Monday, December 14, 2015

Remarkable Shallow Orographic Effects

Don't be fooled by the relatively modest water equivalent rates in the upper elevations of the Cottonwoods today. The mountains are having a dramatic influence on this storm, but more in the lower elevations.

The word orographic means relating to mountains.  When we are talking about orographic precipitation or orographic effects, we mean precipitation or effects related to the mountains.

The orographic effects today are dramatic and pronounced on the radar.  Check out the radar loop below, which covers the period from 1938–2144 UTC (1238–1444 MST).  In the northwesterly flow, there is clear enhancement of precipitation on the windward side of the Cedar, Stansbury, Oquirrh, and Wasatch Ranges as one moves from west to east across roughly the center of the image.

Note also that there's shadowing on the east (downstream) side of those ranges too.

If one looks carefully over the Stansbury, Oquirrh, and Wasatch ranges, however, the highest returns are not found over the crest but over and upstream of the windward slopes.  Totals in the lower Cottonwood Canyons, for example, will be larger for this part of the storm than in the upper Cottonwood Canyons.  What a pity!


  1. Is the situation with the split flow between lower & mid-level flow (as happened with this last storm) a new(ish) thing, or is it just a matter of being better able to see it (model it) to understand what is happening with storms like this? It was really interesting to read your blog and the NWS forecast discussions as this storm evolved- it really helped me to learn more fascinating things about how the weather works. Thanks for your work! PS- I'm still trying to figure out what the conversions are between time measures (e.g., 00Zhrs = what time???) :) Thanks!

    1. There's nothing unusual about the flow yesterday. A consequence of temperature contrasts in the atmosphere is that troughs can have complex vertical structures and one can get situations where the flow is northwesterly at low levels and weak or even from the southwest aloft.

      Utah is 7 hours behind UTC (also called "Z" or "Zulu") when we are on standard time (winter) and 6 hours behind when we are on daylight savings. The conversions drive us all nuts. I find writing this blog that the times from 0000-0600 UTC are the worst because they also have differing dates.