A Tropical Visit with Potentially Big Numbers

Things are coming together for the tropics to pay a visit to northern Utah this weekend.  The loop below shows the 500-mb heights (black contours) and integrated water vapor (a.k.a., precipitable water, color contours, with warmer colors indicating higher values) over the past two days.  Note the finger of tropical moisture known as an atmospheric river extending over Hawaii and into the mid latitudes and the retrogression (i.e., westward movement) of the upper-level trough from southwest Canada to over the north Pacific, which will help push the moisture tap into the western U.S.

Forecasts from the North American Ensemble Forecast System (NAEFS) show the atmospheric river pushing into northern California by 0000 UTC Saturday (5 PM MST Friday) and then into Utah for the weekend.

Even ahead of the AR, we can expect some periods of snow to develop tonight and continue Friday.  After that, the models suggest a free-refill weekend for the resorts with strong winds and heavy snow at times.  For backcountry travelers, this looks like a classic cower in the trees situation and the UAC has already issued an Avalanche Watch with dangerous avalanche conditions expected to develop on Friday and extend through the weekend.

Below is the 12-km NAM forecast for Alta.  Temperatures are expected to rise overnight on Friday  (upper left graph) and this will likely lead to a decrease in the snow-to-liquid ratio (middle right) from about 20-to-1 (i.e., 5% water) to something closer to 10-to-1 (i.e., 10% water content).  Thus, we will probably be looking at upside down snow on Saturday.  Get out the fattys.  

Source: weather.utah.edu

The NAM also forecasts nearly 3.5 inches of water and 45 inches of snow for Alta.  You might recall we got less in the atmospheric river event about ten days ago, but a fundamental difference between the two events is duration as this one looks to be a multi day event and it is a bit colder.  

The Short Range Ensemble Forecast (SREF) system provides a looksee at the range of possibilities for this event.  For Salt Lake City (data for Alta not available) the SREF members generate anything from 0.31 inches to 2.6 inches of SWE, with a mean of over an inch. 

Source: SPC

Based on this, I see the 12-km NAM as perhaps on the high end of what is possible and would probably go with something like 2–3.5 inches of water and 25–40 inches of snow for Alta by 5 PM MST Sunday.  This should help with base, but it will also stress an extremely weak snowpack.  Be careful out there.