Flies in the Ointment

Chances are you've been hearing about the pattern change we are going to experience this weekend for about two weeks.  Chances are you've been dreaming of it for at least two months.  It's going to happen, but for mountain snowfall, there are some flies in the ointment.  

As discussed in the previous post, the medium range forecasts from a couple of days ago exhibited a good deal of spread for precipitation and snowfall at Alta.  The North American Ensemble Forecast System (NAEFS) exhibited three solution clusters, one that produced light accumulations (two inches or less), a second that produced moderate accumulations (about 6-12 inches), and a few others that produced considerably more.  This reflected uncertainty in the speed and depth of the upper-level trough as it moved through the western United States.

There has been some convergence of the large-scale solutions produced by the models since then.  As shown by the thumbnails of the 32 members of the GEFS ensemble below, most ensemble members now call for a lower-amplitude, faster moving trough to sweep across northern Utah.  

Source: Penn State e-wall

Such a solution is fairly close to what the Global Forecast System (GFS) has been advertising the past couple of days.  The European Center for Medium Range Weather Forecasting (ECMWF) Integrated Forecast System has been calling for a deeper trough and more precipitation for northern Utah, but even it has shifted toward a lower-amplitude trough, as illustrated below.

Source: Pivotal Weather

With a shift to a lower-amplitude, faster moving solution, most of the members of our downscaled NAEFS product are calling for an inch or less of water equivalent at Alta (upper left panel below) during the Sunday storm.  Six of the 52 members (12%) go for more than an inch, with one member of the Canadian ensemble generating over 4 inches.  Mean snowfall is about 8 inches, with a majority of members producing in the range from zero to 18 inches.  

I am sometimes asked about the outlier members of the Canadian Ensemble.  The Canadian ensemble has a couple of members that are prone to producing very large localized accumulations.  When this happens in northern Utah, we get a couple of members that produce unrealistic accumulations.  In this instance, the outlier member produces about 50" of snow in 24 hours, which is highly unrealistic for a situation like this.  

I am also asked if a range of 0 to 18 inches is reasonable.  Can't we nail it down more?  This is a 72-96 hour forecast and my answer to that is probably not with current observing and modeling abilities.  In this instance, the driest solutions feature a weak trough that passes to the north, producing just light precipitation in the central Wasatch.  A bit deeper trough and you get more precipitation in the middle of the range.  A bit deeper than that and you start getting totals > 12".  Thus, there are sensitivities in snowfall due to uncertainties in the large-scale trough.  If we included the European ensemble, which has more members favoring the deeper solution, we would probably see an increase in the odds of snowfall in the 6-15 inch range and a decrease in the odds of lower amounts.  However, even in the European ensemble, the greatest range of possible outcomes in the trough intensity is found over Utah, as indicated by the shading in the map below.  

Source: ECMWF

Thus, we can be confident of cooler weather on Sunday (temperature is more predictable than precipitation and snow), but we can't completely nail down the snowfall forecast.  I'm hoping we come in with some valley rain and limited mountain snowfall as this really isn't a long-term pattern change.  The extended forecasts suggest the dice are loaded for below average precipitation after this trough passage.  Without more storms, early snow becomes weak snow on north aspects, and that's a recipe for avalanche problems when the storms return.