Evaluating GFS and ECMWF Ensemble Forecasts of Integrated Water Vapor Transport Along the U.S. West Coast

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Stewart, Briana E.
Atmospheric Rivers (ARs) are long and narrow regions in the atmosphere of enhanced integrated water vapor transport (IVT) and are typically associated with extreme precipitation and high societal impacts. Reliable and skillful forecasts of landfalling ARs in the Western US are critical to hazard preparation and aid in decision support services, such as forecast informed reservoir operations. The purpose of this study is to compare the cool-season water year skill of the NCEP GEFS and ECMWF ensemble forecasts of IVT along the U.S. West Coast for 2017--2020. The skill is analyzed using probability-over-threshold forecasts of IVT magnitudes ≥250 kg m⁻¹1 s⁻¹1 (P₂₅₀) using contingency table skill metrics at a point of interest (38ºN, 123ºW) and along the west coast of North America. Analysis of forecast probability changing with lead-time (dProg/dt) found the ECMWF provided ~1-day of additional lead-time for situational awareness over the GEFS at lead times of 6??10-days. Forecast-based skill showed the ECMWF to lead over the GEFS with success ratios 0.10 to 0.15 higher at lead times >6 days for P₂₅₀ thresholds of ≥25% and ≥50%, while event-based skill analysis using the probability of detection (POD) found that both models were largely similar with minor latitudinal variations favoring higher POD for each model in different locations along the coast. Investigation into whether different types of flow patterns and events result in higher or lower AR-related forecast skill could bring additional situational awareness and further the results of this study.