Dr. Jeff Masters ·

Above: Infrared-wavelength [or visible-wavelength] GOES-16 satellite image of Category 5 Hurricane Irma as of 9 am EDT Tuesday, September 5, 2017. Image credit: RAMMB / CIRA@CSU.

Hurricane Irma intensified into an extremely dangerous high-end Category 5 storm with top sustained winds of 180 mph on Tuesday morning, putting it among the strongest Atlantic hurricanes ever observed. Irma's winds are the most powerful ever measured in an Atlantic hurricane north of the Caribbean and east of the Gulf of Mexico. Measurements from Hurricane Hunter aircraft found peak winds of close to 180 mph, well above the 157-mph threshold for Category 5 strength. At 11:07 am EDT, a dropsonde in Irma's eye measured a central pressure of 927 millibars, 4 mb lower than the previous pass, so Irma is still strengthening.

Figure 1. Radar image of Irma from NOAA hurricane hunter aircraft N42RF, taken at approximately 7 am EDT Tuesday, when the aircraft first observed Category 5 winds. Image credit: Tropicalatlantic.com and Google Earth.

Irma is poised to deliver a punishing blow to the northern Lesser Antilles Islands on Tuesday night and Wednesday. As of 11 am EDT Tuesday, Hurricane Warnings were in effect for the northern Leeward Islands, the U.S. and British Virgin Islands, and Puerto Rico. Tropical storm-force winds are expected to spread into the Lesser Antilles on Tuesday night, reaching the Virgin Islands on Wednesday morning, Puerto Rico on Wednesday afternoon, and the Dominican Republic on Thursday morning (Figure 2). As of 8 am EDT, most of southern Florida, Cuba, and The Bahamas were in the 5-day cone of uncertainty for Irma.

Figure 2. Most likely arrival time of tropical-storm-force winds from Irma, as of the 11 am EDT Tuesday, September 5, 2017 advisory from NHC.

Satellite images on Tuesday morning showed a spectacular hurricane with a large eye surrounded by extremely intense eyewall thunderstorms with very cold cloud tops, indicating that they extended high into the atmosphere. Irma had excellent upper-level outflow on all sides. Conditions were favorable for even more strengthening, with wind shear a low 5 – 10 knots. Sea surface temperatures (SSTs) were a very warm 29.5°C (85°F), and the total heat content of the ocean was a high 60 kilojoules per square centimeter, giving the storm plenty of heat energy to fuel intensification. The surrounding atmosphere has been steadily moistening, as seen on precipitable water imagery, with a mid-level relative humidity near 55%, according to the 12Z Tuesday analysis from the SHIPS model. The eye of Irma was just beginning to be seen on Martinique radar.

Intensity forecast for Irma

According to Dr. Phil Klotzbach, Irma is tied with Rita (2005) and Mitch (1998) as the fifth strongest hurricane in Atlantic records going back to 1851, based on maximum wind speed. Irma is the first Atlantic hurricane outside of the Caribbean and Gulf of Mexico known to attain 180-mph sustained surface winds. The lowest central pressure measured outside the Caribbean and Gulf was 919 mb in Hurricane Gloria (1985), versus Irma's most recent central pressure of 927 mb, but Irma could end up breaking this record as well. The highest winds of any Atlantic hurricane are 190 mph, set by Hurricane Allen (1980), and Irma may approach that record.

For the next five days, wind shear, SSTs, and ocean heat content will remain very favorable for development, with Irma passing over slightly warmer waters of 29.5 - 30°C (85 - 86°F) later this week. Mid-level relative humidity is predicted to slowly rise, reaching 65% by the end of the week. We can expect one or more eyewall replacement cycles (ERCs) this week, which will act to temporarily weaken the hurricane by perhaps 10 mph, followed by re-intensification.

Three of our four most reliable intensity models—the HWRF, COAMPS-TC, and LGEM—predicted in their Tuesday morning runs that Irma would be a Category 4 or 5 hurricane with 130 - 160 mph winds through Saturday, and the official NHC forecast of a Category 4 or 5 hurricane for the remainder of the week looks reasonable. The only major impediment to Irma’s strength would appear to be interaction with land; a close pass or direct hit on Hispaniola or Cuba could potentially damage or destroy the hurricane’s inner core and knock it down to Category 2 or 3 strength.

The NOAA-NASA Suomi NPP satellite took this infrared snapshot of the powerful, Cat. 5 #HurricaneIrma earlier this morning, Sept. 5, 2017. pic.twitter.com/EHvWGm0sbD — NOAA Satellites PA (@NOAASatellitePA) September 5, 2017

Potential impact on the islands

Only three hurricanes in the satellite era (since 1966) have hit the Leeward Islands with winds of 150 mph or greater: David (1979), Hugo (1989), and Lenny (1999). All three hurricanes caused major damage on the islands they hit, and we can expect Irma to cause extreme damage to any islands it makes a direct hit on. At this time, it appears that The Bahamas are at highest risk of receiving the most devastating wind and storm surge impacts from Irma, though the islands at the extreme northern end of the Lesser Antilles chain and the northern Virgin Islands may also receive direct hits.

