A review of the scientific and operational challenges of short-range precipitation prediction over the Great Basin

The intense vertical relief and land-surface contrasts of the Great Basin have a dramatic impact on individual storm systems and largely determine the climatological distribution of precipitation over the western United States. For example, annual snowfall in the Wasatch Mountains of northern Utah approaches 1200 cm, with record storm and 24-h accumulations of 267 and 141 cm, respectively. In lowland regions, orographic and lake-effect precipitation produce heavy accumulations several times each winter. For example, from 24-26 February 1998, lake-effect and orographic precipitation produced up to 130 cm of snow in the Salt Lake City metropolitan area. Over Utah alone, snowfall produced more than $100M of damage from 1993-1997 (NOAA Storm Data).

Although it has been argued that orographic influences are semi-regular and may enhance quantitative precipitation forecast skill, in reality, several studies have shown that the skill of operational forecast models over the Great Basin is lower than over any other region of the United States (e.g., Gartner et al. 1998; McDonald 1998). In fact, Eta model forecast skill is substantially lower over the Great Basin than over California, Oregon, and Washington which lie immediately downstream of the Pacific data void.

This paper will present a summary of the scientific and operational challenges of precipitation prediction over the Great Basin, including an assessment of current short-range (<48 h) forecast skill. The talk will draw heavily from activities conducted at the University of Utah/NOAA Cooperative Institute for Regional Prediction, including findings from:

1. the Intermountain Precipitation Experiment (IPEX), a field program that examined orographic precipitation processes over the Wasatch Mountains;

2. observational and numerical modeling studies of Great Salt Lake-effect snowstorms;

3. studies evaluating the skill of operational and experimental mesoscale modeling systems over the Great Basin;

4. an assessment of the skill of National Weather Service forecasts of heavy precipitation events over the Wasatch Mountains;

5. and investigations of atmospheric predictability over complex terrain.

The paper will educate the community about the unique challenge of quantitative precipitation forecasting over the Great Basin and identify areas of needed advancement in scientific understanding, observing system capability, and forecast system design.