 AM — Time this morning for just a quick update.
• The chance of light snow (coating) has increased for late Thursday afternoon and evening with a warm front expected to move across our area.

ECMWF forecast for 7 PM Sunday. Precipiation north of the multicolored lines will fall as snow. White line is 32º isotherm. (Click on image for a larger view.)

• A secondary coastal low, hinted at by various models, for later Sunday may bring as 1-3 inches Sunday mid afternoon and evening. I’m basing this forecast on the latest ECMWF and ECMWF- AIFS models. There may be some impact on the Eagles game, based on current timing.

Initially, falling as snow as early as 3 PM and melting on some surfaces, temperatures drop late afternoon below freezing, allowing some accumulation of 1-3 inches of snow. Further details later this afternoon. Stay tuned.

Update- This Week’s Weather

Posted Tuesday 01/14/25 @ 5:00 PM — There have been no major changes in the forecast for this week from my post earlier today. Snow showers look less likely for late Thursday and rain is more likely for Saturday afternoon and evening.

There hasn’t been much clarification on the potential light snow later Sunday into Sunday night. The ECMWF is still forecasting 2 to possibly 3 inches of snow, while the GFS is not showing any snow. With the exception of the ECMWF, the trend is towards little (or no) snow as supported by the NBM model blend.

Of interest is the AI version of the ECMWF, which has joined the regular ECMWF in forecasting some snow for us late Sunday—

12z ECMWF-AIFS forecast for Sunday 7 PM. Some light snow for us. (Click on image for a larger view.)

There’s too much model spread and uncertainty to have confidence in late Sunday’s forecast.

Highlights -This Week’s Weather

Posted Tuesday 01/14/25 @ 7:25 AM — Current highlights for this week’s weather:

  • Colder temperatures will be with us for today, Tuesday, and Wednesday
  • A weak clipper disturbance may bring some light snow showers Thursday evening.
  • Somewhat milder temperatures late Friday.
  • By Saturday, milder but with some rain likely.
  • A strong cold front moves through on Sunday.
  • There remains much uncertainty and model spread regarding low pressure expected to develop late Sunday afternoon. Depending upon the position along the coast, we may see up to 3 inches of snow by Monday morning. We may also see little to no snow.

There remains little agreement with the models regarding the second low expected to form later Sunday. Here’s the latest ECMWF—

Latest ECMWF showing low pressure off the coast but considerable surface activity over PA- 3 inches of snow forecast. Contours- WHITE- 32º surface temp; Critical Thickness for SNOW: RED- 1000m-500mb thickness; YELLOW-1000mb-850mb Thickness; MAGENTA- 850mb-700mb thickness

Here’s the latest AI version of the ECMWF forecast—

The ECMWF-AIFS shows little precipitation near us with the same storm Contours- WHITE- 32º surface temp; Critical Thickness for SNOW: RED- 1000m-500mb thickness; YELLOW-1000mb-850mb Thickness; MAGENTA- 850mb-700mb thickness (Click on image for a larger view.)

The GFS had been forecasting something even stronger than the ECMWF, but the latest run has moved closer to the ECMWF-AIFS,

GFS forecast for early Monday Snow in NJ only. Contours- WHITE- 32º surface temp; Critical Thickness for SNOW: RED- 1000m-500mb thickness; YELLOW-1000mb-850mb Thickness; MAGENTA- 850mb-700mb thickness (Click on image for a larger view.)

Of interest is the ICON which is very similar to the ECMWF—

German ICON model forecast Contours- WHITE- 32º surface temp; Critical Thickness for SNOW: RED- 1000m-500mb thickness; YELLOW-1000mb-850mb Thickness; MAGENTA- 850mb-700mb thickness (Click on image for a larger view.)

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So, there’s a range of forecasts and it’s too soon to call. Stay tuned.

Monday’s Weather

Posted Monday 01/13/25 @ 10:16 AM — Today’s clouds weren’t in any of the forecasts I heard last night and for good reason: the models didn’t forecast this level of cloud cover for today. The clouds are ahead of the first cold front expected to move through this afternoon—

Visible Satellite image this morning with superimposed RAP model pressure, 850 RH, and 3 hour pressure change. I’ve drawn the position of the cold front. (Click on image for a larger view.)

Based on GFS relative humidity, (but not the built-in cloud algorithms) this cloudiness may last through 4-5 PM today. Some scattered sprinkles or snow flurries are possible late this afternoon, mostly in far western areas.

The possible storms mentioned in yesterday’s Week Weather Outlook are more uncertain, with the GFS still predicting the Saturday rain and possible change to snow Sunday, while the ECMWF has most of it sliding off to our south, similar to the most recent light snowfall we had. With the models in such disagreement, there’s too much uncertainty to forecast this period.

The latest GFS -AI model (“GraphCast-GFS”) still shows a snowstorm possible next Tuesday instead of Sunday. With the limited experience I’ve had to date with the AI weather models, I haven’t seen them to be the magic bullet for predicting snow storms here that they’ve been touted to be.

Previously Posted Sun @ 10:13 AM — —Following two near average cold temperature days (Sunday, Monday), a series of cold fronts will bring much colder temperatures back to our area.

Generally dry, cold, windy weather will be with us through later Friday. Mostly sunny, but clouds at times due to upper air disturbances and instability.

There may be some snow showers Tuesday evening with the first front.

NAEFS mode version shows large high pressure and a tight pressure gradient to the upper/surface low in the Canadian North Atlantic on Wednesday. . Cold, Windy conditions expected. (Click on image for a larger view.)

The position of the cold air boundary will spawn a series of low pressure systems that will affect our area late Friday into Saturday and into the following week.

The first surface storm, late Friday through Saturday, will likely bring rain to our area, but may start as a mix in the northwest suburbs and may end as a mix or wet snow late Saturday or early Sunday.

NAEFS forecast for Saturday. Large storm north of the Great Lakes and a second low developing near Georgia will bring mostly rain on our region on Saturday, possibly starting as a mix Friday night in the northwest suburbs. Depending upon how the large H (high) in Canada moves in, and the development and track of the second low near Georgia will determine if we get snow on Sunday. The( ? ) denotes another area of potential storm development for the following Monday and Tuesday. (Click on image for a larger view.)

This week’s cold dry weather will likely be a prelude to more active weather systems after Saturday. Possible snow in the period Sunday through Tuesday.