Wednesday, February 16, 2011

And So It Begins...

Morning has broken, the south wind is blowing, and change is coming to the Wasatch Front.

At 1500 UTC 16 Feb, we have a deep upper-level trough centered off the California coast with southwesterly 700 mb flow over California and the Intermountain West.  That 700 mb flow reaches over 50 knots over the Sierra and portions of southern and western Utah.  At the moment, it is -2C at 700 mb over Salt Lake City, but a broad baroclinic zone lies upstream over western Nevada and northern California.  Note that the skies are clear on the warm side of the baroclinic zone, but cloudy within the baroclinic zone.

1500 UTC 16 Feb IR Satellite Image with RUC 700-mb
Temperature and Wind
Over the next several hours, strong frontogenesis will occur over Nevada as the upper-level trough and low-level baroclinic zone move eastward.  Several processes contribute to this frontogenesis.  As can be seen in the NAM model forecast valid 0000 UTC 17 Feb (5 PM MST), strong confluence develops over the Great Basin, which acts to collect and concentrate the isentropes in the pre-existing baroclinic zone as it moves eastward across Nevada.  A similar process was observed during the 2002 Tax Day Storm (see West and Steenburgh 2010).

This confluent frontogenesis will be reinforced by differential diabatic heating as (1) the cloud-free pre-frontal environment experiences stronger daytime heating than the cloudy post-frontal air and (2) post-frontal precipitation develops, leading to evaporative and sublimiational cooling.

The Sierra Nevada play an important role in the frontal strengthening process.  In particular, the interaction of southwesterly pre-frontal flow with the southern "High Sierra" generates a pre-frontal warm anomaly that enhances the cross-front temperature contrast.  The High Sierra also produce a cloud and precipitation shadow (evident in the satellite image above) that that contributes to the aforementioned differential diabatic heating.  We have a recently submitted paper describing these orographic effects during the 2006 Saints and Sinners Storm.  The figure below shows the development of the orographic warm anomaly and Intermountain cold front during that event.

Near-surface potential temperature (contours every 2 K) and
potential temperature and wind anomalies [scales to left of (a)]
based on full terrain and no High Sierra simulations of the
2006 Saints and Sinners Storm (West and Steenburgh 2011).
The bottom line is a strong front should develop over eastern Nevada today and move through Salt Lake City this evening.  Before the front gets here, the Big Bad Wolf is going to be huffing and puffing and strong winds are on tap.  At issue is will we also see dust emissions and transport to the Wasatch Front and Mountains.

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