Friday, February 8, 2013

A Time to Ski

There is a time for everything, but this weekend is a time to ski, especially given the lack of storms over the past 6 weeks.  Where and when to get the goods, however, is a complicated question.

Nobel prize winning physicist Niels Bohr famously quipped, prediction is very difficult, especially about the future, and I've been thinking of that line a lot the past couple of days as I've been surveying the model forecasts.  We have a very complex storm system moving through, one that is fragmented into pieces with multiple troughs and strong contrasts in wind speed and direction.  Small changes in the position and strength of these fragments, troughs, and wind contrasts can make a big difference in how much snow we get in the central Wasatch Mountains, which are the focus of this post.

I'm going to skip some minor details and highlight some of the major issues.  Tonight, the latest NAM forecast calls for a strong surface boundary to setup somewhere between Salt Lake City and the western Uinta Mountains.  In the NAM forecast, this boundary becomes the locus for precipitation overnight as can be seen in the lower right hand panel below (click to enlarge).

If you enlarge the graphics and look at the 700-mb winds (lower left hand panel), which are at roughly 10,000 feet, notice how the flow converges along this line.  There is westerly flow along the Wasatch, southerly to southeasterly flow south of the Uintas, and easterly flow near Evanston.  What a mess!

Many of the models call for heavy precipitation to develop along this boundary, but not all of them.  And there are variations in position and intensity.  The Short-Range Ensemble Forecast System consists of 22 different model forecasts and only 70% of them call for more than 0.25 inches of snow-water equivalent to fall in the area along the Uintas and Wasatch Mountains north of Provo.  While these models generally underpredict how much snow-water equivalent will fall in the mountains, that 70% number does indicate that the models differ in how much precipitation will fall at a given location and, depending on where you are, some of the forecast models are less excited than others.    

When I see a forecast like this, I get excited, but try to avoid irrational exuberance.  There is the potential for several inches of snow tonight in the central Wasatch, and for those of you on the Park City side tonight could be your chance to see more snow than the Cottonwoods (depending on details of the boundary position and local winds), but much will depend on where and how everything comes together.

The technical name for the NAM forecast for Saturday is freaking mess, although I use saltier language behind closed doors.  The pattern is quite complex.  I'm not even going to try and describe it.  Suffice to say that the NAM calls for periods of snow, possibly heavy at times.  Our experimental Little Cottonwoods forecast derived from the NAM tells the story.  

The GFS is also bullish on snow.  Thus, I'm excited, but given the complexities, will keep my emotions in check and will hope everything comes together as advertised.

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