Irma Pounding Cuba as a Cat 3; Likely to be a Cat 3 or 4 in Florida

Hurricane Irma powered ashore on the north coast of Cuba Friday night as a Category 5 storm with 160 mph winds, marking Cuba’s first Category 5 landfall since 1924. Irma has tracked west-northwest along the north coast of Cuba since then, and the encounter with Cuba has significantly weakened the hurricane. As of 11 am EDT Saturday, NHC brought down its estimate of Irma’s top winds to 125 mph, making it a high-end Category 3 storm. An Air Force hurricane hunter aircraft did four center fixes in the storm on Saturday morning, and found a steadily rising pressure, increasing from 935 mb (6 am EDT), to 941 mb (10 am EDT). They found winds of 115 mph, which is the low end of Category 3 strength.

Heavy squalls from Irma were affecting South Florida and the Keys on Saturday morning, and sustained winds above tropical storm-force were already occurring at two stations in the Keys at 10 am EDT: 43 mph, gusting to 46 mph, in the Upper Keys at Fowey Rock; and 40 mph, gusting to 46 mph, in the Middle Keys at Molasses Reef. At 11:30 am EDT, a storm surge height of 1.2’ was reported at Marathon, 0.8’ at Key West, and 1.0’ at Miami. You can follow the storm surge heights by using our wundermap with the “Storm Surge” layer turned on.

Here is how much of Key West went under water in Wilma (2005). This was a Cat 3 that passed 75 miles north. https://t.co/Lu38BAl62o pic.twitter.com/y0ZqzVjv2h

— Bob Henson (@bhensonweather) September 9, 2017

Irma will re-intensify once it leaves the Cuban coast on Saturday afternoon, and is likely to be a very dangerous Category 3 or 4 hurricane when it hits the Florida Keys and west coast of Florida on Sunday. If you live in an evacuation zone in western Florida, get out now if you can safely do so. That is probably not an option in the Keys now, since winds of tropical storm-force are already being experienced there. I recommend that you read storm surge expert Dr. Hal Needham’s Saturday morning blog post outlining his thoughts on the likely storm surge heights and damage. The amount of water pushed by a hurricane like Irma does not decrease very quickly even if the winds at its core decrease. Three of the four most expensive U.S. hurricanes on record dropped by two Saffir-Simpson categories before making landfall: Katrina came ashore as a Cat 3,  Sandy was a post-tropical cyclone near minimal hurricane strength, and Ike was a Cat 2. Do not fixate on Irma's Saffir-Simpson rating.

NHC's storm surge forecast was hiked significantly in their 11 am EDT Saturday advisory, with 10 - 15' of surge now expected along the west coast of Florida as far north as Fort Myers, 6 - 10' from Fort Myers to Sarasota, 5 - 10' in the Florida Keys, and 5 - 8' in Tampa Bay.

Figure 1. GOES-16 Visible satellite image of Irma at 10 am EDT September 9, 2017. Image credit: NOAA/RAMMB.

Category 4 storms cause catastrophic damage

Here is NHC’s characterization of damage from a Category 4 hurricane (focusing on wind-related damage): Catastrophic damage will occur. Well-built framed homes can sustain severe damage with loss of most of the roof structure and/or some exterior walls. Most trees will be snapped or uprooted and power poles downed. Fallen trees and power poles will isolate residential areas. Power outages will last weeks to possibly months. Most of the area will be uninhabitable for weeks or months.

Intensity forecast for Irma

Infrared satellite images and Cuban radar on Saturday morning showed that the areal extent and intensity of Irma’s core area of heavy thunderstorms had sharply decreased since Friday night, but the storm was still maintaining a small 17-mile diameter eye with a fully closed eyewall. We may not yet have seen all the weakening that Cuba may cause to the hurricane, and I expect Irma may be a low-end Category 3 hurricane with 115 mph winds by Saturday evening. Irma’s future intensity depends crucially on whether or not this inner core can survive until the storm starts to separate from the coast on Saturday afternoon. Given the large size and resiliency of the hurricane so far, I think the inner core will survive, and Irma will begin a round of intensification on Saturday afternoon that will continue until at least Sunday morning, when the storm makes landfall in the Florida Keys.

Conditions are favorable for intensification through Sunday evening, with a moist atmosphere, light to moderate wind shear of 5 – 15 knots, and very warm ocean waters near 30°C (86°F). NHC is calling for Irma to increase its maximum winds by 5 mph to 130 mph by Sunday evening, making it a low-end Category 4 hurricane with 130 mph winds when it makes landfall on the west coast of Florida on Sunday evening. A reasonable range of intensities at landfall Sunday would be 115 - 145 mph, and thus I expect Irma will be a major Category 3 or 4 storm for Florida.

Figure 2. WU model map image showing the track for Irma from the 6Z Saturday, September 9, 2017 suite of computer models. All of the models showed a track through the Florida Keys and into Southwest Florida.

Track forecast for Irma

The models have not budged much in their forecasts Saturday morning, and they continue to predict a turn for Irma more to the northwest on Saturday afternoon, followed by a turn to the north-northwest. As of 11:45 am EDT Saturday, the latest available runs (0Z or 6Z or 12Z) of our top models for tracking hurricanes—the European, GFS, HWRF, HMON, and UKMET—had Irma re-intensifying after leaving Cuba, making landfall in the Middle or Lower Florida Keys on Sunday morning near 8 am, then turning north-northwest and making a second landfall on the west coast of Florida between Marco Island and Sarasota between 6 pm – 10 pm Sunday.

The timing and location of this second landfall is uncertain, since Irma will be traveling nearly parallel to the coast. A slight change in the storm’s track could make a 100-mile difference in where landfall occurs. Several of our most reliable models have Irma passing directly over Tampa Bay between 2 am – 4 am Monday. Depending upon how far to the south Irma makes landfall before arriving in Tampa, passage over land should reduce Irma’s top winds by 15 – 35 mph before the hurricane arrives. Due to the configuration of the coast and the expected north-northwest track of Irma, the hurricane should not be able to push a large storm surge into Tampa Bay while it is approaching, since the winds will be blowing offshore. Only after the storm passes and the winds reverse to blow onshore will Tampa Bay see a storm surge, and NHC is warning of a possible 3 – 5’ inundation above ground level. The highest storm surge from Irma is likely to be in the Naples area, where NHC is warning of an 8 – 12’ storm surge. High tide at Naples is at 3 am and 4 pm on Sunday, and low tide is at 9:30 am and 9:30 pm. The difference between low tide and high tide is about 2.5 feet, so Irma will drive a significantly lower storm tide to the coast if it hits at low tide.

Bob Henson contributed to this post.