Irma Reorganizes En Route to Devastating Florida Hit

Hurricane Irma is on the mend Saturday evening, headed for a potentially catastrophic encounter with Florida on Sunday. The hurricane dropped from Category 5 to Cat 3 strength Friday night as it pushed west-northwest along the north coast of Cuba for the better part of a day. Irma’s fierce winds inflicted damage across a broad swath of north central Cuba, including the resort areas of the barrier islands known as the Cuban Keys, though reports thus far have been sketchy. This was the first landfall of a Category 5 hurricane in Cuba since 1924. Fortunately, Cuba experienced the less intense left-hand side of the storm, and chances are the island’s excellent disaster preparedness strategy kept deaths and injuries to a minimum.

Figure 1. Radar image of Irma taken at 8:30 pm EDT September 9, 2017 from the Key West radar.

As of 8 pm EDT Saturday, Irma’s center was located only about 15 miles north of the Cuban mainland and just a few miles north of the Cuban Keys. Hurricane Hunters found on a late-afternoon flight that Irma had developed concentric eyewalls, with an inner eyewall about 17 miles wide and an outer one about 35 miles wide. The dual-eyewall structure means that Irma’s peak sustained winds are somewhat lower than before (120 mph as of 8 pm EDT), but spread out over a broader area. Irma was moving at about 9 mph on a west-northwest track that will bring its outer eyewall completely offshore from Cuba on Saturday night. Heavy squalls from Irma were affecting South Florida and the Keys on Saturday evening, and sustained winds above tropical storm-force were already occurring at a number of stations in the Keys as of 7 pm EDT Saturday:

49 mph, gusting to 56 mph:  Molasses Reef (Upper Keys)
48 mph, gusting to 59 mph:  Vaca Key (Lower Keys)
53 mph, gusting to 56 mph:  Fowey Rock (Upper Keys)
43 mph, gusting to 47 mph:  Pulaski Shoals Light (Lower Keys)

Virginia Key, just east of Miami, reported a wind gust of 53 mph at 4:12 pm, and Key West reported a wind gust of 53 mph at 6:06 pm EDT. Shortly before 8 pm EDT, Marathon International Airport reported a sustained wind of 48 mph and a gust to 67 mph.

At 7:30 pm EDT, storm surge heights of 1.0 to 1.2 feet were bring reported at Marathon, Key West, and Miami. You can follow the storm surge heights by using our wundermap with the “Storm Surge” layer turned on.

Outlook for Irma

Irma’s inner core spent more time over Cuba than forecasters had expected, although the track forecast overall was quite good. The resulting interaction with land disrupted Irma’s core and reduced the strength and coverage of its core thunderstorms. Satellite imagery late Saturday afternoon showed that Irma was having only fitful success at reorganizing its intense thunderstorms around the outer eyewall. Irma will be moving over very warm waters Saturday night (sea surface temperature of around 30°C or 86°F), and wind shear will remain light to moderate. Outflow from the storm’s top will be enhanced by two jets, one on the north and one on the south. Irma’s dual eyewall structure and its large size will put some brakes on rapid intensification, but some strengthening appears likely, and we expect Irma to reach the Florida Keys early Sunday and southwest Florida by midday Sunday as a Category 4 storm, as predicted by NHC.

Horrifyingly spectacular water vapor imagery showing powerful outflow jets helping to strengthen #hurricaneirma-> do not take #irma lightly! pic.twitter.com/pAGWJ96lqx

— Andrew Moore (@OSUWXGUY) September 9, 2017

Expected impacts from Irma

Irma is still predicted to “run the peninsula”, taking an unusual track from south-southeast to north-northwest along the length of the state. This forecast track has steadily nudged west since Thursday, and Irma is now expected to track closer to Florida’s west coast than its east coast. Models are now in very close agreement, and we expect little significant shift to this track outlook.

