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Home > Probabilistic Extra-Tropical Storm Surge documentation

The Probabilistic Extra-Tropical Storm Surge (P-ETSS) model is a modification of the Extra-Tropical Storm Surge (ETSS) model to use an ensemble of wind inputs. Specifically, P-ETSS takes as input the wind and pressure outputs from the 31, 0.25 degree (27 km) Global Ensemble Forecast System (GEFS) ensemble members, along with the 21, 0.5 degree (55 km) Meteorological Service of Canada (MSC)'s Global Ensemble Prediction Systems (GEPS). Each of the resulting 52 ensemble members is run though ETSS to generate 52 hourly storm tide inundation fields. These are then combined via equal weighting to produce probabilities of exceeding a threshold (e.g., probability of > 5 feet) and heights that exceed a given risk threshold (e.g., 10% exceedance = heights that are exceeded by just 10% of the storms) for large extra-tropical storms, and weaker tropical systems.

P-ETSS utilizes an initial water level condition in each computational domain that is derived by the average of the 5-day anomalies for the various stations in those domains, where the station specific 5- day anomaly is defined as the 5-day average hourly anomaly (observation – (surge + tide)), where surge is the ensemble mean. Additionally, the post processing stage combines the P-ETSS surge guidance with station based tidal predictions and observations (where available) to create bias-corrected total water level guidance.

Note that currently MSC’s GEPS system is available only at 00Z and 12Z, so the P-ETSS 06Z and 18Z runs use 1 cycle old GEPS results. Additionally the 00Z and 12Z GEPS may not be available when P-ETSS is run, so P-ETSS may need to use 2 cycle old GEPS results at 00Z and 12Z.

Additional P-ETSS Information:

MDL Storm Surge Webpage


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