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 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.)
