Tuesday, November 29, 2011

Three Lows, Then a Big Blow?

As we discussed yesterday, the atmosphere is in the process of going into outlier mode, and the next few days should prove quite interesting for weather.

Our explosively deepening cyclone is now just south of the Aleutian Islands.  As expected, another low has formed downstream and is just off the Queen Charlotte Islands.  Those of you who are lovers of cyclones will be pleased to know that there is also a third cyclone over the Sea of Okhotsk.  This is what meteorologists call a polar low, because of it's small scale and formation in cold air over a warm body of water.

1500 UTC (0800 MST) 29 Nov 2011 IR satellite image and sea level
pressure analysis
The cyclone near the Queen Charlotte Islands is expected to dig southeastward into Washington State.  Although it will fill at low levels, the upper-level low is expected to be centered near Las Vegas by 1200 UTC (5 AM MST) 31 Nov (Thursday morning).

GFS forecast valid 1200 UTC (5 AM MST) 31 Nov 2011
At this time, northern Utah is experience strong crest-level (700-mb) flow from the east–northeast.  The GFS forecast sounding for the Salt Lake City airport shows a 700-mb wind of 40 knots (20 m/s) from the ENE, and even stronger flow [50 knots (25 m/s)] at 750 mb.

GFS forecast sounding for KSLC valid 1200 UTC (5 AM MST) 31 Nov 2011
The concern here is the potential for strong easterly downslope winds along the Wasatch Front, especially from Olympus Cove through Brigham City.  These winds are sometimes called canyon winds, but it is important to recognize that a downslope windstorm is not necessarily isolated to in, near, and downstream of the canyon mouths.  Instead, during a downslope windstorm, the flow crosses the mountain crest, plunges downward, accelerates, and produces strong winds along much of the leeward side of a mountain range.  For that to occur, one typically needs a stable layer near the mountain crest or an area where the magnitude of the cross barrier flow weakens with height.  Both are present in the morning sounding above.

The strength, duration, and areal coverage of this event will depend on the positioning of large-scale weather features, the strength and height of the stable layer, and other processes that are difficult to predict with precision at longer lead times.  The potential is there, but we will have to see how this event comes together early Thursday morning.

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