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GEFS Precipitation Forecasts and the Implications of Statistical Downscaling over the Western United States


Contemporary operational medium-range ensemble modeling systems produce quantitative precipitation forecasts (QPFs) that provide guidance for weather forecasters, yet lack sufficient resolution to adequately resolve orographic influences on precipitation. In this study, we verify cool-season (October–March) Global Ensemble Forecast System (GEFS) QPFs using daily (24-h) Snow Telemetry (SNOTEL) observations over the western U.S., which tend to be located at upper elevations where the orographic enhancement of precipitation is pronounced. Results indicate widespread dry biases, which reflect the infrequent production of larger 24-h precipitation events (≳ 22.9 mm in Pacific Ranges and ≳ 10.2 mm in the Interior Ranges) compared to observed. Performance metrics, such as equitable threat score (ETS), hit rate, and false alarm ratio, generally worsen from the coast toward the interior. Probabilistic QPFs exhibit low reliability, and the ensemble spread captures only ~30% of upper-quartile events at Day 5.

In an effort to improve QPFs without exacerbating computing demands, we explore statistical downscaling based on high-resolution climatological precipitation analyses from the Parameter-elevation Relationships on Independent Slopes Model (PRISM), an approach frequently used by operational forecasters. Such downscaling improves model biases, ETSs, and hit rates. However, 47% of downscaled QPFs for upper-quartile events are false alarms at Day 1, and the ensemble spread captures only 56% of the upper-quartile events at Day 5. These results should help forecasters and hydrologists understand the capabilities and limitations of GEFS forecasts and statistical downscaling over the western U.S. and other regions of complex terrain.

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