Downslope Wind Verification of the National Blend of Models v4.0 Across the Northern Front Range of Colorado During the 2020/2021 Cool Season

Abstract

Downslope windstorms are common leeward of the Rocky Mountains across the High Plains of Colorado during October through March and can cause property damage and travel disruptions. This region is renowned for two primary types of strong downslope winds: (1) chinooks that feature warm, dry winds and are often associated with the development of mountain waves, and (2) boras that are colder and often associated with the passage of a mid- and upper-level trough. The National Blend of Models (NBM) is regularly utilized by forecasters at the National Weather Service Forecast Office (WFO) in Boulder and is one tool used by the WFO to forecast downslope windstorms. No study, however, has quantified the performance of the NBM during downslope windstorms along the Colorado Front Range. In this exploratory study, downslope windstorms from October 2020 through May 2021 were identified using ten observation sites along the Colorado Front Range. Windstorms were subsequently analyzed to quantify forecast statistics of wind speeds and gusts during windstorms to provide a better understanding of NBM v4.0 performance. On average, we found that the overall maximum magnitudes of wind speeds and gusts are on average 38% and 35% too low in the NBM output, respectively, when averaged across 24 to 72-h lead times. Weather and Research Forecasting (WRF) model simulations of varying vertical resolution were also completed to determine if WRF, a constituent model of the NBM, could provide a robust representation of select downslope windstorms across the Colorado Front Range.