THIS WEEK’S WEATHER

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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.


WEEKEND WEATHER FORECAST

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Sunday Forecast Update

Posted Sunday 02/18/24 @ 9:42 AM — Sunny and windy today. The cloudiness that I thought would impact northern areas today will remain even further to our north. Highs 41-43º. Wind gusts near or over 30 mph.

NBM wind meteogram for Wings Field, Blue Bell, PA (Click on image for a larger view.)
This Week’s Peek

The coming week looks tranquil. A coastal storm has been consistently forecast for this Friday. It looks like rain, not snow here—

02-18-24 06z NAEFS statistical “mode” version model combined with GEFS “bias-corrected” precipitation model forecast for Friday (Click on image for a larger view.)

Philadelphia Inquirer Article on the Snow

Sunday’s Philadelphia Inquirer had an article on the snow forecast that didn’t quite make the grade. I would have added a comment about the ground temperatures that were well above freezing and melted much of what fell initially and allowed accelerated melting in the morning.

More Thoughts about the Storm

It looks like many areas did receive snowfall as predicted by yesterday’s forecasts. (The snow totals reported to the NWS can be found here.)
Most were in the 2-4 inch range. Areas far north received over a foot of snow.

Montgomery county: 1″ – 4.2″
Delaware county: 3″ – 4″
Chester county: 1″-4.8″
Bucks county: 1″-13″
Philadelphia county: 2.3″ -3.5″

With soil temperatures and surface temperatures so warm and radiant energy from the sun causing a melt, it’s been amazing how fast the snow has disappeared today. I measured 2.7 inches at my home at 8 AM, but by noon, there wasn’t any snow to shovel from our sidewalk. The part of our driveway that gets the sunshine had nothing to shovel either.

I’ve learned something from this storm. Surface (skin temperatures) and ground soil temperatures play a large part in how impactful a snowfall will be. Most of my focus has always been on temperatures at different levels of the atmosphere and whether the precip will fall as rain, sleet, or snow.

In the future I am adding a review of soil ground temperatures to my forecast checklist. (The models have that information and today I added it to my HRRR data downloads.) With our milder winters and longer periods of warm weather between storms, the residual heat in the ground will require greater attention.

Sat 9:05 AM —Forecast Review — A beautiful sight out the window this morning and a not-so-good forecast. Let me cut to the chase: what went wrong with the forecast—

• Temperatures remained well-above freezing before the storm (35º Blue Bell Wings Field) until it started snowing at 12:15 AM.
• Temperatures remained somewhat warmer than predicted throughout the storm.
• Ground temperatures (‘skin temperatures and soil temperatures”) were above freezing for much of the storm near the city.
• Precipitation amounts (in water) were very different from what was predicted.
• Banding of precipitation and snowfall occurred far north of our area.

The MRMS summary gives a handle on how much precipitation (water or water equivalent) actually fell and where—

MRMS combined rain gauge & radar-based measurement of actual rainfall received. Color shading is in inches. Line contour numbers are in mm (25.4 mm= 1 inch) These are liquid water measurements, not snow depth (Click on image for a larger view.)

From the above, we can see that the storm’s precipitation occurred in the city and immediate suburbs in the range forecast (0.30″) , but south of the city in Delaware county and Chester counties, precipitation was much lower than forecast. The banding occurred far north with heavy snow in that region.

It’s funny. I pointed out that the media weather people were under-forecasting the snowfall. Eventually they raised their forecast to a “Winter Storm Warning” last night, only to see that their original forecast was probably better.

Meanwhile, I saw signs of this forecast change, in last night’s HRRR, but I was reluctant to call it and attributed the reduction in snow to model spin up errors. Go figure.

Snow Update

Posted Friday 02/16/24 @ 9:00 PM — Some trends are becoming apparent in reviewing additional models. The high snow accumulation values predicted this morning and this afternoon have been eroding downward.

The latest HRRR (00z) just available more than captures this significant downward tend—

02-17-24 00z HRRR just available shows a significant decrease in overall precipitation.

When I saw the trends with tonight’s HRRR and other afternoon models, I wasn’t sure if I would post it. I can think of many good forecasts that were derailed by my posting the ‘latest model’ and going with it.

There is a known phenomenon called “model spin up time”. Basically, it takes several hours before model forecasts become accurate They can take as long as 6 to 10 hours to be valid in an area, especially when a storm is on one’s doorstep. Put another way, the ‘latest model’ is often inaccurate in the short term. That could be the case here.

The latest satellite and radar shows the storm may be splitting into two centers, explaining the HRRR’s corridor of low snowfall above.

MRMS and Theta E (thermal potential energy levels) contours at 8:45 PM (Click on image for a larger view.)

I guess we’ll find out when we wake up tomorrow morning.


Originally Posted Fri @ 4:57 PM — —A fast moving storm will bring snow to our area tonight (Friday) late evening and night. The storm is visible on WV imagery—

Current( 4 PM) 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. (Click on image for a larger view.)

The light snow will move in from the west as early as 11 PM, but most models have it moving in about 1 AM.

Some banding in snowfall has been expected to develop and the heavier snow is now forecast to be north of the city, instead of south of the city as forecast earlier. A generalized 1.5-5 inches of snow is expected.

I just wanted to note that several models, specifically the German ICON and the Canadian HRDPS/RGEM, are showing lesser snow amounts, closer to 1.5-2 inches overall. But I’m staying with the HRRR and NAM.

As is always the case, the models differ in their placement of the heaviest snowfall. Here’s the latest HRRR (18z) —

02-16-24 18z HRRR snow accumulation forecast (Click on image for a larger view.)

Many long time followers of this blog know my preference for the NAM for snowfall forecasts here. Here’s the latest NAM (18z)—

02-16-24 18z NAM snow accumulation forecast. (Click on image for a larger view.)

Saturday

The storm ends by 7-9 AM in most areas Saturday morning, but instability cloudiness and snow flurries may be with us for much of the morning and even some of the afternoon. From this morning’s update—

• 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 skies – Clouds with breaks of sun during the afternoon, but with another possible area of snow flurries passing through.

NBM high temperatures: Blue Bell 37º Philadelphia 38º
uncertainty average (based on standard deviation): ± 1.8º

Sunday

Sunny early but with cloudiness moving in late morning, especially areas north of the city due to an upper air disturbance. Rather windy.

NBM high temperatures: Blue Bell 38º Philadelphia, 40º
uncertainty low (based on standard deviation): ± 1.6º


Monday

Sunny but still with temperatures a bit below season averages

NBM high temperatures: Blue Bell 41º Philadelphia, PA 43º
uncertainty average (based on standard deviation): ± 2.0


Weather… and Other Things 'Up in the Air'