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 (Figure 1.) 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.

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.

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.

Irma plowed through the Turks and Caicos Islands (population 31,000) on Thursday evening, and was headed west-northwest at 16 mph just south of the central Bahamas on Friday morning. An eyewall replacement cycle that began on Thursday evening continued into Friday morning, and finally knocked down Irma’s peak winds below Category 5 strength. Irma was a Category 5 storm a remarkable 3 days, ending at 2 am EDT Friday. The only Atlantic hurricanes to be at Category 5 strength as long or longer were the 1932 Cuba hurricane (3.25 days) and Hurricane Allen of 1980 (3 days, and Allen's period as a Cat 5 was broken into three intervals.)

The weakening of Irma overnight was due to a major eyewall replacement cycle, where the inner eyewall (diameter 23 miles) collapsed, and was replaced by a new outer eyewall with a diameter of 45 miles. The reduction in Irma’s peak winds is not necessarily a good thing, since the hurricane hunter data shows that the hurricane-force winds of the storm have spread out over a larger area, which will increase the storm surge; infrared satellite images on Friday morning showed that the areal extent and intensity of Irma’s core area of heavy thunderstorms made a dramatic expansion in areal extent (Figure 3 and 4.) A NOAA hurricane hunter plane in the storm Friday morning found top surface winds near 125 mph on their first pass through the eye near 8:20 am EDT, and 145 mph on their second pass through near 9:20 am EDT. The pressure stayed nearly constant in the two passes, at 927 and 928 mb, respectively.

It is uncommon for a major hurricane that suffers such a large hit to its strength and organization to make a quick recovery and re-intensify back to its former strength. I’m not expecting Irma to regain Category 5 status, particularly since part of its circulation is now over Cuba, and the core of the hurricane will come very close to the Cuban coast on Friday night and Saturday. Conditions are otherwise favorable for intensification through Sunday, with light wind shear less than 10 knots and very warm ocean waters near 30°C (86°F.) Irma is likely to be a large and very dangerous Category 3 or 4 hurricane with 115 – 140 mph winds when it makes landfall in the Florida Keys between 1 am and 7 am Sunday morning.

Slight shifts westward and southward occurred in models overnight, resulting in most of them now showing a track for Irma much closer to the north coast of Cuba on Saturday. This is bad news for Cuba, but good news for Florida, since it means the hurricane will not be able to intensify as much as it could have. There is a chance (perhaps 20%) that Cuba could disrupt Irma enough to reduce it to a Category 2 storm by the time of landfall in South Florida on Sunday morning. As of 10 am EDT Friday, the latest available runs (0Z or 6Z) of all of our top models for tracking hurricanes—the European, GFS, HWRF, HMON, and UKMET—had Irma making landfall in South or Southwest Florida Sunday morning between 1 am and 7 am, then turning north or north-northwest. Low tide is 6 am Sunday, and high tide is 12:15 am. All of the members of the high-probability cluster of the European model ensemble (Figure 6) also showed this track. The only ray of hope for South Florida could be found in the ensemble members of the 0Z GFS ensemble (Figure 5), which had a few members missing the peninsula. Since the average error in a 2-day forecast is about 90 miles, it is important to remember that the models may still have additional shifts, and one must pay attention to the NHC cone of uncertainty! If you are in the cone of uncertainty, you are in danger of a direct hit. In the case of Tampa, which is still in the cone, if a direct hit came, it would most likely be from over land, and thus only at Category 1 strength.

We will have more later today on Hurricane Jose (now a Category 4 storm with 150 mph winds) and Hurricane Katia (now a Cat 2 storm).We'll also look in more detail at Irma's storm surge in Florida.