Irma will assume a more west-northwesterly track over the next day, which would bring the core of the hurricane just north of the Virgin Islands and Puerto Rico on Wednesday. The northern portions of both locations are in the cone of uncertainty, and could recieve a direct hit. Irma is expanding in size, and is predicted to increase the radius of its tropical-storm force wind area by about 10 - 15 miles every day. As of 11 am EDT Tuesday, tropical storm-force winds extended out 140 miles from the center, and hurricane-force winds extended out 35 miles from the center. Most of the islands along Irma’s path will be on the weaker left side, where the wind and storm surge impacts will be less than on the right side of the storm.

The 11 am EDT Tuesday Wind Probability Forecast from NHC highlighted a number of islands that might be at risk of hurricane-force winds on Tuesday and Wednesday. The highest odds were for Barbuda and Saint Maarten, with an 87 - 90% chance of hurricane-force winds. For the northern Virgin Islands and northern Puerto Rico, a 41 - 66% chance was given.

The 6Z Tuesday run of the HWRF model predicted that much of The Bahamas and eastern portions of Cuba may receive rains of 8 – 16” from Irma, and these rains will be capable or causing life-threatening flash floods and mudslides. Hopefully, the core of the 8 - 16" rain swath will stay offshore from Puerto Rico and Hispaniola, but these islands could well be affected by torrential rain.

Figure 3. The 20 track forecasts for Irma from the 0Z Tuesday, September 5, 2017 GFS model ensemble forecast. Image credit: CFAN.

Figure 4. The 0Z September 5, 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 0Z), along with the track of the average of the 50 members of the European model ensemble (heavy black line), and the 50 track forecasts from the 0Z Tuesday European model ensemble forecast (grey lines). Image credit: CFAN.

Figure 5. The 0Z September 5, 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 0Z), 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 European model ensemble members that have performed best with Irma thus far. Image credit: CFAN.

Long-range outlook for Irma

Irma poses the most serious hurricane threat to northern Cuba and Florida since at least Hurricane Andrew (1992). Since Sunday night, computer models have agreed that Irma will continue west-northwest before making a fairly sharp right-hand turn in the vicinity of the Florida Straits over the weekend. The level of agreement among models and over time has been quite high for a forecast in the 5-day range. Given this agreement and Irma’s Category 5 strength, residents of Florida must take this hurricane with the utmost seriousness.

What is not yet certain is whether Irma will travel along Florida’s west coast or its east coast, offshore from one or the other, or along the spine of the Florida peninsula. Any of these paths could bring significant and potentially devastating impacts to large parts of the state. There remains a small chance that Irma will make a sharp enough turn to miss Florida and head north through The Bahamas, but the stakes are too high for Floridians to count on that possibility.

Based on model guidance from Monday night (00Z Tuesday), it appears that the most likely outcome is for Irma to arc slightly leftward as it approaches the Florida Straits, moving just inland over northern Cuba for perhaps 12-24 hours. Much of Irma’s circulation would remain over water, and northern Cuba is a much less mountainous region than southern Cuba, so it is less likely to disrupt Irma’s circulation and cause a major drop in intensity. Cuba has a well-organized hurricane warning and response program that will go a long way to ensure public safety should Irma make landfall. Still, severe damage would be possible if Irma does strike northern Cuba, and Irma’s intensity could easily remain at Cat 4 after it leaves Cuba, as indicated by our most reliable models. If Irma stays just north of Cuba, it will likely maintain Category 4 or 5 strength through at least Sunday, as predicted by NHC.

NHC's official 5-day forecast as of 11 am EDT brings Irma to the Florida Keys by Sunday morning. At this point, it appears the most likely course for Irma after its right-hand turn is to move northward near Florida’s west coast or up the western side of the peninsula from around Sunday into Monday. This is the scenario favored by the operational 00Z Tuesday run of the European model, as well as two of the highest-probability ensemble members from that run. The other three highest-probability Euro tracks keep Irma offshore, either to the west or east of the Florida peninsula; two of those three tracks results in a landfall in the Florida Panhandle, and the other would be a devastating blow to The Bahamas. The operational 00Z, and the ensemble members from the 00Z Tuesday GFS model run, are more tightly clustered around a track near or just off Florida’s west coast, but again with some variation. The 06Z run of the GFS takes Irma near Miami and along Florida’s east coast.

The take-home message: while it is too soon to rule out other possibilities, Irma has a good chance of moving northward close enough to the Florida peninsula for significant impacts to large parts of the state, potentially devastating in some areas. Irma may be moving at 10 mph for a day or more after it makes its northward turn, which will prolong the period of high winds and heavy rains within its circulation. Even if it moves along Florida’s west coast, residents on the East Coast could still receive hurricane-force winds, significant storm surge, and torrential rains of 10 - 15” or more. Depending on Irma’s track, some areas could experience 8 hours or more of hurricane-force wind and 24 hours or more of tropical-storm-force wind. The National Hurricane Center reminds us not to focus on the exact forecast track, though, especially at the longer ranges, since the average NHC track errors are about 175 and 225 miles at days 4 and 5, respectively.

