#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—

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 —

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—

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—

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—

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 —

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—

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—

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)—

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

Specifics:

• 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—

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—

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 —

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.

Here’s the NBM 75 Percentile amounts—

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—

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—

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—

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.

The Friday night potential storm—

Tuesday Storm

Show More

Posted Tuesday 02/13/24 @ 11:06 AM — Back edge of snow moving through before noon here. Visible satellite image superimposed shows sunny breaks in Lancaster County will be here likely between 12:30 PM and 2 PM.

Posted Tuesday 02/13/24 @ 9:26 AM — Current radar/satellite view as of 9 AM Back end of edge visible.

The storm is rapidly moving to our east-northeast.

Posted Tuesday 02/13/24 @ 9:03 AM — Latest HRRR (12z) has the sun breaking out between 11 AM and 12 noon

Posted Tuesday 02/13/24 @ 8:05 AM — A quick update with the 11z HRRR. Snow should be tapering off by 11 AM. This is likely closest to the actual final snow totals—

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Update Tue 2/13 4:23 AM — Woke up at 4:15AM and took a quick look at the 06z HRRR.

Large snow accumulations again being forecast – 8 inches!!? Near Philadelphia!

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Posted Monday 02/12/24 @ 9:27 PM — Tonight’s NAM and HRRR have moved in the same direction, trending downward from the rather extreme snow accumulations forecast this afternoon. The reason for the change in snow totals— the main axis of the heaviest precip (total water) has changed to be over NJ where it will be warmer. Here’s the latest—

I’m going to stick with this forecast. The change might reflect model “spin up” error, but I’m going with this.

Addendum: The heaviest precip (total water) axis is now in NJ instead of over Philadelphia. Here’s tonight’s NAM-NEST—

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Tuesday Storm Update

Posted Monday 02/12/24 @ 7:20 PM — This afternoon’s Canadian High Resolution model (HRDPS) has trended towards the NAM and HRRR forecasts—

Here’s the key points posted earlier today and somewhat revised—

• Heavy rain at night changes to snow. (Storm Total Liquid 1.0-1.6″ water equivalent.)
• A changeover to snow occurs between 6 AM and 8 AM, perhaps with a period of heavy sleet. (earlier changeover far west/northwest)
• A period of heavy wet snow for about 2-3 hours between 8-11 AM.
• Snow tapers off by before noon with perhaps some lingering snow showers or rain showers.
• Temperatures may get closer to freezing than previously forecast.
• Dynamic cooling will bring the air temperature to 33º during the heaviest precipitation.
• Strong wind gusts expected in the morning hours and into the afternoon.
• Rapid melting expected on roadways in the early afternoon. Solar insolation (radiant energy through clouds) through clouds and warm existing ground temperatures will come into play.

I was curious if they would run the RRFS model today (new, experimental model under development). It looks like today’s 18z RRFS ran and here’s its snow totals—

I’ll have the next HRRR about 8:40 PM and the next NAM about 9:05 PM. I’ll update around 9:30 PM.

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Posted Monday 02/12/24 @ 5:09 PM — Earlier, I posted the models with the highest snow totals. The latest GFS just available remains in the 1-3″ range. I don’t think it is capturing the dynamic cooling. I’m leaning towards the NAM and the HRRR posted earlier and a likely average of the two models.

Here’s the latest GFS at the lower end of the spectrum—

The GFS also shows 1.1-1.4 inches of water (rain plus snow water equivalent) falling with this storm.

Posted Monday 02/12/24 @ 4:09 PM — Additional confirmation of the trend set by the HRRR. This afternoon’s NAM is also forecasting higher snow totals—

Tuesday Storm may have More Snow

Posted Monday 02/12/24 @ 3:47 PM — Since last night, the HRRR has been forecasting greater accumulating snow than many other models.

That trend continues with the latest 18z HRRR which has increased the precipitation rate in the morning, resulting in total liquid precipitation in the form of water (and snow water equivalent) to 1.5. Due to the high precipitation rate, “dynamic cooling” will be significant and temperatures will approach 32º allowing greater accumulation in our area.

The latest HRRR total snowfall is an eye-opener—

As mentioned this morning, when I posted the HRRR large snow totals last night, it was absolutely an outlier. I think these snow accumulation amounts make sense, considering the extremely high precipitation rate and the amount of dynamic cooling that will occur. So far, I haven’t seen other models join this forecast, but I’m sure we’ll see some confirmation by early this evening.

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Monday: Snow Update

Posted Monday 02/12/24 @ 11:57 AM — The morning models are out and the spread in snow totals is quite large. Most models (GFS, NAM-NEST, HRDPS and RGEM have 0.5-1″ near the city to as much as 3″ northern Montgomery and Bucks counties. The HRRR and NAM continue to forecast a burst of snow between 8 AM and 11 AM with significantly higher totals. I’ve posted the 06z NAM earlier this morning and the 12z NAM is similar.

Rain moves in around 6:30 to 8 PM tonight.

• Heavy rain at night changes to snow. (Storm Total 1.0-1.3″ water equivalent.)
• A changeover to snow occurs between 6 AM and 8 AM, perhaps with a period of heavy sleet.
• A period of heavy snow for about 2-3 hours between 8 AM and 11 AM from the city westward.
• Snow tapers off by noon with perhaps some lingering snow showers.
• Temperatures won’t get to or below freezing. Dynamic cooling will bring the air temperature to 33º.
• High wind gusts expected in the morning hours and into the afternoon.
• Rapid melting expected on roadways. Solar insolation through clouds and warm existing ground temperatures come into play.

Here’s the 12z HRRR—

Updates later this afternoon.

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Posted Monday 02/12/24 @ 8:33 AM — Heavy rain moves in tonight with the aforementioned coastal storm that develops this evening and then moves off to the east-northeast. The latest models are still in the 0.5-2 inch range, including what I deemed to be an outlier, last night’s HRRR, posted last night. (Too many tortilla chips and salsa during last night’s game probably impaired my judgement in having posted that likely outlier model.)

However, regular readers here know my mantra, “never ignore the NAM”

The latest NAM has rain changing to snow about 7AM with heavy precip through about noontime Tuesday. Today’s 06z NAM is looking closer to the last night’s outlier HRRR —

I was hoping to be able to post the experimental RRFS, but apparently this still-experimental model is down for repairs for having crashed.

One thing not mentioned are the high wind gusts expected with this storm, in the 35 -40 mph.

More models, of course coming out all day. I’ll update later. Stay tuned.

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Originally Posted Sun 10:56 PM — A strong southwest to northeast jet flow will bring a deepening low pressure system to our area late Monday into early afternoon Tuesday.

NAEFS jet flow —

Here’s what the satellite water vapor image for the above time frame—

A cloudy day Monday with rain from the approaching low arriving here in the Delaware Valley around 6 PM, earlier than previous forecasts.

Here’s the ECMWF forecast for Tuesday at 4 AM, as the primary low spawns a secondary coastal low—

The models are still forecasting 1.1- 1.3″ of water to fall, mostly as rain in our area. Only a small portion of that will fall as wet snow. Temperatures near surface will be above freezing the entire event in both the immediate PHL area and the immediate suburbs which will reduce accumulation.

The snow will fall during the height of solar insolation (solar radiant heat through the clouds), which becomes more of a factor in February, further reducing lingering accumulations, especially on roadways.

The latest HRRR this evening appears to be an outlier. Most models still show a slushy 1-2″. Tonight’s HRRR is an eye-opener and I’m posting it because the HRRR is probably our most advanced model.