Category Archives: Philadelphia Weather Outlook


#Philadelphia #weather #PAwx

Thursday – Friday Outlook

Posted Thursday 02/22/24 @ 9:54 AM — Clouds will be on the increase later this morning and it should be mostly cloudy by early afternoon. Light showers expected to move in from the west between 5 PM and 7 PM tonight.

There’s still a trend for the rainfall to be on the light side. The latest model blend shows these amounts—

02-22-24 12z NBM model blend – total rainfall through Friday. (Click on image for a larger view.)

NOAA’s new model under development, the still-experimental RRFS, has show some big advances in its development over the past month with its own data inputs and some advances to its ensemble version. Just to see, here’s the current RRFS -Ensemble total rainfall forecast —

02-22-24 06z RRFS Time-Lagged Ensemble Model total rainfall (Click on image for a larger view.)

It looks like light rain/drizzle lingers into mid afternoon Friday, with some clearing before sunset Friday.

The weekend looks cold, although somewhat less so on Sunday.

Posted Wednesday 02/21/24 @ 7:06 PM — Sometimes the latest models are not the best in forecasting the immediate upcoming 6 hours. Such was the case with this morning’s HRRR and NAM-NEST which forecast more clouds than we had. Forecasting can be a challenge, even if supported by satellite imagery.

As for Thursday, clouds moving in ahead of a system to our west will have Thursday becoming increasingly cloudy. By late morning it will be mostly cloudy. The models are fairly consistent forecasting some very light rain moving in by 5 -7 PM. There’s a wide spread in forecast high temperatures: 46º – 48º ± 2.6º.

The big question continues to be the amount of rain that will fall here in the immediate Philadelphia area Thursday night through early Friday. There’s tremendous model differences, with the majority of the models having most of the rain bypassing our area to our north and south. The amount of rain forecast varies from a few hundredths of an inch to almost about 0.60″ inch of rain. The NBM (model blend) falls in the the low range of about 0.20″.

Here’s the latest GFS rainfall forecast. The Philadelphia area is in a doughnut hole—

02-21-24 18z GFS accumulated rain forecast by Friday evening (Click on image for a larger view.)

As was the case with today’s cloud cover, the models are having difficulty resolving the current weather pattern. As was the case yesterday, I expect further changes in the forecast.

Wednesday Update

Posted Wednesday 02/21/24 @ 9:24 AM — A change in the forecast. The low pressure system in the Atlantic, originally expected to bring some cloudiness only to eastern NJ has pushed moisture further west. This morning’s HRRR and NAM-NEST show cloudiness pushing westward. The latest visible satellite image—

Visible Satellite image 9 AM today (Click on image for a larger view.)

On water vapor imagery, a wind change line has set up, which I’ve drawn in as warm front. East of this line, we’ll have increased cloudiness—

Water Vapor image this morning. (Click on image for a larger view.)

So today looks to have more cloudiness, with sunshine through high and mid level clouds, than previously forecast especially from just west of the city and eastward.

As alluded to yesterday, I expect to see forecast changes over the next day or so. The second system in the Atlantic is probably complicating the model forecasts.

Wednesday – Friday Outlook

Posted Tuesday 02/20/24 @ 7:50 PM —— Continued near -seasonable temperatures Wednesday with highs in low 40s with plenty of sunshine.

Things warm up a bit on Thursday, but with clouds moving in early afternoon. Highs in the mid to upper 40s. Rain should wait until late evening, but there have been some hints that we may see it start earlier.

There’s still some uncertainty about the rain expected late Thursday evening into Friday. Most models have very light rainfall for our area (0.20″), but there have been some model runs that crank out much more (0.60″)

The GFS has the rain ending by noon, but the NBM shows light rain showers lasting into Friday afternoon, perhaps from another low that forms along the cold front —

02-20-24 18z GFS shows weak low pressure over our area Friday noontime with much of the rain missing us. I’m just not yet convinced of this. There’s plenty of ‘bagginess’ in the isobars (white box) that often suggests formation of another low. (Click on image for a larger view.)

It would not surprise me if the light rain forecast for Friday changes in the coming days. Regardless, plenty of wind is expected later Friday afternoon.

Tuesday-Thursday Outlook

Posted Monday 02/19/24 @ 7:37 PM — There’s been clarification with the forecast for this week. We’ll be mostly sunny Tuesday through very early Thursday. (Some clouds from large storm system in the western Atlantic may spill back into NJ on Wednesday.) Temperatures will approach 50º by Thursday.

Low pressure approaches on Thursday. Thursday will have increasing cloudiness with rain moving in from the west as early as late afternoon or early evening. It will be rainy on Friday. It’s not clear how strong the storm will get on Friday. The ECMWF has only 1/2 inch of rain falling from this system—

02-19-24 12z ECMWF forecast for Friday at 1 PM. Low pressure moves almost over us and the northeastward. No cold air in place with the freeaing line (white contour) in Canada! (Click on image for a larger view.)

