Friday, December 14, 2012

Forecast Tools: Little Cottonwood Model Guidance

Alta and Upper Little Cottonwood Canyon from Mt. Superior
January 27, 2009
If one wishes to access specific forecasts for Little Cottonwood Canyons, you can always turn to the National Weather Service Cottonwood Canyons Forecast or  But what if you want to make your own forecast or look under the hood of the forecast process?  You can always access the model forecasts at sites like or Weather Underground's Wundermap.  These sites are great, but if you are like me, you want to also look at numbers.  How much precipitation is the model producing?  What are the snow levels?  Etc.

An option is to turn to our experimental model guidance page for upper Little Cottonwood Canyon.  The data on this site, which was developed by Trevor Alcott when he was a graduate student here at the University of Utah, is based on model soundings obtained directly from the NAM and GFS models (known as "bufr" soundings).   For example, here's the guidance from the NAM model through noon tomorrow based on the Alta bufr sounding.  

Snow level is 500 feet lower than the NAM "wet-bulb zero" level.  Studies have shown that this is usually a good approximation for the snow level.  Snow ratio and water content are based on a non-linear version of the algorithm described by Steenburgh and Alcott (2010) applied to the NAM forecast.  The Mt. Baldy temperature, RH, and wind forecasts involve simple statistical adjustments to the NAM forecast, which does not adequately resolve local topographic effects, to produce a better point-specific forecast.  

The QPF (quantitative precipitation forecast) tells you how much snow-water equivalent the NAM is producing.  Snowfall amounts are based on the QPF forecast combined with our snow ratio algorithm.  

The web site also includes similar guidance from the GFS model, but uses the bufr sounding from the Salt Lake Airport since there is no GFS bufr sounding for Alta.  The GFS bufr data is also available at less frequent time intervals.  

The use of the GFS bufr sounding from Salt Lake City instead of Alta probably doesn't affect results much since the effective horizontal grid spacing of the GFS is only 25-km and the terrain is pretty pathetic.  Thus, there's probably not much difference in the forecast produced by the GFS for Salt Lake and Alta.  

It is important to recognize that this page provides access to forecast guidance and not a forecast.  A meteorologist would take a look at these numbers, as well as those from other models, and combine it with their situational awareness and past experience to produce a forecast.  One of the biggest problems is with the QPF and snowfall amount forecasts, which are frequently (but not always) underdone because neither the NAM nor the GFS adequately resolves the Wasatch Mountains, let alone the fine-scale details of the terrain in and around Little Cottonwood.  Use it with caution for your own forecasting purposes.