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 06zNAEFS 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.
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
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.
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.)
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—
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 belowthat 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 Thursdayevening. 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.
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.
Back edge of precipitation (snow) moving through shortly with breaks of sun to follow. (Click on image for a larger view.)
Posted Tuesday 02/13/24 @ 9:26 AM — Current radar/satellite view as of 9 AM Back end of edge visible.
9 AM 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. (Click on image for a larger view.)
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—
02-13-24 11z HRRR snow totals by 11 AM (Click on image for a larger view.)
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!
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—
02-13-24 00z HRRR snow accumulation by 11 AM Tuesday. (Click on image for a larger view.)
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—
Tonight’s NAM-NEST shows the heaviest precip (rain plus snow water equivalent) has shifted to be over NJ instead of westward into Philadelphia. Over two inches of liquid ( as snow and rain) eastward into NJ. (Click on image for a larger view.)
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—
02-12-24 18z HRDPS shows unusual banding of snow right over parts of Philadelphia White inch labels refer to the blue contours that are spaced at 1 inch increments. (Click on image for a larger view.)
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—
02-12-24 18z Experimental RRFS model snow totals by noon Tuesday. The RRFS isn’t ready for prime time and while I look at them daily, I don’t use them yet for my forecasts. I just find them interesting.) Like the HRDPS, it also suggests heavy banding of precipitation near the city. (Click on image for a larger view.)
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.
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—
02-12-24 18z GFS forecast snow depth by 11 AM (Click on image for a larger view.)
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—
This afternoon’s NAM (18z) Snow Depth totals by noon Tuesday. Contours are 0.5″ increments.
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—
Latest HRRR (18z) forecast snow depth at noon Tuesday. (Click on image for a larger view.)
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.
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—
02-12-24 12z HRRR snow depth forecast Tuesday at noon. Blue contours are 1″ increments. (Click on image for a larger view.)
Updates later this afternoon.
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 —
02-12-24 06z NAM snow depth forecast by noon Tuesday. Still likely another outlier, but can’t be ignored as a possibility. (Click on image for a larger view.)
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.
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 —
02-11-24 12z NAEFS statistical “mode” version model combined with GEFS “bias-corrected” precipitation model forecast for Sunday afternoon (Click on image for a larger view.)
Here’s what the satellite water vapor image for the above time frame—
Sunday’s corresponding satellite water vapor image with superimposed RAP model 500-1000 mb thickness lines (yellow), Mean Sea Level Pressure isobars (black contours) potential vorticity (fine violet contours) with superimposed MRMS RADAR. The dashed red line is the 540 thickness line, a general rain-snow line. (Click on image for a larger view.)
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—
02-11-24 12z ECMWF forecast for 4 AM Tuesday morning. Secondary low forms. Note the position of the 32º surface isotherm (white) and the 540 thickness line, the traditional rain-snow dividing line based on average temperatures of the lower half of the atmosphere. I’ve also drawn in the 36.5º line, sort of a minimal near surface temperature for snow to fall.
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.
Sunday night’s 00z HRRR has rather extreme snow totals. Probably an outlier. (Click on image for a larger view.)
Posted Saturday 02/10/24 @ 10:16 PM — The storm for late Monday night into Tuesday will be a forecasting challenge. A secondary low will form off of the Delmarva coast, perhaps bringing in more cold air aloft. 1.1″ of water will fall, initially in the form of rain, mixing with and changing to wet snow, then tapering off during Tuesday afternoon. The changeover to snow or a wet mix near the city will occur between 5 and 8 AM Tuesday morning.
Here’s tonight’s NAM snow depth forecast by Tuesday afternoon—
02-11-24 00z NAM showing snow accumulation. Blue contours are in 1/2″ increments Above freezing near surface air temperatures, warm ground temperatures and rapid precipitation rate with dynamic cooling will make forecasting snow accumulation a challenge. (Click on image for a larger view.)
Sat 5:40 PM —Forecast Review — It looks like the temperatures at the two local airports didn’t get above 57º (KLOM -Blue Bell) and 54º (KPHL- Philadelphia). The NBM, my go-to-model for temperatures, really didn’t impress today, while the GFS and HREF were right on the mark. Not surprised with the lack of any really strong sunshine. For the record, the RRFS model (experimental, being developed) was too warm.
Posted Saturday 02/10/24 @ 10:33 AM — Many models are showing highs today in the 60-62º range, but the GFS and HREF continue to keep us in the mid to upper 50s.
Here’s the latest NBM—
02-10-24 12z NBM high temps for Saturday. All highs are ± 1.6º (Click on image for a larger view.)
As for the ‘snow’ for Tuesday morning, littleaccumulating snow is expected. The latest GFS captures it well—
02-10-24 06z GFS accumulated snow totals Tuesday morning. (Click on image for a larger view.)
Updated Fri 9:12 PM — Tonight’s early models are trending a bit more towards some widely scattered light sprinkles during the day Saturday and a few heavier showers early evening with the cold front. The NBM shows low 60s in NJ.
Previously Posted Fri 6:05 PM —
The blocked pattern giving us this past week’s fair weather and now increasingly warm weather will become a thing of the past by late Monday. But first things first.
A slow moving cold front will approach Saturday, preceded by clouds and the chance of light widely scattered sprinkles in mostly far western and northern suburbs. (An area of very light showers may move in during the early evening Saturday just ahead of the cold front. )
Most of the showers are expected to stay to our north and to our south on Saturday. Current water vapor shows the setup—
Current satellite water vapor image (Friday afternoon) with superimposed RAP model 500-1000 mb thickness lines (yellow), Mean Sea Level Pressure isobars (black contours) potential vorticity (fine violet contours) with superimposed MRMS RADAR. Most of the activity on Saturday stays to our north and to our south. (Click on image for a larger view.)
On Sunday, the front stalls to our south and moisture moving to our south will spill up towards us in the form of cloudiness. No rain expeacted.
The moisture plume to our south will be the launch pad of a developing storm for late Monday into Tuesday morning.
Saturday
Cloudy. Showers mostly north west and south of our area as described above. The latest NBM has temperatures close to 60º, despite the cloudiness. The Canadian HRDPS, which tends to run warm, has us up to 60º while the GFS has us only in the mid 50s. Windy during the evening hours.
NBM high temperatures: Blue Bell, PA 59º Philadelphia, PA 60º slightly above average uncertainty (based on standard deviation): ± 2.0º
Sunday
Cloudy, cooler, but still temps above average.
NBM high temperatures: Blue Bell, PA 49º Philadelphia, PA 51º slightly above average uncertainty (based on standard deviation): ± 2.1º
Monday into Tuesday Outlook
Another rainstorm, begins towards midnight Monday and changes to wet snow from northwest to southeast into the city. A heavy 1.4″ of water falling (rain) with about 0.5″- 1.5″ (preliminary estimate) wet snow accumulation upper Montgomery, western Chester and northern Bucks counties.
Forecasts have trended to show increased intensification of this low pressure system. Current lack of cold air will be a major factor in snow totals. The latest GFS shows secondary coastal low formation. Stay tuned for updates.
GFS shows secondary low formation. Precipitation may ‘skip over our area’ (Click on image for a larger view.)
