Saturday, December 2, 2017

Sunday Night's Postfrontal Crapshoot

Source: Juancho10.0
As I sift through this morning's forecasts, I don't see much change from yesterday.  Ultimately, how well we do depends on the post-frontal crapshoot. 

A surface trough and accompanying cold front will move through northern Utah Sunday afternoon.  This morning's 12Z NAM has that front just through the central Wasatch by 2100 UTC (2 PM) tomorrow afternoon, with cold air pushing into the region.  It's a little unclear from the latest runs if precipitation will begin with the frontal passage or perhaps lag somewhat behind it.  

The real meteorological headache is what will happen Sunday night and Monday morning behind the front.  Moist, unstable, northwesterly flow is typically good for snow in the Cottonwoods.  The problem is, the various forecast systems and ensembles are generating a wide range of predictions concerning the gory details of that post-frontal flow.  

The loop below shows several consecutive NAM forecasts valid at 1200 UTC (5 AM MST) Monday morning, illustrating the variations in crest-level flow direction and relative humidity (bottom left) and precipitation (bottom right).  Although the models generally agree on a trough being over the area, the smaller scale details, what meteorologists would call the mesoscale, differ.  Those differences matter.  

An additional complicating factor for snowfall forecasts is that conditions look near optimal for low-density snow, with 700-mb (crest-level) temperatures from about -13ºC to -16ºC Sunday night.  What falls should contain plenty of dendrites, although if it goes strongly convective, such as in a lake band, there could be periods of moderate to heavily rimed crystals or some graupel.  Such low densities mean one can get a lot of snow from a little moisture.   A half inch of water equates to 8.3" of snow if it is 6% water content and 12.5" if it is 4% water content, so the water content prediction matters a great deal in scenarios like this.

Lookin gat this morning's NAM, we see it is going for 0.27" of water from late Sunday through Monday morning.  Note the increase in snow ratio (and hence density) during the period, with a total snowfall of 5 inches.  

Looking at the downscaled SREF, there is a very large spread from about 0.1 to almost 1.2 inches of water.  The major difference in these runs is what happens after the frontal passage.  The wettest members produce post-frontal precipitation overnight Sunday and in some cases into Monday.  The driest aren't very productive with the front, but also cut things off after frontal passage and keep it dry thereafter.  

Given that the NAM is generating about 0.28" of water and the mean of the downscaled SREF members is around 0.4", I would lean toward 0.3 to 0.6" of water and 6-12 inches of snow being the most likely range of outcomes at Alta.  If things come together, we will do better.  The odds of coming in below 6" aren't zero, but they are smaller than the odds of coming in above 12.  The NWS Cottonwood Canyons forecast issued about 3 AM last night calls for 9-15, so perhaps I'm a dry foot. 

Beyond this storm, no changes in the long term.  Looks dry next week. 

Bottom line: Continue prayers, ski burning, and goat sacrifice.  If you are sacrificing anything larger or more intelligent than a goat, I don't want to know about it.  

1 comment:

  1. Can you go away on a trip or something? I feel like if you sacrificed yourself and went somewhere tropical, that might bode well for us.