Friday, November 28, 2014

Screwed by the High Sierra

The Wasatch are a remarkably snowy interior mountain range, but they would be even snowier if not for the High Sierra.

One way to illustrate this is to look at the fraction of cool-season (November to April) precipitation produced over the western U.S. when atmospheric rivers (ARs) are present.  ARs are filaments of strong water vapor transport characterized by high water vapor content and strong winds.  As can be seen in the figures below, more than half the cool-season precipitation along much of the U.S. west coast is produced when ARs are present.  ARs sometimes extend inland, especially over the Columbia Basin and panhandle of Idaho, the Snake River Plain and southwest central Idaho Mountains, and much of Arizona, where >30% of the cool-season precipitation occurs when ARs are present.  In contrast, AR penetration into central Nevada and Utah is less common and AR-related precipitation is less common.

Source: Rutz et al. (2014)
This is primarily the result of the High Sierra, which form a formidable barrier running through eastern California south of Lake Tahoe.  The High Sierra directs low-level airmasses to the north or south and generates heavy orographic precipitation that removes water vapor from any airmasses traversing the range.  As a result, it is very difficult for ARs to penetrate directly across the High Sierra.

One can see this very clearly in the forecasts for the coming week.  In the GEFS ensemble mean integrated water vapor transport forecast for 0600 UTC 2 December 2004, there is a broad area of strong water vapor transport over and upstream of the U.S. west coast.  This is basically a fat AR.  The AR able to penetrate inland across portions of Oregon and Idaho, but can't survive transit across the High Sierra.  What a pity!

As the system moves southeastward, higher integrated water vapor transport values can sneak inland to the south of the High Sierra, but once again, there's not penetration inland across the highest portion of the range.

And here's the GFS total precipitation forecast for the next week showing a band of heavy precipitation to the north of us and an arm of heavier (but weaker than that to the north due to the shorter duration) precipitation to the south.  In this forecast, we get clipped by the occasional system so we do get something, but it would have been much more were it not for the High Sierra.

To do better, we need a system to draw moisture in from the north or south and concentrate it over northern Utah (a weak front sagging in on Sunday does this to some degree, but right now doesn't look like it will produce a major snowfall in the southern Wasatch).  A passing system on Wednesday might do the trick too, although we'll have to see it it comes together as the event approaches.  Alternatively, you could drive north and out of the shadow of the High Sierra.

Announcement: I'll be giving a talk on my new book Secrets of the Greatest Snow on Earth at 12:15 on Tuesday, December 2nd in the Sutherland Moot Courtroom at the University of Utah College of Law, with a book signing immediately following.  The talk is free and open to the public and King's English Bookstore will have books available for purchase.  Click here for more information.

1 comment:

  1. One interesting aspect about this case is that the primary AR plume is sourced directly from the ITCZ (Inter-Tropical Convergence Zone) in the eastern Pacific, which I suspect is only true of a minority of all the AR events. Note the 700 mb temperatures of +4 C and above associated with this plume from about the latitude of LA / Phoenix and southward in most of the forecast model data.