An assessment of the North American Mesoscale model forecasts during valley cold pools in the western United States

Persistent valley cold pools (PVCPs) are topographically trapped layers of air that are cooler than the overlying air mass and that last more than 24 h. They can lead to several forms of hazardous weather including freezing rain, fog, pollution, and even avalanches. Historically, operational numerical weather prediction models have provided unreliable guidance on key thermodynamic properties of PVCPs, making the forecasting of hazardous weather difficult. In June 2006, the National Weather Service implemented the Weather Research and Forecasting Nonhydrostatic Mesoscale Model (WRF-NMM) as a part of its operational suite. This model's efficacy at predicting PVCPs has hitherto been untested. Forecasts of PVCPs from the 2006/07 winter season in 3 major valleys and basins in the western United States are considered herein. While the model has a nocturnal warm bias, it captures daytime high temperatures reasonably well during cold pools. Dew point forecast errors during cold pools can be quite large with no preference for dry or moist biases. Stability profiles are generally very well handled, with a few notable exceptions. The effects of typical errors on forecasting hazardous weather during PVCPs is also discussed.