Anticipating a Rare Event Utilizing Forecast Anomalies and a Situational Awareness Display: The Western U.S. Storms of 18–23 January 2010

Abstract

anuary to 23 January 2010, a series of winter storms impacted the western United States. During this period, a record-setting system produced severe convection, high winds, and heavy rain and snow on 21?22 January. The severe weather included tornadoes in California and gusts in excess of 40 m s?1 associated with an intense squall line affecting southeast California and Arizona. One of the primary impacts of the storms was a heavy precipitation event across Arizona. Rainfall amounts of 125?250 mm were recorded along the Mogollon Rim in central Arizona, while higher elevations in northern Arizona received 100?150 cm of snow, with one site setting the state's 24-h snowfall record. The heavy snow and high winds resulted in widespread power outages and paralyzed travel across portions of northern Arizona. All-time minimum pressure records were set across a large portion of the western United States from Oregon to Arizona. This was an extraordinary event that was well predicted. Standardized anomalies derived from the GFS Ensemble Forecast System (GEFS) indicated a potentially historic storm one week in advance. The forecast synoptic-scale anomalies were well correlated with high-impact weather across the western United States. This case demonstrates the utility of using standardized anomalies to increase situational awareness, which enables operational forecasters to provide decision makers with information regarding the potential significance of pending weather events. The event will also be utilized to demonstrate an anomaly-based situational awareness display for streamlining the identification, and analysis, of significant forecast anomalies.