Previously Posted Sun 9:17 PM —

Quiet weather is expected the early part of this week and the thought was there would be a general moderating trend in temperatures by Wednesday or Thursday. Looking at the latest NAEFS, however, I’m seeing large forecast changes from previous model runs in the overall jet level pattern, especially by Thursday.

More so, the coast storm that was expected on Friday may not develop as previously forecast.

Frankly, the current forecast upper air map is a bit confounding.

Here’s the NAEFS jet level forecast for Wednesday—

02-18-24 18z NAEFS jet level and surface level combined forecast for Wednesday at 4 PM. Developing warm upper ridge in the center of the country doesn’t make big progress into our area with a persistent trough (blue diagonal line) over the coastal northeast hanging tight through Wednesday. It may reach us by Thursday. Low pressure forecast over Kansas may not trigger coastal development near the question mark.

So, until things clarify, I don’t have my usual big picture summary of the coming week’s weather. Right now, I think we can safely forecast through Tuesday. And maybe expect some milder weather by Thursday.

For Monday we can bank on sunny skies.

Tuesday will be sunny in the morning with some high clouds later in the afternoon as a warm front tries to approach.

For the rest of the week, we’ll have to wait to see if the warm front makes it here and we’ll have to determine what will happen with the low pressure system. Some time next weekend looks colder again.


#Philadelphia #weather #PAwx

Why are the TV/Radio Snow Forecasts So Low? (Again)

Posted Friday 02/16/24 @ 9:40 AM —I’ve been scratching my head trying to understand why the TV/radio snow accumulation forecasts for tonight’s storm are too low again. Most models are cranking out 0.3-0.5″ of water precipitation tonight. Even with a conservative snow to water ratio of 10:1, it would mean 3 to 5+ inches of snow for us, not the 1″-3″ I keep hearing on the broadcasts. With snow accumulation, one usually has to account for ‘snow compaction’, but that’s a lot of compaction to bring it down to 1-3″ of snow.

Hey, I might be wrong with this forecast, but long time followers of this blog know I have a pretty good track record.

I’m inclined to go with the HRRR model again.

Here’s the latest HRRR (12z run, just available)—

02-16-24 12z HRRR snow forecast by Saturday at 7 AM Some additional snow possible to this. (Click on image for a larger view.)

Here’s the NAM with a 10:1 water snow ratio—

02-16-24 12z NAM snow totals based on a snow to liquid ratio of 10:1 without compaction. (Click on image for a larger view.)


• Snow starts about midnight tonight (Friday).
• Snow tapers off west to east about 7-9 AM Saturday.
• Some light snow showers or flurries possible, especially western sections, until about noon.
• Windy and gusty mid day Saturday.
• Unsettled weather – Clouds with breaks of sun during the afternoon, but with another possible area of snow flurries passing through.
• High of only 36º-38º

Friday Night into Saturday Snow Update

Posted Thursday 02/15/24 @ 5:25 PM — The fast moving storm expected here late Friday into Saturday morning has just come into the forecast range of the HRRR model. (The HRRR did extremely well with our last snowfall.)

Here’s the latest HRRR snow totals forecast—

02-15-24 18z HRRR Snow accumulation forecast by 9 AM Saturday. It’s showing a band of the most snow from the city southward. (Click on image for a larger view.)

Both the NAM and NAM-NEST model built-in snow depth parameterization are forecasting less, about 1.6 -3.0″. However their forecast precipitation amounts with a snow water ratio of 11:1 brings their snow total closer to the above HRRR.

As mentioned, unlike the last storm, this will be a lighter snow, easier to shovel, and immediate melting will be slower, due to the colder temperatures that will move in behind the system.

As for of winds, tonight (Thursday night) will be quite windy after the first system moves through with cold front and a few showers. Sunday looks windy too—

NBM wind meteogram for Blue Bell, PA (Click on image for a larger view.)

Friday Night into Saturday Snow Update

Posted Thursday 02/15/24 @ 10:16 AM — We’re just getting into the range of some of the higher resolution model forecasts (60 hrs) and some of the NBM snow accumulation forecast stats (up to 60 hours). Not yet in the range of the HRRR (48 hours).

The current track and general intensity of the storm is unchanged from my earlier update last night. The ECMWF has bumped up its total precipitation forecast up to 0.33 inches of water which translates into higher snow totals.

The NBM mean snow total forecast for Friday night into Saturday —

NBM mean (or average) snowfall model prediction. (Click on image for a larger view.)

The NBM mean and median snowfall forecast snow amounts are similar, suggesting a tendency towards the mean.

However, several models that comprise the NBM have higher snow accumulations. The NBM 75 percentile covers those models.

(The concept of percentiles is often hard to get one’s head around. The 75th percentile means that 75% of the models that comprise a group of models are at or below that value.

It does NOT mean that 75% of the models are predicting that amount, nor does it mean that there’s a 75% chance of that amount.)

