Monday, April 16, 2012

Backside Meteorology

The Wasatch Weather Weenies don't spend much time on the Wasatch Back during winter.  We like being in the Cottonwood Canyons where there's a greater variety of storms and a deeper snowpack.      However, thanks to some free lift tickets from a friend, we ran laps yesterday at Park City Mountain Resort (PCMR) where we got a great look at some of the leeward effects of the Wasatch Mountains.

As shown by the 1800 UTC (1200 MDT) 15 April 2012 NAM analysis, yesterday featured large-scale northwesterly flow at crest level and through most of the troposphere.

This led to the development of low clouds over the western (windward) slopes of the Wasatch Mountains, including the Cottonwood Canyons (in lower left-hand corner of red box below).  In contrast, a pronounced cloud shadow was found downstream of the Wasatch Crest (right side of red box).

Early in the morning, this led to beautiful sunny skies at  PCMR, with low clouds hanging over the Wasatch Crest to the west.  

Later in the morning, perhaps due to surface heating, the contrast was less abrupt, but still evident from the top of Jupiter bowl near the Wasatch Crest.  Looking southeastward across upper Big Cottonwood Canyon, it was mostly cloudy.

In contrast, looking eastward toward Park City, it was partly sunny with a pronounced cloud-free hole over the lowlands between the Wasatch Mountains and the Uinta Mountains further east.

In the early afternoon there was a nice example of what we call precipitation spillover.  Looking northward, shallow cumulus clouds were well developed and producing snow over the Wasatch Mountains.

The snow was subsequently carried eastward by the prevailing flow and "spilled over" into the lowlands around Park City.

This provides a visual example of what happens in more substantive Wasatch Mountain snowstorms.  Precipitation is generated as air is forced upwards over the windward side of the Wasatch Mountains.  However, some of the precipitation is carried downstream by the prevailing flow and spills into the lee. Precipitation typically decreases as one moves downstream of the mountain crest (there are some exceptions depending on storm characteristics and terrain size and shape).  Contributing to this decrease in precipitation is the warming and drying of the air as it sinks over the lee slopes, which results in evaporation and sublimation.  For example, in the images above, the clouds never penetrate eastward over the lowlands, despite the prevailing northwesterly flow because they are comprised of small droplets that evaporate quickly.  Only the larger snowflakes and ice crystals generated within the clouds  survive long enough to fall out on the lee slopes.

In a future post, we'll take a look at how these processes affect the average snowfall and snowpack across the Wasatch Mountains.


  1. Hey Jim,

    I have a quick question on semantics: In this post, it sounds like you're describing spillover precipitation to be whatever is advected by the flow into the downstream environment. I always thought of it as precipitation that is actually being generated downstream of the barrier. Is spillover the term for either?

    1. In my view, spillover is precipitation that is generated upstream and is advected downstream into the lee of the barrier. Situations where precipitation may be generated on the "leeward side" aren't included in this definition. For example, when there is low-level flow reversal causing localized upslope flow in the "lee" of an isolated obstacle, I don't refer to it as spillover. Similarly, the formation of a lee-side convergence zone, such as happens downstream of the Olympics, doesn't count.

      I first ran across the term in graduate school when I was thinking about precipitation in the New Zealand Alps. Mark Sinclair and compadres were using the term (;2). The definition in the Glossary of Meteorology is similar to mine: "That part of orographic precipitation that is carried over the peaks by the wind so that it reaches the ground on the lee side of the barrier."