Friday, October 18, 2019

Assessing the Likelihood of Weekend Skiing

The likelihood of skiing this weekend is dependent on several factors, including how much snow the next storm produces, your willingness to sacrifice p-tex, and your tolerance of bodily harm. 

I'll focus on the former, beginning with where we are right now. 

Prior to last nights storm, snow cover in the Wasatch was scant.  The Alta web cam at the top of Collins shows just a few patches of snow left over from last season.

Last nights storm, however, produced near the top of expectations at upper elevations.  Radar imagery below shows the passage of the frontal system across the area with initially scattered showers followed by a well organized precipitation band. 

As a result, Alta is covered by a blanket of white. 

Automated observations from the Alta-Collins site show a total of 0.62 inches of water.  I suspect that the initial snow depth yesterday wasn't actually 4 inches and that this reflects a calibration error.  However, overnight, the total snow depth increases 8 inches and the interval snow depth (measured on a board) goes up 6 or 7, the latter being the peak right at the end of the storm before settlement.  Based on the mean, we'll call it 7 new with 9% water content.

Source: MesoWest
Other than perhaps a flake or two this morning, it should be dry through tomorrow (Saturday) morning.  Then the next upper-level trough and frontal system approach, with the chance of precipitation with a snow level near 6500 feet increasing in the afternoon.  The NAM forecast valid 0000 UTC 20 October (1800 MDT Saturday) shows precipitation from the previous 3 hours down to roughly the Cottonwoods.  

Passage of the trough will result in a lowering of snow levels to the valley floor Saturday night.  The models are producing both orographic and lake effect snow in the post-frontal northwesterly flow late Sunday night and Sunday morning.  The example below is from the NAM at 1200 UTC (0600 MDT) Sunday morning.  

Models like the NAM don't adequately resolve the influence of the lake or mountains, but this is a scenario where the likelihood of lake effect is higher due to the relatively warm lake (about 11.5˚C) and forecast for a relatively cold and moist post-frontal flow.  

Our downscaled forecasts based on the SREF are generating 0.25 to nearly 2.2 inches of water and about 4 to 35 inches of snow at Alta Collins through 1800 UTC 20 October (1200 MDT Sunday).  For snow, most of the members lie between 10 and 20 inches.  

Ultimately, the forecast distribution is one that I would describe as "heavily skewed."  There is a high likelihood of something in the 6-10 inch range, but a long skinny tail of lower probability outcomes with greater amounts.  Something on that long skinny tail verifying above 12 inches will require Alta to be in the cross hairs of well organized post-frontal convection or lake-effect.  

The bottom line is that this is an event worth watching.  A likely outcome is that the snow from this storm, combined with that from last night, will push the total snow depth to something like 10 to 16 inches by 1200 MDT Sunday.  Given that's all relatively fresh, unsettled snow with a limited water content, it's certainly below my threshold for skiing, and it should be below yours too.  

On the other hand, if all the ingredients came together — the trough track is right, the post-frontal and lake-effect convection goes big, and the central Wasatch are in the cross hairs — this could be a more significant storm.  

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