Parts of the Florida Keys will experience extreme winds and storm surge. On its west-northwest heading late Saturday, Irma was aimed toward the west of Key West, which would put the city on the storm's more dangerous right-hand side. However, Irma is beginning to angle rightward, and this will most likely bring its core somewhere near or just east of Key West between around 2 and 8 am Sunday. Winds of 120 mph or more can be expected just east of the eye, and storm surge is predicted to range from 5 to 10 feet.

A catastrophic storm surge is increasingly likely from Fort Myers to Naples on Sunday afternoon as Irma passes along or just to the west of this coastline. Surge values of 10 to 15 feet are expected from Cape Sable to Captiva. Vanderbilt Beach at Naples experienced a peak surge of 10-13 feet during Hurricane Charley, and Captiva Island experienced a surge of 6.5 feet. Irma’s surge will be more widespread and devastating to southwest Florida than the surge experienced during Charley, since Irma is a much bigger storm and has had more time to build up a larger surge. Parts of this coastline may also experience destructive Category 2 or 3 winds if Irma intensifies as expected and its center remains just offshore. Assuming that Irma moves inland between Fort Myers and Tampa as predicted, surge values northward through the Tampa area will be somewhat lower, but still potentially 5 to 8 feet. It would take a significant jump westward in the forecast track to put Tampa on the more dangerous right-hand side of the storm.

Figure 2. Worst-case storm surge inundation values predicted by NHC as of Saturday evening, September 9, 2017. Values shown are those that would exceeded about 10% of the time in situations like the current one. Inundation values assume high tide, but they do not include the effects of waves or of inland rainfall flowing toward shore. 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. Image credit: NHC.

If Irma’s track holds, the Miami area can expect top sustained winds of 60 - 80 mph, with a long period of tropical-storm-force winds. Expect widespread tree damage and power outages, and perhaps extensive damage to the facades and windows of the upper stories of skyscrapers, where winds will be considerably stronger. Winds will be somewhat weaker northward into Fort Lauderdale and Palm Beach, but a few hours of 40 - 60 mph sustained winds are possible here. Surge in the Biscayne Bay area is not expected to be catastrophic, but it could be significant (4 to 6 feet), with sharp local variations as the storm moves by to the west.

A pocket of 60 – 80 mph sustained winds may develop Sunday night across central Florida as Irma passes just to the west. These winds could be as strong or stronger than those produced in the Orlando area by Hurricane Charley in 2004. Winds may also approach or exceed hurricane force along the northeast Florida coast, including the Jacksonville area, late Sunday night.

Significant storm surge of 4 to 6 feet remains possible in Georgia and southern South Carolina. The higher end of this range could rival some all-time records at coastal tidal gauges, including 5.06 feet set at Fort Pulaski, Georgia, during Hurricane Matthew in 2016. Expect widespread areas with damaging tropical-storm-force winds across northern Florida and southern Georgia, enough to down trees and power lines.

Torrential rains of 7 – 15”, with local totals exceeding 20”, can be expected across most of Florida and southeast Georgia. These amounts will not rival the 40 – 50” totals from Hurricane Harvey, because Irma will not be stalling over Florida as Harvey did over Texas. Rains of 2 – 5” may affect northern Georgia, including Atlanta, from Monday into Tuesday as Irma angles toward the west.

Figure 3. Track of the Tampa Bay Hurricane of 1921, one of only two major hurricanes ever recorded to hit the city. This Category 3 storm with 115 mph winds brought a storm tide of 10 - 11.5 feet (3 - 3.5 meters) to Tampa Bay.

Tampa’s hurricane history: only two major hurricanes since 1848

Tampa Bay doesn't get hit very often by hurricanes. This is because the city faces the ocean to the west, and the prevailing east-to-west trade winds at that latitude make it uncommon for a storm to make a direct hit on the west coast of Florida from the ocean. This is fortunate, since the large expanse of shallow continental shelf waters offshore from Tampa Bay (less than 300 feet deep out to 90 miles offshore) is conducive for allowing large storm surges to build. Tampa Bay is most vulnerable to large storm surges from storms approaching from the southwest or west and passing just north of the city, since the westerly winds in the hurricane's eyewall will force a massive storm surge directly into the bay. Tampa Bay is much less vulnerable to large storm surges from a storm approaching from the south, like Irma, since the hurricane's winds will be blowing offshore until the eye of the storm passes to the north. At that time, the winds will reverse and bring a storm surge into Tampa Bay.

