Evaluation of weather forecasts and observations using surface analyses over the western United States

Surface analyses created by the Advanced Regional Prediction System Data Assimilation System (ADAS) valid during the 2003/04 winter season (18 November 2003-7 March 2004) are used to verify National Weather Service (NWS) gridded temperature forecasts and evaluate the sensitivity of objective analyses to Remote Automated Weather Station (RAWS) temperature and wind speed observations. The ADAS analyses are based on Rapid Update Cycle analyses that are adjusted by over 3000 surface observations from MesoWest. Accuracy and skill of the NWS gridded forecasts are determined by point verification against the MesoWest observations and gridded verification against the RUC and ADAS analyses. The impact of RAWS observations on the ADAS analyses is assessed by three metrics (accuracy, sensitivity, and areal influence) that rely on results from a series of data withholding experiments.; For the West as a whole, the NWS gridded temperature forecasts issued at 0000 UTC during the 2003/04 winter season exhibit skill at lead times of 12, 24, 36, and 48 h on the basis of several verification approaches. The root-mean-square errors (rmse) of the NWS gridded temperature forecasts are estimated to be on the order of 3°C at lead times shorter than 48 h and greater than 4°C at lead times longer than 120 h. Incremental improvements in forecast accuracy as a result of forecaster adjustments to the 0000 UTC temperature grids from 144- to 24-h lead times are estimated to be on the order of 13%.; Random withholding of observations from the ADAS analyses provides a baseline estimate of the analysis quality. Relative to this baseline, removing the RAWS observations degrades ADAS temperature (wind speed) analyses by an additional 0.5°C (0.9 m s-1) when evaluated in terms of rmse over the entire season. RAWS temperature observations adjust the RUC background the most during the early morning hours and during winter season cold pool events in the western United States while wind speed observations have a greater impact during active weather periods. The average analysis area influenced by at least 1.0°C (2.5°C) by withholding each RAWS observation is on the order of 600 km2 (100 km2).