Shift in the Weather

The models have been hinting at a pattern shift for the start of February, with the west-coast ridge weakening and a new high-over-low blocking pattern developing farther upstream over the Pacific.  

The GFS forecast valid 0000 UTC 2 February (5 PM MST Saturday) shows the high-over low block well upstream of the Pacific Coast with the confluence of two very different airstreams downstream of it over western North America.  The first is a colder airstream originating over the high latitudes that turns counterclockwise (cyclonically in meteorological speak) and into the Pacific Northwest (see northernmost yellow arrow below).  The second is a warmer, moister airstream originating over the subtropics that extends northeastward and eastward over the Great Basin (see southernmost yellow arros).  This second airstream is also associated with an Atmospheric River that extends across northern California and into northern Utah (see red arrow).

The confluence of these two airstreams results in strong to westerly flow over the Great Basin.  This is a pattern that can be good for precipitation generation in the Wasatch if the location of the atmospheric river and the confluence are right.  If it sets up right, we could get a good deal of wet, high density snow at upper elevations (with rain at lower and possible mid elevations at times).  On the other hand, a slight shift to the north and we'll be warm with some scraps.  

As a result, there is enormous spread in the forecasts from the Utah Snow Ensemble, with members of the GEFS generally producing more water and snow than members of the ECMWF for Alta-Collins.  The red box below highlights the forecast period from 0000 UTC 1 Feb (5 PM MST Friday) to 00000 UTC 4 Feb (5 PM MST Monday). Several members of the GEFS are going for more than 2 inches of water (20 inches of snow), including one that is over 4 inches (40 inches of snow).  The GEFS mean is near 2" water/18" of snow.  The Euro members are, however, much drier, with a mean of about 0.6" of water and 5" of snow. 

A look at the forecasts for the 0.5ºC wetbulb level shows a big shift from the cold weather we've had of late.  By 0000 UTC 3 Feb (5 PM MST Sunday) the median wet-bulb 0.5ºC level is over 8000 ft, which would put the snow level at maybe 7000-7500 feet.  There are a few members that are lower, but the violins show a tight clustering between 7500 and 9500 feet (I suspect the highest forecasts are from the ECMWF and they aren't as wet).  Regardless, this is a big warm up and the net result are snow-to-liquid ratios that are fairly low.  In fact, medians are below 7.5 to 1 and in the Cascade Concrete Range.  

So, high confidence of a shift in the weather, but the details of just how warm and how much snow remain uncertain, especially given the lack of agreement between the ECMWF ENS and the GEFS.