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.

Wednesday, November 26, 2014

Beautiful Thanksgiving in the Offing

Ideal weather will prevail over Utah tomorrow for your Thanksgiving festivities with high pressure firmly in control.

I can think of only a few individuals who might be unhappy.

Happy Thanksgiving!

Tuesday, November 25, 2014

Valley Cloud Storm

The radar loop for this morning shows some very weak echoes forming over and upstream of the Wasatch Mountains.  If you look specifically at the eastern Salt Lake Valley, you'd probably think it is raining or snowing.

Very little of that precipitation, however, is reaching the valley floor (although it's beginning to get close as I write this).  It's mostly snow that is falling and sublimating (when ice turns directly into vapor) before it reaches the ground.

The reason that this precipitation shows up on radar is that the beam originates on a mountain peak (Promontory Point) and is oriented with a slight tilt relative to the horizon (0.5ยบ).  The starting elevation, combined with the slight tilt and curvature of the Earth, overwhelms the slight tendency of the atmosphere to bend the beam back towards the Earth's surface.   As a result, the KMTX radar samples the atmosphere well above the valley floor, meaning it sees the snow aloft rather than what's happening at lower levels.

Source: Secrets of the Greatest Snow on Earth
This is an important consideration when interpreting radar imagery over the Salt Lake Valley and many other valley locations over the Intermountain West.  It pays to consult web cams and other sources in addition to radar.

Sunday, November 23, 2014

Natural Snowmaking Today

It doesn’t need a reason to snow in Little Cottonwood Canyon; it needs a reason to stop.” 
- Alta Meteorologist Mike Kok

The natural advantages of upper Little Cottonwood Canyon and the high terrain surrounding the Cottonwoods are on display today.  It doesn't look like much, but notice the weak radar reflectivities that have been lingering in the central Wasatch this afternoon.  Alta and upper Little Cottonwood Canyon identified with a red box.

Those weak radar returns have been producing about an inch an hour of low-density fluff at the Alta-Collins site this afternoon.  

Most of that snow growth is probably occurring at very low levels, some of it below the radar beam.  This is often a problem in shallow orographic storms, as illustrated conceptually in the figure below from my book Secrets of the Greatest Snow on Earth.

Source: Secrets of the Greatest Snow on Earth
If you look carefully at the radar loop above, you can see another area of light but persistent radar returns in the mountains southeast of north Salt Lake and Bountiful.  Here and in the Cottonwoods further south, the northwesterly flow is encountering pronounced concavities in the terrain thanks to the Avenues foothills and the Traverse Range, respectively, which extend westward from the main body of the Wasatch.  Coincidence?  I think not.  

Storm Update

Pretty good storm totals in the upper Cottonwoods so far (as of 8 am) including 20 inches at our site in upper Albion Basin and 19 inches at Alta-Collins.  Water totals are also great for base building with 1.96 inches of water equivalent at the former and 2.21 inches at the latter.  That works out to a water content of about 10-12%, which is high by Utah standards, but it's stacked right and should ski great this morning, although there's lots of buried monsters (stumps rocks, etc.) and high backcountry avalanche danger to contend with.  Let's be careful out there!

This morning we've had a nice round of orographic snowfall add to the totals, with some possible lake enhancement to the southeast of the Great Salt Lake.  This particular period of snowfall appears to be winding down as I write this (about 8:30 am).  If you look carefully at the radar loop, you can see one of the reasons why Park City gets so much less precipitation than the upper Cottonwoods.  In shallow, northwesterly flow storms like this one, very little of the precipitation is able to spill east the Wasatch Crest.  Note how widespread and persistent the radar echoes are over the Cottonwoods compared to east of the Wasatch crest.

Saturday, November 22, 2014

Forecast and Secrets Update

Not much change from yesterday in the forecast for today and tonight (compare time-height section below with that in the previous post).  Looks like a pretty good dump for the Wasatch.  I suspect most of you will be happy in the morning.   I'm still perplexed about what to expect thereafter. It's the weekend, so I'll let you surf around and look a the divergent forecasts out there and pick what you want.

A quick update on my book Secrets of the Greatest Snow on Earth.  Amazon is finally reporting that it will be in stock on November 25.  I've heard delivery dates ranging from November 28th to 30th.  Thanks for your patience!  In addition, it is now available in Nook and paperback (shipping with 24 hours) from Barnes and Noble.  I'm a huge fan of e-books, but in this case, I recommend the paperback.  For those of you on or near the University of Utah campus, I will be doing a lecture and book signing (Thanks to King's English who should have the book in stock, but call ahead to be sure) at the Wallace Stegner Center at 12:15 PM on December 2nd.  Click here for more information.