Evaluation of the NSSL-WRF model in an Operational Colorado Flood Threat Prediction Program

Evaluation of the NSSL-WRF model in an Operational Colorado Flood Threat Prediction Program John F. Henz Dewberry Denver CO 80224 The Colorado Water Conservation Board (CWCB) developed a Flood Threat Bulletin (FTB) program to complement and supplement National Weather Service flash flood/flood predictions for the emergency response community. The program was developed in 2006 and provides county- and basin-specific flash flood threat predictions from May 1 through September 30. Over ninety percent of all Colorado flash floods have occurred during this period. The FTB is a Google-Earth based and internet accessed web page (www.coloradofloodthreat.com) that provides three services. First a daily Storm Total Precipitation product is provided by 930AM daily. It combines STP fields from five National Weather Service radars (KFTG, KPUX, KGJX, KCYX and KGLD) with MADIS-based and CoCoRahs based rainfall observations to identify for emergency managers where flash flooding and heavy rainfall occurred the prior 24 hours. The FTB provides specific forecasts for the numerous fire burn areas within Colorado. The Flood Threat Bulletin (FTB) is issued daily before 100AM and provides a county-specific and basin-specific prediction of flood threat, rainfall intensities, prime time for flood threat, associated severe weather and storm probabilities. An interactive map serves as a platform to access the information. On Mondays and Thursdays a 15-day flood threat and precipitation outlook is issued before 330PM as a planning tool for emergency managers and water suppliers within the state. All predictions were prepared by Dewberry's meteorologists. Figure 1 Dewberry Flood Threat Bulletin example During the 2012 FTB operational season Dewberry's meteorologists evaluated the use of the experimental high resolution, 4km NSSL WRF model (V3.1.1) and its PUB zoom products. Initial and Boundary conditions were obtained from interpolation of the routinely available 40km NAM model fields obtained from EMC/NCEP, using the WRF Preprocessing System (WPS). Initialization time is 00 UTC and the forecast length is 36 hours. The model runs over a domain that cover the entire United States and the PUB zoom products cover the entire state of Colorado. Figure 2 NSSL WRF-PUB 26 hour forecast of Max reflectivity for 02Z 07 June 12 Dewberry meteorologists used the NSSL WRF-PUB products for developing basin/county specific flash flood threat predictions. Model radar reflectivity forecasts were used to provide valuable timing, intensity and location estimate of storm systems. Model precipitation estimates were enhanced using a combination of the predicted reflectivity fields, observed GPS-IPW (Integrated Precipitable Water) observations and the Henz Atmosphere-Truth Z-R (AT-ZR) QPF process. The figure shows the predicted maximum reflectivity field for storm activity in Colorado that was remarkably accurate. Early results from May to July 2012 operations period have yielded some remarkable improvements in the prediction of storm event timing, intensity and movement when even weak synoptic scale forcing was present. Weaker results were obtained when outflow boundary forcing was the key storm triggering mechanisms. The model provided more reliable and consistent results for areas east of the Continental Divide.