Thursday, February 24, 2011

Intermountain Cyclogenesis and Forecast Uncertainty

As discussed yesterday, we're on track for an Intermountain cyclogenesis event tomorrow and Saturday.  Yesterday the NAM and the GFS forecast cyclone tracks and frontal positions for the event that were quite different.  Today, they have converged on a solution that moves the low center across southern and central Utah.

Over the next 24-36 hours, a surface front intensifies and slides eastward and southward into Nevada and northern Utah.  As shown by the 1200 UTC 24 Feb initialized NAM, the passage of this front through northern Utah (see the cyclonic wind shift in the image below) produces a good round of snow for the Salt Lake Valley and Wasatch mountains tomorrow afternoon.

Concurrently, cyclogenesis occurs over central Nevada, downstream of the High Sierra along the western portion of the frontal boundary.  

Although both the NAM and the GFS eventually move the cyclone across southern Utah, the real forecast problem for Friday night concerns the placement of the frontal trough tomorrow evening and night.  During this period, a band of snow will likely develop along and to the north of the trough.  This can be seen above and in the forecast analyses valid for 0300 and 0900 UTC 26 Feb (8 PM MST Friday and 2 AM MST Saturday).

Vigorous, slow-moving fronts of this type can produce substantial snowfalls in the valleys of northern Utah, but positioning is everything.  The precip band can be 50-100 km wide and a shift in north or south can influence whether or not Ogden, Salt Lake, Provo, or Nephi get the goods.

The National Centers for Environmental Prediction Short Range Ensemble Forecast system (SREF) provides forecasts from several models with different initial conditions.  The SREF can be used to help assess forecast uncertainty.  Meteorologists often will view the mean and standard deviation of a forecast variable to do this.  The 0900 UTC 24 Feb initialized SREF forecast valid 0300 UTC 26 Feb drapes the sea-level pressure trough over central Utah, perhaps a bit south of the NAM position.

Note that the standard deviation of sea level pressure is high across the Intermountain West, indicating that there is quite a bit of spread amongst the model forecasts.  In other words, the forecasts that make up the SREF vary in the intensity and position of the frontal trough.  For these forecast details, the atmosphere has low predictability for a forecast of 36-48 hour lead time.  Be prepared, but stay tuned.

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