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| IR satellite imagery from 0600–1400 UTC 29 Jan 2016 |
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| GFS integrated water vapor transport forecast valid 0600 UTC 30 January 2016. Source: NWS. |
The integrated water vapor transport associated with this atmospheric river is quite high for late January and lies at the outer edge or outside values observed in the climate forecast system reanalysis for this time of year.
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| Source: NWS |
Bottom line #1: This is an unusual event for this time of year. And, a strong cold front will be moving through northern Utah tomorrow morning, giving us a potent double whammy.
The NAM continues to put out some big water numbers for Alta Collins. The 0600 UTC run (I'm writing this early in the morning) goes for 2.22" of water through 5 PM tomorrow. As discussed yesterday, I'm a bit concerned the flow direction overnight will not be optimal for Alta and thus think this number may be high for the Cottonwoods. In contrast, the northern Wasatch might do better.
Let's instead have a look at the higher-resolution NCAR ensemble. For the 48-hour period from 0000 UTC 29 Jan - 0000 UTC 31 Jan (1700 MST Yesterday - 1700 MST Saturday), you can see that the mean of the members (upper left hand panel) keys in a bit more on the northern Wasatch and to some degree the area near Mt. Timpanogos, which are favored in the predominantly westerly or west-southwesterly flow accompanying the atmospheric river. The numbers for Snowbasin and Ben Lomond aren't comparable to the central Wasatch, but I suspect even at 3-km, the model isn't quite picking up on the narrow Wasatch Range in that area.
Looking at Alta-Collins, there is remarkably little spread in the ensemble, with 9 members putting out between 1.95 and 2.75 inches.
I'm still worried that the NAM and this ensemble are too productive during the AR portion of this event for upper Little Cottonwood and thus rather than bump these numbers up, I'll continue to go for 1.5-2.25 inches of water equivalent at Alta Collins from late this afternoon to 5 PM tomorrow. The NWS drops the water totals further to 1.1-1.5", making me a wet foot once again. Since low expectations are the key to good skiing, so maybe you should stick with the NWS numbers, but hope I verify. Snowbasin and Ben Lomond might do a little better, but there's a catch.
The catch is that the atmospheric river portion of the event is going to be warm. Snow levels derived from the NAM are near or above 8000 feet in upper Little Cottonwood tonight, with a peak at 9000 feet.
My experience with these NAM-derived snow levels, however, is they tend to be too high by about 1000 feet, so I suspect we'll see snow levels for much of the night near or above 7000 feet and perhaps reaching as high as 8000 feet. Thus, the base of Snowbasin could see rain. Snow levels will drop tomorrow morning.
For elevations above the snow level, the warmth of the early phase of the storm means high density snow with water contents of 12-16% (possibly higher in the melting layer). Yup, Sierra Cement. However, the snow will be drier with the frontal passage tomorrow, making for a right-side-up snowfall that probably won't ski to bad tomorrow afternoon. For storm total snowfall, I'll go for 12-20" at Alta-Collins by 5 PM tomorrow.
Bottom Line #2: This is a sloppy storm with complications from warmth and local effects on precipitation.
How are the model snow levels calculated?
ReplyDeleteIt's the height of the wet-bulb zero derived from model BUFR soundings. In most instances around here, the snow level is about 1000 feet below that level. I should put a fudge factor in to account for that.
DeleteI thought that might be the case. I would imagine that the snow level is decently below that nearly everywhere, especially in heavy snow because of the cooling caused by melting and the time it takes snow to melt. I would imagine that there is a diurnal signal too related to surface cooling/heating. Complex terrain obviously complicates things much more.
DeleteFYI that I've changed the labels on plots and table to say "wet-bulb zero" instead of "snow level."
DeleteHi Jim,
ReplyDeleteLove the blog! I will be in SLC area from Feb 6th through Feb 13th. Is there anything on the long range forecast that is predicating any kind of storms? I am hoping the active weather you guys have been receiving will still be around for my trip. Thanks!
Sorry, don't do individual forecasts or I'd never get anything done. I don't have much faith in forecasts beyond 7 days and rarely look (and that's the case today for out that far).
DeleteThanks for the kind words, really like the blog. I will hoping the for the best and dreaming of a powder week :) Cheers
DeleteInteresting discussion on the R/S elevation. Given the atmospheric river inducing heavy precip rates, wonder if that would bring down snow level a bit? Trying to decide if I can go to Canyons area (free with pass) or use discount pass for Alta.
ReplyDeleteBy 9 AM tomorrow morning, the snow level should be down to the base of PCMR. Bigger issue is overnight, when snow levels could pop up to 8000-8500 feet. You could check the reports in the morning and decide.
ReplyDeleteThanks Jim. But was still wondering, given the very heavy precip rates overnight, do you think that would bring snow level down a bit lower than models expect, or in some of the higher resolution models do you think that factor is already baked in?
ReplyDeleteThat factor is "baked in" so to speak in the sense that the models account for cooling from sublimation, evaporation, and melting. However, how well that's handled is an open question. For example, a 3-km model might not fully capture the terrain variability and thus a locally lower snow level due to cooling from melting in a quiescent valley might be poorly handled. Given the origins of this storm and the overall temperature forecast, I'm still inclined to think that we'll see snow levels getting up to 8000-8500 feet tonight. There's always hope though that the diabatic effects (i.e., cooling from evaporation, sublimation, and melting) come through.
DeleteThanks Jim. But was still wondering, given the very heavy precip rates overnight, do you think that would bring snow level down a bit lower than models expect, or in some of the higher resolution models do you think that factor is already baked in?
ReplyDeletePlease disregard duplicate question accidentally sent.
ReplyDeleteYour reply on R/S line and model variability was very detailed.
Much appreciated. Basil
If I go to PC I'll let you know the verdict. Either way, the "good stuff" at Canyons is 8000 ft and up, so I suspect I'll have a good morning of Sierra Cement which I skied on for years quite happily. But like you said, by afternoon perhaps several inches of drier snow on top, although I suspect that will be most likely in the Cottonwoods.
ReplyDelete