Predictability of drought using short-term climate information from dynamical weather prediction models

The U.S. Drought Outlook from the Climate Prediction Center (CPC) forecasts drought conditions through a 3-month period. Often, such information is not representative of drought conditions during the period, which may fluctuate from week to week. There is interest by various sectors in drought information on shorter timescales, as well as a need for forecast information for the quick onset of drought conditions, or “flash drought”. As an experimental product of the North American Ensemble Forecast System (NAEFS), the CPC is producing two-week bias-corrected ensemble model accumulated precipitation forecasts, combining the NCEP and Environment Canada dynamical ensemble models. Raw ensemble member precipitation is bias-corrected and downscaled using the CPC Unified Precipitation dataset. Multiple realizations of possible precipitation amounts from the combined ensemble system allows for better identification of model biases for various precipitation thresholds such that model bias is derived stratified by precipitation amount. Verification of these forecasts has shown them to provide a significant gain in skill over raw model precipitation. An advantage of the ensemble forecast system is that it allows for the prediction of a full probability density function (PDF) and the probability of exceeding any precipitation threshold. From this information, it is possible to generate probabilistic predictions of various climatological percentiles of precipitation. Combining the short-term bias-corrected model precipitation with recent precipitation observations, using the CPC Unified Precipitation dataset, a standardized precipitation index (SPI) change forecast system is developed. These forecasts are then verified against the observed SPI values on 1 to 3 month time scales. In addition, the predictability of future 1 to 3 month SPI values from 15-day precipitation forecasts is assessed.10-->