26 Great Salt Lake-Effect Precipitation: Observed Frequency, Characteristics and Associated Environmental Factors

Monday, 20 August 2012
Priest Creek AB (The Steamboat Grand)
Trevor I. Alcott, University of Utah, Salt Lake City, UT; and J. Steenburgh and N. F. Laird

This climatology examines the environmental factors controlling the frequency, occurrence, and morphology of Great Salt Lake-Effect (GSLE) precipitation events using cool season (16 Sep - 15 May) WSR-88D radar imagery, radiosonde soundings, and MesoWest surface observations from 1997/98 – 2009/10. During this period, the frequency of GSLE events features considerable interannual variability that is more strongly correlated to large-scale circulation changes than lake area variations. Events are most frequent in fall and spring, with a minimum in January when the climatological lake-surface temperature is lowest. Although forecasters commonly use a 16°C lake–700-hPa temperature difference (ΔT) as a threshold for GSLE occurrence, GSLE was found to occur in winter when ΔT was only 12.4°C. Conversely, GSLE is associated with much higher values of ΔT in the fall and spring. Therefore, a seasonally varying threshold based on a quadratic fit to the monthly minimum ΔT values during GSLE events is more appropriate than a single threshold value. A probabilistic forecast method based on the difference between ΔT and this seasonally varying threshold, 850–700-hPa relative humidity, and 700-hPa wind direction offers substantial improvement over existing methods, although forecast skill is diminished by temperature and moisture errors in operational models.

An important consideration for forecasting because of their higher precipitation rates, banded features—with a horizontal aspect ratio of 6:1 or greater—dominate only 20% of the time that GSLE is occurring, while widespread, non-banded precipitation is much more common. Banded periods are associated with stronger low-level winds and a larger lake-land temperature difference.

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