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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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