Using Ensembles to Predict Storm Surge During Hurricane Michael and Hurricane Maria.

Currently, the Coastal Surge and Inundation Prediction System (CSIPS) used by the US Navy for operational surge predictions provide only deterministic results. The system generates surge and inundation products based on forecast storm track without taking into account the errors in track and intensity associated with the cone of uncertainty that is part of the forecast storm track. The forecasts typically provide storm center locations, storm translation speed, magnitudes of maximum sustained winds, and radii of 34kt, 50kt, 64kt and 100kt winds in each geographic quadrant of the storm. To provide a more realistic representation of surge and inundation, it is necessary to account for uncertainties associated with each of these forecast parameters that cannot be captured using a single, deterministic run. To account for the effects of these uncertainties, ensemble runs with representative storm tracks are necessary. In this presentation, using Hurricanes Maria (2017 and Michael (2018) as case studies, the ensemble forecast of surge will be compared to the deterministic results. Comparisons of water levels at available NOS tide stations will be used to validate the model and quantify errors in the system. At present, waves are included in the deterministic forecasts. However, wave modeling consumes a significant amount of the computational resources. As a result, it is not operationally feasible at this time to run all instances of the ensemble members with coupled waves and circulation. Since wave effects are important for surge calculations, we will evaluate and present results from using a set of representative members of the ensemble to provide wave information to the probability maps.