Update 11am: The NHC reports Irma remains a powerful category 4 hurricane with 115 knot winds and 933mb central pressure. A science-based storm surge prediction looks bad for Naples, Florida. Note that the water levels will go from very low levels to very high levels within a few hours as the eye passes by offshore. Moreover, there is likely to be a captured wave fetch that will bring waves surging from the south southwest as the tides rapidly rise. This will be an extraordinarily dangerous situation for anyone trying to ride the storm out near Naples.
The combination of a captured wave fetch and rapidly rising tides will cause water to rapidly surge ashore. Surf waves will come in wave groups called sets and those sets will create wave surges that may last several minutes long and drive past barriers that stop normal surf waves. Combined with the rapidly rising storm surge these infragravity waves will create a very dangerous situation.
Unfortunately, Irma is approaching Naples around high tide which is 4pm today.
NWS Key West determined, based on this clip that Cudjoe Key had a storm surge of 10-14 feet at Irma’s landfall. This may be bad news for Naples. A large surge may be coming at high tide there.
-end late morning & early afternoon update-
Irma’s eye has made landfall at the Florida Keys just east of Key West. Wind shear, land interaction with Cuba and dry air at midlevels have contorted Irma’s eyewall but Irma is still a large powerful category 4 hurricane with 130mph winds and a central pressure of 929mb according to the National Hurricane Center.
Do not believe reports that the Gulf coast of Florida will be spared from potentially deadly storm surge. They are not true. Fort Myers and Naples are in harm’s way. Offshore winds ahead of Irma will not protect them from a deadly surge and a potential “shelf wave” that could move up the coast.
The NHC’s official storm surge forecast is here. “Hurricane Hal” , storm surge expert has said that he thinks the NHC’ forecast is good.
HAZARDS AFFECTING LAND----------------------STORM SURGE: The combination of a dangerous storm surge and thetide will cause normally dry areas near the coast to be flooded byrising waters moving inland from the shoreline. The water isexpected to reach the following HEIGHTS ABOVE GROUND if the peaksurge occurs at the time of high tide...Cape Sable to Captiva...10 to 15 ftCaptiva to Ana Maria Island...6 to 10 ftCard Sound Bridge through Cape Sable, including the Florida Keys...5 to 10 ftAna Maria Island to Clearwater Beach, including Tampa Bay...5 to 8 ftNorth Miami Beach to Card Sound Bridge, including Biscayne Bay...3 to 5 ftSouth Santee River to Fernandina Beach...4 to 6 ftClearwater Beach to Ochlockonee River...4 to 6 ftFernandina Beach to Jupiter Inlet...2 to 4 ftNorth of North Miami Beach to Jupiter Inlet...1 to 2 ftThe deepest water will occur along the immediate coast in areas ofonshore winds, where the surge will be accompanied by large anddestructive waves. Surge-related flooding depends on the relativetiming of the surge and the tidal cycle, and can vary greatly overshort distances. For information specific to your area, please seeproducts issued by your local National Weather Service forecastoffice.The combination of a life-threatening storm surge and large breakingwaves will raise water levels ABOVE NORMAL TIDE LEVELS by thefollowing amounts within the hurricane warning area near and to thenorth of the center of Irma. Near the coast, the surge will beaccompanied by large and destructive waves.
The southwest coast of Florida is particularly vulnerable to hurricanes because the continental shelf is very wide and the water is very shallow. Hurricane force winds pile up water and drive it into bays and estuaries potentially several miles inland in places like Fort Myers where much of the town is below ten feet in elevation. And on top of that a wave of water trapped by the winds may develop on the continental shelf.
From Hurricane Hal’s storm surge blog: hurricanehalssb.blogspot.com
2) If Irma's storm surge reaches 12 feet in Naples and my house in Naples is 10 feet above sea level but inland 1-2 miles, will I flood?
If given enough time, a 12-foot storm surge will inundate everything under 12 feet with salt water. Storm surge can flow inland quite rapidly, but surge needs at least several hours of strong onshore winds to get pushed inland. In general, if your property is located within 2-3 miles of the coast, the surge level will reach its elevation contour.
Keep in mind that waves can travel on top of the storm surge and this is not included in the storm surge forecast. Areas exposed to a bay or ocean will often observe waves of at least 1-2 feet during a hurricane (and often times higher wave heights).
This was Hal’s forecast made last night. Tampa Bay might get a break because wind shear is forecast to weaken Irma before it gets to Tampa. However, Irma is forecast to stay offshore of Naples and Fort Myers so a wedge of water could be trapped on the continental shelf and driven northwards by the Powerful southerly and southwesterly winds on Irma’s right hand trailing quadrant. This trapped wave of water, called a shelf wave was observed in Hurricane Dennis in 2005.
Storm surge near Tampa is highly dependent on the storm track. However, expect a negative surge followed by a positive surge and the possibility of a 4-8 foot storm surge from hurricane force winds blowing in from the southwest after the eye passes.
Keep in mind that storm surge refers to the elevated height of sea level, but does not include the large, destructive waves that may ride on top of a storm surge. So if your first floor elevation is 12 feet and storm surge as high as 10 feet are forecast for your area, imagine a new sea level two feet below your house, with even 3-foot waves washing into your first floor. Scary stuff.
If Irma's track stays just offshore, this will mean higher storm surge for communities along Florida's west coast. It could also lead to a "shelf wave" developing from Clearwater north, as a bulge of water essentially gets trapped between the storm and the coast. This happened in Hurricane Dennis (2005) and it exacerbated storm surge for areas like Cedar Key and St. Marks.
Please be aware that there may be heavy flooding well away from the center of Irma made much worse by the interaction of heavy rainfall with prolonged storm surge.
Again from Dr. Hal’s storm surge blog:
Along the Atlantic Coast, storm surge levels should be more moderate, but onshore winds will persist throughout the event, keeping a low-moderate surge persisting for more than one day. This may be a problem in places like coastal Georgia, where a 4-6 ft storm surge can be expected, with elevated water levels likely exceeding 36 hours.
This creates a problem with "compound flooding", when prolonged storm surge impedes rainfall drainage. In a compound flood event, an area may flood from 10 inches of rain, when in similar rainfall events the location did not flood. Places like Savannah may observe compound flooding.