Tuesday, December 30, 2014

Why Don't the Cottonwoods Get "Canyon Winds"?

I am reminded this morning of the Norwegian saying, "there's no such thing as bad weather, only inappropriate dress."  Peak gusts this morning of 77 mph in Farmington (Site AP611 @ 7:52 AM), 68 mph in Centerville (CEN @ 7:00 AM), and 60 mph at the University of Utah (WBB @ 4:35 AM).  Temperatures along much of the Wasatch Front are in the low teens or single digits.  There's great skiing to be had in the central Wasatch, but check out the temperatures in the latest MesoWest plot below.  Sub zero at nearly all locations.  The contrast between the east side and west side is most impressive.  Below -10ºF at most mid-mountain observing sites from Deer Valley to the Canyons.  In contrast, it's balmy in Little Cottonwood where temperatures are only in the minus single digits.

Surface winds (barbs) and temperatures (digits) at 15:56 UTC (08:56 AM MST).  Source: MesoWest
One thing you will notice in the plot above is that the winds in the central Wasatch really aren't all that bad.  In fact, it's far windier along the Wasatch Front from Parley's Canyon to Ogden, as can be inferred by the map below in which the digits are now wind gusts.

Surface winds (barbs) and wind gusts (mph, digits) at 15:56 UTC (08:56 AM MST).  Source: MesoWest
It's not all that unusual for the central Wasatch and the Cottonwood Canyons to not be as windy as the northern Wasatch Front during canyon wind events, for a few reasons.  First, the name "canyon winds" is a misnomer.  It gives the false impression that these winds are caused by the funneling of cold air through the canyons, when in reality we are experiencing a downslope windstorm with cold air is spilling across the entire Wasatch Range and causing strong winds from Parleys Canyon northward.

So, a critical component of these downslope wind events is the strength of the easterly flow impinging on the Wasatch and its ability to cross the crest and plunge down the west side, and there are at least two reasons why the Wasatch Mountains north of I-80 take the brunt of these events and not the central Wasatch.

First, there is the influence of the Uinta Mountains to our east.  The Uinta Mountains are a huge barrier and they block and steer the cold air over Wyoming eastward toward the northern Wasatch.    Second, the easterly flow can penetrate more easily across the Wasatch Mountains north of I-80 because they are lower.  It is possible that one of the reasons why Farmington is so windy in these events isn't because of funneling through Farmington Canyon, but the fact that the crest of the Wasatch is a bit lower at the head of Farmington Canyon [perhaps one of our adventurous graduate students can do some super high resolution modeling in the future to investigate this possibility].

The net effect of this is that the Cottownwood Canyons and the area to their west, including Sandy, Draper, and Cotttonwood Heights, are basically in a wake during downslope wind events.  In fact, there is often an eddy near Sandy that we call the Sandy Eddy and it shows up plain as day this morning, although it's centered a bit west of Sandy at the time below.

Surface winds (barbs) and wind gusts (mph, digits) at 15:56 UTC (08:56 AM MST).  Source: MesoWest
In fact, one can see a thin band of stratus cloud forming where the westerly flow on the south side of the Sandy Eddy is being forced upward where it converges with southerly flow nearer the Wasatch Range.

If all of this sounds like the kinds of features that you find in a river, you are right!

Addendum @ 10:00 AM: Although the central Wasatch aren't directly affected by the so-called canyons winds, I should note that the easterly flow there will be on the increase today as low pressure moves southward and intensifies.

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