The Colorado Flood Threat Bulletin Program – Utilization of WRF-based Models for Flash Flood Threat Prediction

The Colorado Flood Threat Bulletin Program – Utilization of WRF-based models for flash flood threat prediction. John F. Henz Dewberry Denver CO 80224 The Colorado Water Conservation Board (CWCB) and an engineering company–based CCM co-developed a Flood Threat Bulletin (FTB) program to complement and supplement National Weather Service flash flood/flood predictions for the Colorado emergency response community. County- and basin-specific forecasts of flash flood threat are produced daily from May 1 to September 30 for the entire state. Over ninety percent of Colorado's flash floods and floods occur during this period of time. These predictions became even more critical this year when over a dozen significant fires charred the state and produced large areas primed for rapid runoff into populated areas. The FTB is a Google-Earth based and internet accessible 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. During the 2012 FTB operational season Dewberry's meteorologists began daily use of two WRF-based models, the University of Arizona AZ WRF-NAM and AZ WRF-RUC and the 4km NSSL WRF(V3.1.1), to assist in the prediction of the flash flood threat. Both models provided predicted fields of radar reflectivity, precipitation and accumulated precipitation. Each model had additional other predicted fields of forecast value: NSSL-WRF (lightning threat, pbl moisture and wind fields and CAPE) and UofA WRF (PWI, 700MB RH, MCAPE and 600mb vertical velocity). Figure 1 shows predicted radar reflectivity fields for 02Z August 09 2012 for both the NSSL-WRF PUB Zoom and U of A WRF-NAM. Figure 2 Figure 1 Predicted radar reflectivity fields for 02Z August 09 2012 for both the NSSL-WRF PUB Zoom and U of A WRF-NAM. Dewberry meteorologists used the NSSL WRF-PUB and U of A WRF's 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/predicted GPS-IPW (Integrated Precipitable Water) observations and the Henz Atmosphere-Truth Z-R (AT-ZR) QPF process. In all cases the predicted model fields were valuable to the operational meteorologists' decision-making. Early results from May to July 2012 operations period have yielded some remarkable examples of improved 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 NSSL-WRF model provided more reliable and consistent results for areas east of the Continental Divide while the U of A WRF's were much better west of the Continental Divide. In all cases model output was used in a relative sense rather than quantitatively by the operational meteorologist in preparing forecasts. Plans exist to apply the model output to decision support systems for the next year's operations.