The last time Tampa suffered a direct hit by any hurricane was 1946, when a Category 1 storm came up through the bay. The Tampa Bay Hurricane of October 25, 1921 was the last major hurricane to make landfall in the Tampa Bay Region. This low-end Category 3 storm with 115 mph winds at landfall brought a storm tide of 10 - 11.5 feet (3 - 3.5 meters), causing severe damage ($10 million in 1921 dollars.) The only other major hurricane to hit the city occurred on September 25, 1848, when the Great Gale of 1848, the most violent hurricane in Tampa's history, roared ashore as a Category 3 or 4 hurricane with 115 - 135 mph winds. A 15-foot storm surge (4.6 meters) was observed in what is now downtown Tampa, and the peninsula where St. Petersburg lies, in Pinellas County, was inundated, making St. Petersburg an island. A large portion of what few human structures were then in the area were destroyed.

Two mass evacuations in Tampa in the past 35 years
Two hurricanes have prompted mass evacuations of more than 300,000 people from the Tampa Bay area over the past 35 years. The first was Hurricane Elena of 1985, a Category 3 hurricane that stalled 80 miles offshore for two days on Labor Day weekend, bringing a 6 - 7 foot storm surge, wind gusts of 80 mph, and torrential rains. On August 13, 2004, another mass evacuation was ordered for Hurricane Charley. Thanks to a late track shift, Charley missed Tampa Bay, and instead hit well to the south in Port Charlotte as a Category 4 storm with 150 mph winds. More limited evacuations of low-lying areas and mobile homes in the 4-county Tampa Bay region were ordered for three other hurricanes in the past twenty years--Hurricane Georges of 1998, Hurricane Frances of 2004, and Hurricane Jeanne of 2004.

Figure 4. A sensible hurricane awareness effort: Hillsborough County, Florida got a $30,000 grant to post 30 of these signs around the Tampa Bay area to show how high a storm surge from a major hurricane might reach. This sign by a McDonald's at 19th Ave. and Highway 41 is positioned 13 feet above ground level. Image credit: photonews247.com.

Tampa Bay's vulnerability to hurricanes
When the 1921 hurricane hit Tampa Bay, there were 160,000 residents in the 4-county region, most of whom lived in communities on high ground. Today there are 2.8 million residents in the region, and that number is growing by about 50,000 people per year. Most of the population in the 4-county Tampa Bay region lives along the coast in low-lying areas; about 50 percent of the population lives at an elevation less than ten feet. Over 800,000 people live in evacuation zones for a Category 1 hurricane, and 2 million people live in evacuation zones for a Category 5 hurricane, according to the 2010 Statewide Regional Evacuation Study for the Tampa Bay Region. Only 46% of the people in the evacuation zones for a Category 1 hurricane evacuated when an evacuation order was given for 2004's Category 4 Hurricane Charley.

Global warming may make extreme Tampa hurricanes up to 14 times more likely
Using a detailed hurricane model embedded within six different global climate models, hurricane scientists Kerry Emanuel of MIT and Ning Lin of Princeton University showed in a 2015 paper that the risk of extreme “grey swan” hurricanes in Tampa may increase by up to a factor of fourteen by the end of the century, thanks to human-caused climate change. Grey swan hurricane are storms so violent that they have never been observed in the historical record, but can be anticipated to occur in the future. See Jeff Masters’ blog post on the subject.

We'll be back late Saturday night with an update on Irma and on Hurricane Jose, which continues to rage at Category 4 strength just northeast of the Leeward Islands.

Bob Henson co-wrote this post.