Sunday, January 8, 2012

Skiing in an Upper-Level Front

The upstream side of an upper-level trough can, in some instances, be a frustrating place to ski.  The storm is over, you're hoping the sun will come out, but the clouds and strong ridge-top winds linger.  That was the case in the upper Cottonwoods today.  The morning dawned with blue skies over the Salt Lake Valley, but, despite a bit of clearing in the morning, things just stayed cloudy in the Cottonwoods.  By afternoon, it was cloud everywhere.  

The setup for this persnickety weather was a slow moving upper-level trough over the 4-coners with an upper-level ridge over the west coast and Pacific Northwest.  Both of these features are what meteorologists call "positively tilted" in that they tilt toward the east as you move northward.  Often one can find an upper-level front between the ridge and trough in a scenario like this.   An upper-level front is a region with a strong temperature gradient and wind speed in the middle and upper troposphere that is not necessarily connected with the surface.

NAM analysis valid 1800 UTC (1100 MST) 8 Jan 2011
Indeed, at 700-mb, there is a strong temperature contrast between southwest Wyoming and western Nevada that indicative of the upper-level front at roughly mountaintop level (10,000 ft).  The upper-level front can also be seen in this mornings's sounding from the Salt Lake City Airport as a layer of high static stability (i.e., temperatures nearly constant with height) with strong vertical wind shear between about 775 and 650 mb.

Notice how the winds at the base of the stable layer (near 775 mb) are light (5 knots), but at 700 mb they are much stronger (30 knots).  700-mb is near 10,000 feet.  These are the winds that buffeted the ridges during the day.  If you were skiing in the Wasatch today, you were embedded in the upper-level frontal zone and bore the full brunt of the strong winds in the associated wind-shear layer on the ridge tops.  These winds were particularly nasty along ridges and saddles that were oriented perpendicular to the northerly flow.

Pacific moisture sneaking over the ridge, which can be seen in the 700 mb analysis (lower-left hand panel above) as an area of higher relative humidity crossing southern BC and then diving southward through Montana, Idaho, and Wyoming, provided the moisture for the altostratus clouds that refused to dissipate today.  A few sun breaks in the morning provided the best ego-building turns of the day.

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