Ferocious Irma Pounding Florida, But It Could Have Been Worse

Florida experienced widespread damage and disruption, and dodged several major bullets, as Hurricane Irma made landfall and ground northward along the state’s west coast on Sunday. As of 9 pm EDT Sunday, Irma was located about 50 miles southeast of Tampa, packing top sustained winds of 100 mph as it moved north at 14 mph. Irma was predicted to continue north on a gradually accelerating pace while increasing wind shear and land interactions sap its strength. The storm will move along or just inland from Florida’s west coast on Sunday night and Monday as a weakening hurricane, then arc leftward into Alabama on Monday night into Tuesday as a decaying tropical cyclone. 

Irma’s mayhem is not yet over. Damaging winds of sustained tropical-storm strength with gusts to 50 - 90 mph overspread much of the southern and eastern Florida peninsula on Sunday. These winds will work their way toward parts of Georgia overnight, bringing down countless trees and power lines. More than 2 million customers in South Florida alone have lost power; it may take weeks to restore service to all customers, said officials with Florida Power & Light on Sunday evening.

Figure 1. Infrared GOES-16 satellite image of Hurricane Irma as of 10:52 pm EDT Sunday, September 10, 2017. Image credit: NASA/MSFC Earth Science Branch.

Irma made landfall at around 9:10 am EDT near Cudjoe Key, about 30 miles east of Key West, while classified as a Category 4 storm with 130-mph sustained winds. Officials at the National Weather Service office estimated a peak wind gust of 94 mph before the site's anemometer was knocked out of service. Power was out across the Keys on Sunday night, and communications were sketchy, but it appears storm-surge flooding in parts of the Keys was considerably more severe than during Hurricane Wilma in 2005. On Sunday, the emergency director for Monroe County, Martin Senterfitt, called Irma’s impact on the Keys a “humanitarian crisis.” A series of C-130 flights involving the U.S. Air Force and Air National Guard will be staged from Florida Keys Marathon Airport.

If a powerful hurricane had to approach the Florida peninsula from the south, Irma took one of the better possible routes. Irma made its second Florida landfall at 3:35 pm EDT Sunday at Marco Island while classified as a Category 3 hurricane, then moved north along the west coast. This landfall location kept the bulk of the eastern eyewall’s intense winds over the largely empty Everglades rather than over the state’s densely populated east and west coasts. Moreover, Irma reached Florida without having fully recovered from the disruption caused to the storm by having rolled along the north coast of Cuba for the better part of a day. Had Irma tracked just 20 miles off Cuba's northern coast instead of riding right along the coast, we might have been experiencing a Cat 5 landfall in Florida today.

Figure 2. A huge wave breaks near the Morro Castle in Havana, Cuba, on Sunday, September 10, 2017. More than a million people were evacuated in Cuba from the path of Irma, the first Category 5 storm to hit the nation in almost a century. Image credit: Yamil Lage/AFP/Getty Images.

Irma’s northern eyewall socked the coast from Fort Myers to Naples with the strongest winds reported anywhere in Florida, but very high gusts were also recorded along the southeast coast. A personal weather station (non-wunderground) at Naples Municipal Airport reported an unofficial gust to 142 mph; Naples Municipal Airport recorded gusts as high as 76 mph before the site stopped reporting. Other gusts compiled as of late Sunday by weather.com included:

• North Perry Airport (Broward County): 109 mph
• Miami International Airport (tower observation): 99 mph
• St. Lucie Nuclear Power Plant: 99 mph
• Ochopee: 92 mph
• Coral Gables: 90 mph
• Fort Myers (Southwest Florida International Airport): 89 mph
• Marathon Key: 88 mph
• Deerfield Beach: 86 mph
• Miami-Opa Locka Executive Airport: 84 mph
• Juno Beach: 83 mph
• Miami NWS/NHC office: 81 mph
• Tampa Bay: 78 mph
• Clearwater Beach: 78 mph
• West Palm Beach International Airport: 77 mph
• Fort Lauderdale/Hollywood International Airport: 75 mph
• Port Charlotte (Charlotte County Airport): 74 mph

Irma’s surge still pouring onto Florida’s west coast Sunday night

As of Sunday evening, storm surge from Irma was less severe than feared from Naples to Fort Myers. Storm tides (storm surge plus astronomical tides) of 4’ to 6’ were common, less than the worst-case inundation forecast of 10’ – 15’. This is likely because the angle with which Irma hit the coast brought the greatest surge to the south of Naples. It may also be, in part, because Irma’s southern eyewall, which brought onshore winds to the west coast, reached Florida in a weakened state. However, initial photos out of Marco Island, where Irma initially made landfall in Southwest Florida, show that a significant storm surge arrived there.

Some more photos of storm surge on Marco Island — photos from: https://t.co/hyBM5ShHwI#MarcoIsland #StormSurge #HurricaneIrma pic.twitter.com/qwfMZmh6Ae

— Zachary Halaschak (@zhalaschak) September 11, 2017

ALERT: Water surrounding Tampa Bay is very low due to the storm. It is dangerous to be walking out there. pic.twitter.com/yUY0YqyPRA

— Alert Tampa (@AlertTampa) September 10, 2017

The quick switch from offshore to onshore flow had dramatic effects along Florida’s southwest coast. Adam Dean (The Weather Channel) noted that Naples went from its second-lowest to its highest water level in 52 years of data, all in the span of eight hours on Sunday. Very low water was also observed in the Tampa area (see embedded tweet above).