Figure 6. Track of Hurricane Donna of 1960.

Comparison with Hurricane Donna of 1960

The best historical analogue for a hurricane that follows the current NHC forecast for Irma may be Hurricane Donna of 1960, which tore through The Bahamas and the Florida Keys just northeast of Marathon as a Category 4 storm with 140 mph winds. The hurricane continued to the northwest along the southwest coast of Florida, passing over Naples and Fort Myers before turning inland to the northeast. Donna maintained Category 2 strength into central Florida, then weakened to a Category 1 storm as it passed near Orlando, and exited the coast near Daytona Beach. Donna then made landfall near Wilmington, NC and on Long Island, New York as a Category 2 storm. Donna killed 148 and caused $387 million in damage (1960 dollars).

If Donna were to hit today, damage would likely be more than $50 billion, according to three separate estimates. ICAT estimates a loss of $66 billion; according to a 2006 AIR Worldwide publication, “What would they cost today? The estimated impact of historical catastrophes on today’s exposures”, a repeat of Donna in 2005 would have caused $26 billion in insured losses. This includes loss to property, contents, direct business interruption, and additional living expenses for residential, mobile home, commercial, and auto exposures. Since uninsured losses from a hurricane are typically roughly equal to insured losses, this would put the cost of a repeat Donna at $52 billion in 2005. That was 12 years ago, and according to a 2006 report by AIR Worldwide, catastrophe losses should be “expected to double roughly every 10 years because of increases in construction costs, increases in the number of structures and changes in their characteristics.” Thus, a repeat of Donna in 2017 could be expected to generate roughly $100 billion in losses. An independent analysis done in 2012 by Karen Clark & Company, “Historical Hurricanes that Would Cause $10 Billion or More of Insured Losses Today”, found that a repeat Donna in 2012 would have done somewhat less damage (but still a staggering amount): $25 billion in insured losses, or roughly $50 billion in total losses.

Figure 7. Enhanced infrared image of 94L as of 1315Z ( 9:15 am EDT ) Tuesday, September 5. Image credit: NOAA/NESDIS.

Tropical Storm Jose forms in central Atlantic

A tropical wave located about 1500 miles east of the Leeward Islands developed into Tropical Storm Jose on Tuesday morning. As of 11 am EDT, Jose was moving west-northwest at 10 - 15 mph with top sustained winds of 40 mph. Satellite images on Tuesday morning showed plenty of spin, and heavy thunderstorm activity was gradually increasing and growing more organized. Conditions were favorable for development, with moderate wind shear of 15 - 20 knots, SSTs near 28.5°C (83°F), and a moist surrounding atmosphere.

The 0Z Tuesday operational runs of our three reliable models for predicting tropical cyclone genesis—the GFS, European and UKMET models—all predicted further develompent of Jose. Residents of the Lesser Antilles Islands should keep an eye on this system, since approximately 40% of the 50 members of the 0Z Tuesday European model ensemble forecasts showed 94L affecting the Lesser Antilles late this week. The official NHC forecast as of 11 am Tuesday takes Jose well north of the Leeward Islands as a strong Category 2 hurricane by Saturday. The super long-range GFS model forecasts of 94L/Jose show it performing an unusual clockwise loop in the mid-Atlantic next week, but such long-range forecasts are of low reliability.

Because Jose and Irma are more than 1000 miles apart, their tracks are unlikely to be influenced by the Fujiwhara effect. However, outflow from Irma could produce vertical shear that may slow Jose's development later this week.

Figure 8. Enhanced infrared image of 95L as of 1342Z (9:42 am EDT) Tuesday, September 5. Image credit: NASA/MSFC Earth Science Branch.

Gulf of Mexico disturbance 95L may develop

A trough of low pressure in the southwestern Gulf of Mexico’s Bay of Campeche was designated Invest 95L on Monday night. The system was producing increasingly organized heavy thunderstorms on Tuesday morning, as seen on satellite imagery. SSTs are very warm, near 30.5°C (87°F), but wind shear is high, 20 – 30 knots. Our three reliable models for predicting tropical cyclone genesis all developed the system in their 0Z Tuesday runs, predicting that it would affect the coast of Mexico between Veracruz and Tampico with heavy rains late this week. Strong upper level winds out of the northwest over the Gulf of Mexico should keep 95L bottled up in the southwestern Gulf of Mexico near the coast of Mexico this week. None of the 0Z Tuesday operational model runs nor the 70 members of the GFS and European model ensemble runs intensified 95L into a hurricane. In its tropical weather outlook issued at 8 am EDT Tuesday, the National Hurricane Center gave this system 2-day and 5-day odds of development of 50% and 60%, respectively.

Bob Henson co-wrote this post.