Probabilistic Extra-Tropical Storm Surge Guidance

The National Weather Service’s (NWS) Meteorological Development Laboratory (MDL) developed the Extra-Tropical Storm Surge (ETSS) model in 1995 by applying the Sea Lake and Overland Surges from Hurricanes (SLOSH) model to Extra-Tropical storms. Over the last two years, MDL, with Hurricane Sandy Supplemental funding, has enhanced the ETSS model to meet the anticipated requirements of a potential extra-tropical storm surge watch. The latest such enhancement, implemented in October 2015, enabled ETSS to operationally provide deterministic inundation guidance four times a day based on storm surge and tide in all of its model domains.

Storm surge guidance has various uncertainties associated with it such as (a) the atmospheric forcing (wind speed, wind direction and atmospheric pressure), (b) the initial water conditions, (c) the included physical processes, (d) the numerical scheme, etc. While some of these can be reduced by enhancing the storm surge model, others, such as atmospheric forcing, rely on external inputs. Uncertainty in atmospheric forcing is particularly challenging as it is the main source of uncertainty in storm surge based inundation guidance. Ensemble techniques are necessary to produce quantitative estimates of storm surge based inundation risk.

To create such an ensemble technique, MDL has developed the Probabilistic Extra-Tropical Storm Surge (PETSS) model by using atmospheric inputs from the 21 Global Ensemble Forecast System ensemble members. This paper describes the details of this effort and provides statistical verification of the PETSS products for several case studies.