Smithfield, Cache Valley (4 PM Sun): 12"
Ogden (5 PM Sun): 2"
West Valley City (6 AM Mon): 5.2"
Salt Lake Airport (5 AM Mon): 3.6"
Cedar Hills, Utah County (5 AM Mon): 9"
Powder Mountain (5 AM Mon): 12"
Brighton Crest (5 AM Mon): 16"
Alta-Collins (3 AM Mon): 14"
Spruces (4 AM Mon): 10"
Through 8 AM, Alta Collins is up to 18", blowing my 7-14" expected through 9 AM out of the water.
With the KMTX radar up-and-down overnight (and currently down), getting a good handle on the action is more difficult than usual. One thing that caught my attention is described in the tweet below, issued last night. From 8-10, Alta-Collins recorded 6" of snow (3"/hr mean rate) and I saw some tweets of impressive accumulations near Alpine as well. During this period, a very pronounced band extended across northern Utah County and far northwest Wasatch County, and stronger radar reflectivities also lingered over the upper Cottonwoods.
One option when KMTX is down is to examine data from the Terminal Doppler Weather Radar (TDWR) operated near the east shore of the Great Salt Lake west of Farmington. The primary purpose of this radar is to detect hazardous weather and wind shear over the airport. It is blocked significantly by the surrounding topography, and provide little to know information about what is happening over the Wasatch and the broader region. You can see, however, returns related to snow showers over portions of the Salt Lake Valley.Recent radar loop showing band of persistent higher returns just south of Utah/Salt Lake County line extending across extreme NW Wasatch County. Also, higher returns hanging on over high terrain surrounding LCC including @AltaSkiArea and @Snowbird pic.twitter.com/Re8CA4djap— Jim Steenburgh (@ProfessorPowder) February 19, 2018
Periods of snow look to continue today and tonight in the mountains and also in the valley. The latest NAM forecast shows us transitioning into deeper northwesterly flow during the day today, with the flow transitioning to westerly overnight.
The situation for tonight is really interesting. The latest forecasts show a pocket of remarkably cold air moving over northern Utah, with the latest NAM forcasting -22ºC over the Great Salt Lake at 1200 UTC.
In fact, the absolute minimum is an astounding -22.9ºC at 1500 UTC (8 AM). That is a remarkably low 700-mb temperature. It would not be a record for February (-25.9ºC is the all-time low), but we don't see too many days around here with temperatures that cold.
The Great Salt Lake is actually not that warm compared to climatology, but still, the average lake-surface temperature is almost 4ºC.
As a result, the HRRR is fairly excited about a possible lake-effect event tonight.
Our statistical forecast system, based on the 6Z NAM, shows elevated lake-effect probabilities ovrenight tonight, peaking at 90% at 2 AM. Note how the affected area shifts from the Cottonwoods to the northern Wasatch with the flow shift overnight.
There are, however, three important issues to keep in mind. The first is that wind directions beneath these upper-level troughs are tricky to forecast, so one can't count on that forecast precisely.
The second is that we tend to fixate on lake effect, when we need to keep in mind that we could see post-frontal snow showers generated by other processes.
Finally, the third is the cold. This is a remarkably cold airmass. Temperatures at crest level are going to be so cold that instead of favoring dendrites, they will favor higher density crystals. I've seen situations where this has put a damper on snow amounts in the past. There aren't a lot of days at Alta that are this cold, but if you look at the snow-to-liquid ratios when the 650-hPa temperature is below -20ºC, there is a tendency for lower values.
I'm thinking another 3-6" in the upper Cottonwoods through 6 PM this afternoon. After that, I'm happy to sit in my ivory tower and watch this one play out. Hopefully the radar will come up.
Jim -- Snow today (at PCMR) was much denser/wetter than last week's 18" surprise at Alta. Any comments about why, especially given the fairly cold air temps? Thanks. CHH
ReplyDelete
DeleteI worked today and didn't get a first hand look.
A few thoughts. First, although it was cold today, much of the snow fell overnight when temperatures where higher. Nevertheless, it wasn't a warm storm by any measure.
A few other possibilities. Riming could have played a role given the intensity of the band and the fact that thundersnow was reported in some areas. Stronger updrafts allow ice crystals to become more rimed. Another is that the storm was deep, so there may have been at least some non-dendritic, higher density crystals.
Sometimes wind plays a role. I wasn't out today to get a handle on that.
Jim
Snow at the Bird was Utah blower, no wet/dense there.
DeleteAfter the storm, Snotel is showing 53 percent of 1981-2010 base water year, down from 70 percent at beg of month. Kurt
ReplyDeleteThe 2.5" of water in the past week at Snowbird is a bit more than average for a week in February, but for the month as a whole, we are still losing ground to climatology.
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