80 A Review of the Significant Coastal Flooding Associated with Hurricane Irma along the Southern South Carolina and Northern Georgia Coastlines

Tuesday, 17 April 2018
Champions DEFGH (Sawgrass Marriott)
Steven P. Rowley, NWS, North Charleston, SC; and C. E. Barnes

The storm surge event associated with Tropical Storm Irma along south coastal South Carolina and north coastal Georgia was a complicated, but well-forecast event. The synoptic set-up ahead of the event, featuring zonally elongated high pressure centered over the Great Lakes as Irma approached the area from the south, led to persistent onshore flow. This onshore flow resulted in increasingly positive tidal departures for days prior to the arrival of tropical storm force winds.

Significant coastal flooding occurred with the high tide Monday afternoon, resulting in the second highest crest of record at the Ft. Pulaski tide gauge on the Savannah River, and the third highest at the Charleston Harbor tide gauge. This high tide occurred as a tropical rainband producing the strongest winds of the event arrived. When superimposed on already elevated water levels, these strong onshore winds helped produce a rapid rise in tide levels which created significant storm surge inundation of 4 to 6 feet in many coastal locales.

Because of the changing track of Irma (and other factors), the Probabilistic Hurricane Storm Surge model (P-Surge) significantly underestimated the storm surge inundation potential, while the Probabilistic Extratropical Storm Surge model (P-ETSS) produced a much more realistic forecast, and per collaboration with the NHC Storm Surge Unit, we shifted our focus to this extratropical guidance. Initially, P-ETSS indicated significant uncertainty, as portrayed by a large envelope between the 10% and 90% exceedance forecast. However, this envelope began to narrow within about 48 hours of the event, and we were able to deliver consistent messaging regarding a high confidence, life-threatening storm surge event. Ultimately, this model verified very well along our coastline.

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