Friday, November 9, 2012

Where Is the Orographic Enhancement?

This has been a very impressive precipitation event thusfar in the Salt Lake Valley.  As of 2 PM MST, 0.94 inches of precipitation (snow-water equivalent) has fallen at the Salt Lake Airport.

Most skiers typically expect that when it snows this much in the valley, it must be snowing a lot more in the mountains, but that is simply not the case so far today.  On average during the month of November, Alta receives about 5 times as much precipitation as the Salt Lake City airport, but that's the average.  Today, we're seeing very limited orographic enhancement of precipitation.  For example, Alta-Collins has received only 1.17 inches of precipitation (snow-water equivalent) thusfar, or only 1.25 times more than Salt Lake City Airport.

Why is the enhancement so small?  It's because the precipitation is being generated primarily by the front and typically the orographic enhancement of precipitation is smallest over the Wasatch Mountains during periods when large-scale processes, such as those associated with fronts, dominate the precipitation dynamics.  For example, during the event below, which occurred during the 2001 Hundred Inch Storm, the ratio of precipitation (snow water equivalent) at Alta to Salt Lake City was lowest during the frontal storm stage [apologies for the poor resolution, but you are getting this for free :-)].

Source: Steenburgh (2004)


  1. Jim,
    How much is it a lack of orographic enhancement versus orographic enhancement in less common and less celebrated locations, such as Park City and Heber. Am I correct in thinking that a southwesterly flow results in favorable orographics for this locations? Precipitation estimates for this storm suggest so - .

  2. Byron:

    That is an interesting point. As of 2 PM, there was 1.7" recored at Thaynes Canyon and 1.5" at Brighton. The former is about 45% greater than CLN (although Alta-Guard had 1.43", but I think that's high). Thanks for pointing that out. Using Thaynes would up the orographic enhancement relative to SLC to about 1.8, higher than indicated by the CLN-SLC comparison, but still low. Curiously, amounts at lower elevations on the PC side are a bit lower than on the Salt Lake side (e.g., .74" at Park City Golf Course). Ah, what a tangled web that precipitation weaves.

    Thanks for pointing out this adjustment!


  3. Ah, let me add further to the intrigue. I pulled the numbers in my comment above off the NWS page, but here's what the stations report from midnight - 2pm:

    Brighton Snotel: 0.4" based on guage, 1.5" based on snow pillow
    Thaynes: 1.8" based on gauge, 1.7" based on pillow

    Not sure why the Brighton gauge is off by so much. Typically the pillow is problematic at that site (it often reads low).

  4. And more. Here are snow depth increases from several sites since midnight.

    PC Summit House: 13"
    Thaynes SNOTEL: 13"
    PC Jupiter: 13" (wow...almost too consistent)
    DV Ontario: 10"
    DV Ruby: 7"
    Solitude Summit: 12"
    Alta-Collins: 11"

    These are broadly consistent with the precip being slightly greater along the Brighton-PC ridgeline.