Expanded HRRR Domain Testing and Evaluation


As computing resources at NCEP continue to increase in coming years, we foresee that the current regional models, the NAM and RAP, will be replaced (within 5-7 years, perhaps sooner) by regional cloud-resolving configurations of similar domain size to that of the current NAM and RAP. These will be nested within the then operational global model. Looking toward that day, and in view of the importance of accurate short-term forecasts for vulnerable coastal areas, particularly along the Gulf and Atlantic coasts, we investigated the value of an initial expansion of the HRRR domain in all directions, but mainly toward the east and south. Coastal storms (aka Nor’easters) that impact the heavily populated east coast with high winds, heavy precipitation, and often a very disruptive “wintry mix” of precipitation are often close enough to the current lateral boundaries of the HRRR, particularly during their formation and deepening stages in the Gulf of Mexico and in the southwest North Atlantic between the southeast US and Bermuda, and as they pass seaward of New England as they track north or northeastward, that the circulation is not well described within the model domain itself. The lateral boundaries of the HRRR domain are often within the storm circulation, leading to flow distortions. Such an expanded domain would also allow for improved prediction of tropical systems that are within, say, 48-h striking distance of the US mainland. At the time this work was originally proposed, the operational NCEP HRRR forecasts only extended to 15 h, but owing to considerable interest on the part of forecasters within and outside of the NWS to see an extension of the HRRR, ESRL is now running the HRRR experimentally to 36 h, every 3 hours. Expanding the boundaries of the current HRRR domain has thus become increasingly important for these longer forecast lengths.