A Deficit of Seasonal Temperature Forecast Skill over West Coast Regions in NMME

This study investigates the forecast skill of seasonal-mean near-surface (2 m) air temperature in the North American Multimodel Ensemble (NMME) Phase 2, with a focus on the West Coast of the United States. Overall, 1-month lead time NMME forecasts exhibit skill superior or similar to persistence forecasts over many continental regions, and skill is generally higher over the ocean than the continent. However, forecast skill along most West Coast regions is markedly lower than in the adjacent ocean and interior, especially during the warm seasons. Results indicate that the poor forecast skill along the West Coast of the United States reflects deficiencies in their representation of multiple relevant physical processes. Analyses focusing on California find that summer forecast errors are spatially coherent over the coastal region and the inland region individually, but the correlation of forecast errors between the two regions is low. Variation in forecast performance over the coastal California region is associated with anomalous geopotential height over the lower middle latitudes and subtropics of the eastern Pacific, North America, and the western Atlantic. In contrast, variation in forecast performance over the inland California region is associated with the atmospheric circulation over the western United States. Further, it is found that forecast errors along the California coast are linked to anomalies of low cloudiness (stratus clouds) along the coastal region.