The center of Irma was expected to pass just east of Tampa early Monday morning at hurricane strength, and it may be the first direct hit on the city by a hurricane since a Category 1 hurricane in 1946. Once Irma's center passes the city, onshore winds will funnel water into Tampa Bay, bringing a storm surge expected to be in the 2- to 4-foot range. The shores of Tampa Bay are heavily populated, with large portions of the metro area less than ten feet in elevation, so this storm surge flooding will have some impacts. It would have been much worse for Tampa, though, if Irma had hit the city head on, which would have resulted in a storm surge closer to 15 - 20' in Tampa Bay.

Well east of Irma’s center, southeast Florida experienced significant and prolonged surge on Sunday. Parts of downtown Miami flooded, and Virginia Key in Biscayne Bay maxed out with around 3.8 feet of storm surge just after 3 pm EDT.

Figure 3. A pedestrian walks through a flooded street in the Brickell area of downtown Miami as Hurricane Irma passed west of the city on Sunday, September 10, 2017. Image credit: Joe Raedle/Getty Images.

A swarm of mesocyclones (and waterspouts?) along the Space Coast

Tornadoes often form within rainbands across the right front quadrant of landfalling hurricanes, because of the effects of friction on near-surface winds and the resulting enhancement of vertical wind shear favorable for mesocyclones (small mid-level rotations). A veritable swarm of mesocyclones developed near the Space Coast of east central Florida on Sunday afternoon—more than 100 in all, estimated John Monteverdi (San Francisco State University). Forecasters at the NOAA/NWS Storm Prediction Center (SPC) had predicted an enhanced risk of tornadoes in east-central Florida as far back as Saturday. Meteorologists had their hands full on Sunday dealing with Irma, and many spin-ups would have been obscured by heavy rain, so we’ll never know exactly how many mesocyclones produced brief waterspouts offshore, or short-lived tornadoes onshore. Only three tornado reports had been compiled by SPC as of 8 pm EDT Sunday.

Mesocyclonic spin-ups are also common within hurricane eyewalls, where they can enhance the winds produced at the heart of the storm. On Sunday afternoon, as the Weather Channel’s Mike Bettes was reporting live from Naples within the northern eyewall of Irma, it appears a tiny spin-up passed just behind him.

Amazing embedded supercell structures in hurricane #Irma. Many waterspouts likely! pic.twitter.com/5H4nLytuvj

— Victor Gensini (@gensiniwx) September 10, 2017

Still ahead: torrential rain, potential flooding

Rainbands cascading toward Irma will dump huge amounts of rain from the Florida peninsula into parts of Alabama, Georgia, and South Carolina over the next couple of days. Totals will be far short of the 40 – 50” that fell in southeast Texas during Hurricane Harvey, because Irma is now moving at a healthy clip, but widespread 5” -10” totals with pockets of 10  - 20” can be expected. Major river flooding is possible in coastal areas from northeast Florida to southeast South Carolina, where runoff from heavy rain will meet a persistent storm surge from onshore winds flowing toward Irma. Localized flash flooding will be a risk across the east slopes of the Southern Appalachians, as rainbands associated with Irma stream northwest into the mountainsides.

Figure 4. GOES-16 view of Hurricane Jose at 4 pm EDT September 10, 2017. Image credit: NOAA/RAMMB. GOES-16 data is considered preliminary and non-operational.

After Irma, we have Jose to reckon with

After threatening the northern Leeward Islands hard-hit by Hurricane Irma, Hurricane Jose has moved off to the north of the islands, and is destined to spend the remainder of the week performing a slow clockwise loop several hundred miles north of Puerto Rico, far from any land areas. When Jose completes this loop late this week, it will be close to the eastern Bahamas, which are now in the 5-day cone of uncertainty for Jose.

Jose is under high wind shear of 20 - 30 knots, and this high shear has degraded the storm over the past two days. Jose was a high-end Category 4 storm with 155 mph winds on Saturday morning, but had weakened to a low-end Category 3 storm with 115 mph winds by 11 pm EDT Sunday. Moderate to high wind shear, combined with dry air, will likely continue to weaken Jose to Category 1 strength during the week, despite very warm sea surface temperatures (SSTs) near 29.5°C (85°F).

Long-range models suggest that a strong ridge of high pressure will build in to the north of Jose by Friday, forcing the hurricane to move west-northwest towards the U.S. East Coast. Once Jose approaches the U.S. this weekend, it may encounter some steering influence from the remnants of Hurricane Irma, which will still be over the Southeast U.S. If Irma’s remnants are cohesive enough, Jose would tend to move northward towards New England or Canada. Jose could also be influenced by stronger upper-level flow along the jet stream north of Irma’s remnants, but it’s too soon to tell exactly how the features within that flow will take shape over the eastern U.S. a week from now. It does appear we will be tracking this hurricane at least another ten days.

Figure 5. The 20 track forecasts for Jose from the 12Z Sunday, September 10, 2017 GFS model ensemble forecast. About half of the solutions resulted in an eventual landfall in the U.S. or Canada. Image credit: CFAN.

Figure 6. The 12Z September 10, 2017, track forecast by the operational European model for Irma (red line, adjusted by CFAN using a proprietary technique that accounts for storm movement since 12Z Sunday), along with the track of the average of the 50 members of the European model ensemble (heavy black line), and the track forecasts from the “high probability cluster” (grey lines)—the five European model ensemble members that have performed best with Irma thus far. Not much of a consensus here! Image credit: CFAN.

Dr. Jeff Masters contributed to this post.