Satellite image of Hurricane Irma as of 9:30 am EDT Friday, 8 September 2017. Southern Florida is at top left. Photo: RAMMB / CIRA@CSU

Dr. Jeff Masters
8 September 2017

(Weather Underground) –  Hurricane Irma weakened from Category 5 to Category 4 strength on Friday morning, but still represents an extreme danger to much of Southern Florida, as well as portions of northern Cuba and the southernmost Bahama Islands.

If you live by the coast in an area that that been ordered to evacuate due to storm surge, get out now if you can safely do so.

In the U.S., the worst of the storm will be felt in South Florida. The Florida Keys will take the full force of the storm, with Category 3 or 4 winds and a dangerous storm surge of up to 10 feet. Southwest Florida is expected to see a storm surge of 6 - 12 feet. Category 2 winds will potentially affect the east coast of Florida from Miami to West Palm Beach, and the west coast of Florida from the Keys to at least Naples. There is more than a 30% chance that Category 1 hurricane-force winds may affect the Florida west coast from Naples to Tampa, and the Florida east coast north of West Palm Beach, as well as the Orlando area. A large and dangerous storm surge is likely to affect the entire east coast of Florida, and well as Georgia and South Carolina. The surge may exceed the records set last year during Hurricane Matthew.

Key points:

If you are in a mandated evacuation zone, evacuate. These zones are created for a reason, and Irma is not a storm to be toyed with.

Irma will be tracking north-northwest along the Florida peninsula. The most recent consensus of models is that this track will run inland along the peninsula from south to north, which would keep the East Coast in the more dangerous right-hand side of the hurricane.

Impacts will extend along the length of the Florida peninsula. There are no recent precedents for Irma’s expected north-northwest track. The best analog would be Hurricane Cleo (1964), which took a similar NNW track along the eastern peninsula. Cleo was a much weaker storm than Irma, though, weakening from a Cat 2 at landfall near Miami to a tropical storm near Savannah, GA. Hurricane Donna (1960) traveled over land from the SW to NE sides of the peninsula; Donna was a Cat 4 near Marathon and still a Cat 1 at Daytona Beach.

The highest risk for the worst winds is in South Florida, including the Miami area. The very strongest winds will be in the eyewall just east of Irma’s center as it moves north. Parts of Broward and Palm Beach counties have not experienced sustained Cat 2/3 winds since the 1940s. Winds will be stronger at the upper stories of high-rise buildings. Many windows not up to current code will be blown out.

In southeast Florida, take shelter outside the evacuation zone in as sturdy a building as possible. Mobile homes are not safe shelter in the winds of a major hurricane.

Dangerous storm surge is expected across Miami, the Florida Keys, and the Everglades. Inundation of 5 – 10 feet above ground level is possible. The surge may be highly variable and quickly changing along and near Biscayne Bay in the Miami area.

Dangerous storm surge is expected in the Naples and Marco Island areas, especially if Irma tracks on the west side of model forecasts. Surge may peak here during the southwesterly onshore winds after Irma has passed just to the north. Residents must heed all evacuation orders.

Dangerous storm surge—potentially higher than the surge during Matthew—can be expected from northern Florida to southern South Carolina, especially along the Georgia coast. The concave coastline in this area tends to concentrate storm surge. The surge may be even higher in Georgia than in South Florida, even if Irma has weakened by the time it reaches Georgia. The surge may be similar in magnitude to what was observed last year in Hurricane Matthew.

Power outages will affect millions of Floridians and could last for days or weeks.

Tropical storm force winds may extend well inland across parts of southern Georgia. These could be strong enough to topple trees and knock out power. [more]

Extremely Dangerous Cat 4 Irma Headed for Florida

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