Here’s the NBM 75 Percentile amounts—

NBM 75 Percentile snow accumulation. This percentile takes into account some models leaning towards higher snow amounts. (Click on image for a larger view.)

While it’s too soon to hang one’s hat on snow amounts and the locations of likely higher snow totals, I’m leaning toward 3″, possibly 4″ in much of the area. I base this on the higher snow:liquid ratios and total precip amounts being forecast.

Additionally, we’re forecast to be near what’s called the left exit region of a jet streak. It’s an area of upward vertical motion and enhanced precipitation. We’re close, but not in an ideal position.

We really won’t have a real handle on snow totals until Friday morning’s models.

Friday Night Snow Update

Posted Wednesday 02/14/24 @ 8:03 PM —Fast moving low pressure systems will move (1) north of us Thursday evening and (2) south of us Friday night into Saturday morning—

Current satellite water vapor image with superimposed RAP model 500-1000 mb thickness lines (yellow contours), Mean Sea Level Pressure isobars (black contours) potential vorticity (fine violet contours) with superimposed MRMS RADAR. System 1 will be minor. System 2 is looking more interesting. (Click on image for a larger view.)

To focus on System 2 (Friday night into Saturday), it will be a fast moving system, but the latest models are showing a trend towards increased moisture and snowfall.

More importantly, the latest NAEFS shows the track somewhat closer to our area than previous model runs, meaning more snow for us—

NAEFS statistical “mode” version model combined with GEFS “bias-corrected” precipitation model forecast for Saturday 4 AM (Click on image for a larger view.)

Models are cranking out 0.20 to 0.40 inches of water, falling as snow. The NBM shows a snow:liquid ratio that high, on the order of 12:1-15:1. So I’m thinking we’re possibly dealing with 2-4 inches of snow, possibly a bit higher.

The storm exits quickly and we should see some sun Saturday afternoon. Unlike the previous storm, it will turn very cold and windy Saturday, so melting will not occur as it did the last storm.

The storm will fall into the range of the higher resolution models tomorrow. Stay tuned.

Update: Wednesday through Saturday

Posted Wednesday 02/14/24 @ 9:04 AM — Today, Wednesday, will be sunny but quite cold and windy. Highs only in the mid 30s.

Thursday will become mostly cloudy by mid to late morning, as mild air moves in aloft and a cold front approaches from the west. High 42-45º with above average uncertainty.
A few widely scattered sprinkles (rain, not snow) move in Thursday evening. The cold front passes through before midnight. Winds pick up during the night.

Friday will be very windy in the morning and sunny. Clouds move in late afternoon ahead of the next system. High temp 40-43º

It now appears that the low pressure system passing by to our south Friday evening be be a bit further south. Light snow develops towards midnight Friday evening and ends around early Saturday morning. Best guess about snowfall is about 1.5-2″, slightly more further south of the city.

It’s really too soon to be posting any forecast snow totals, but here’s the current NAM snow totals by Saturday morning—

02-14-24 06z NAM model forecast snow depth at 10 AM Saturday. White contour is the 32º line . (Click on image for a larger view.)

I would be posting the HRRR snow totals, given the model’s success with the recent storm. The HRRR only forecasts out to 48 hours. The NAM forecasts out to 84 hours.

Saturday will be fairly cloudy, windy with some snow flurries. Cold with highs in the upper 30s.

Wednesday and Thursday Outlook

Posted Tuesday 02/13/24 @ 4:21 PM — Cold high pressure builds in for Wednesday. Two disturbances will affect our weather later this week. A clipper type disturbance to our north may bring some light snow showers late Thursday, as a cold front moves through.

A somewhat more robust but fast moving system shows a good chance of bringing additional snow late Friday into Saturday morning.

Current satellite (Tuesday afternoon) water vapor image with superimposed RAP model 500-1000 mb thickness lines (yellow), jet level wind streamlines (orange contour-arrows) potential vorticity (fine violet contours) and superimposed MRMS radar. Disturbance 1 is a clipper type with another cold front for Thursday. Disturbance 2 shows the potential for snow late Friday into early Saturday. (Click on image for a larger view.)

The Friday night potential storm—

NAEFS statistical “mode” version model combined with GEFS “bias-corrected” precipitation model forecast for Saturday 7 AM (Click on image for a larger view.)
Tuesday Storm
Tue 11:12 AM —Forecast Review — So why did the radio and TV weather people wait so long to update their forecast for today? I’m not sure but they were going with a low accumulation range forecast through much of Monday, then they did a fast change in forecast without explanation last night.

My guess is they were highly influenced by the ECMWF model forecast. I think they’re enamored with this model (German engineering, Italian design and high cost), despite evidence that our US – NOAA models are often better. I know, in this case, our HRRR was forecasting the possibility of larger snow totals as early as Sunday night.

The new RRFS model that’s currently being developed is an evolution of the HRRR/NAM-NEST/HREF and while it currently is not ready for prime time, I’m hoping that it will be a go-to model for snow storms here.

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