At 2:00 AM AST on Sat Sep 9, 2017 the eye of Hurricane Irma is moving over the Camaguey Archipelago of Cuba as a Category 5 Hurricane with maximum sustained winds of 160 mph.
The most common metric for understanding the strength of a hurricane is the Saffir-Simpson scale, which is determined by maximum wind speed. Irma, a Category 5 as of Saturday morning, September 9, broke the record for consecutive hours at Category 5 on Thursday. But hurricane strength is about more than just maximum wind speed. Energy is an important metric to measure, because it can help determine the impacts, how bad they will be and how far they will extend.
A very large storm with moderate winds may have more total energy than an intense but small storm. The map above, provided by RMS, a company specializing in risk assessment, compares the total energy of Irma with other historic and recent hurricanes in the Atlantic basin. Irma’s wind speed along with its size carries an energy value of 112 terajoules, which is similar to the energy carried by Katrina in 2005. How much energy is that? A lot. Check out the table above. Hurricane Sandy, not depicted in this map, was a Category 1, but according to RMS carried 330 terajoules of energy before its landfall in New Jersey because its wind field was so big.
Irma’s maximum sustained winds were measured at 185 mph Tuesday. Only one Atlantic Ocean storm in recorded history has exceeded that level: Hurricane Allen, which slammed into Mexico and Texas in 1980, had top wind speeds of 190 mph.
About 400 miles wide, Irma is significantly larger and, at least for now, a bit more powerful than Hurricane Andrew, which devastated parts of South Florida in 1992. Andrew’s top sustained winds were 175 mph, as were Hurricane Katrina’s in 2005.
But while Katrina had declined to a Category 3 storm by the time it struck the Gulf Coast, Andrew strengthened to a Category 5 just before it barreled into Florida. (Exact numbers for Andrew are sketchy because the storm destroyed much of the measuring equipment in its path.)
As of Wednesday, Irma was much smaller than Katrina at its peak but larger than Andrew, and its hurricane-force winds stretched about 100 miles across — roughly the width of Florida in many places. It is impossible to know what it will be like if it hits Florida.
Current projections from the National Hurricane Center have Hurricane Irma Coming ashore on Estero Island, Fort Myers Beach area with sustained winds of 130-150 mph. The Fort Myers metro area is home to a population of over 1,000,000.
Besides Irma, there are two other hurricanes in the Atlantic Ocean basin. This is the first time since 2010 there have been three active hurricanes. Hurricane Katia currently Category 1, in the Gulf of Mexico, should make landfall Saturday morning in Mexico. Like Irma, Jose formed in the far eastern Atlantic, and is now a Category 3 hurricane with sustained winds of 125 miles per hour as of Friday morning. A hurricane watch is in effect for Antigua, Barbuda, and Anguilla, St. Martin, St. Barthelemy.
Irma sets a record as strongest hurricane in open Atlantic in two ways
Irma’s peak sustained surface winds of 185 mph are the highest observed in any hurricane north of the Caribbean and east of Florida, topped only by Allen (1980) in the Caribbean (190 mph). Two hurricanes have notched 185-mph winds in the Caribbean: Gilbert (1988) and Wilma (2005). The Labor Day hurricane of 1935 hit the same peak winds in the Florida Straits.
Irma set another record late Wednesday afternoon: its central surface pressure dropped to 916 mb, as extrapolated from dropsonde data collected by Hurricane Hunters. This beats the previous Atlantic record outside of the Caribbean and Gulf of Mexico, a 919 mb reading from a dropsonde with Hurricane Gloria (1985).
Irma's central pressure at 2 pm EDT Tuesday, when the hurricane's top winds first hit 185 mph, was 926 mb. Interestingly, the other four Atlantic hurricanes with winds at least that strong had significantly lower central pressures:
Wilma: 882 mb
Gilbert: 888 mb
Labor Day 1935: 892 mb
Allen: 899 mb
Why the difference? Wind speeds are driven by the contrast between the central pressure of a hurricane and its surrounding environment. As a rule, the sharper the contrast, and the smaller the distance over which it occurs, the stronger the peak winds. Normally, one would expect a medium-sized hurricane like Irma with a central pressure of 926 mb to have slightly weaker winds. However, surface pressures around Irma are considerably higher than usual, which appears to have boosted the pressure gradient.
As a hurricane matures, its wind field often spreads out while the peak winds slowly decrease. The hurricane’s storm surge potential does not necessarily drop, though, because the broadened wind field can be pushing just as much water as before. Hurricanes such as Ike (2008) and Sandy (2012), whose winds weakened before landfall, produced much more surge than one might have expected from their Saffir-Simpson rating, which depends only on maximum wind speed. A storm as powerful as Irma will have the potential to create devastating storm surge even if its wind field eventually expands and its top winds weaken.
Maximum Potential Intensity (MPI) is a theoretical measure of the maximum strength a hurricane can achieve, based on the existing ocean temperatures and the thermal profile of the atmosphere. Above is a plot of the MPI as computed from the GFS model run from 0Z Tuesday, September 5, 2017. Irma hit 185-mph winds in a region where theory said it should only have been able to attain peak winds of about 165 mph. According to an email from hurricane scientist Dr. Kerry Emanuel, “in numerical simulations, rapid intensifiers often overshoot their potential intensity, but eventually settle back to it.” Note that the MPI along the track of Irma peaks at about 210 – 215 mph in the Florida Straits. This implies that Irma has room for still more intensification as it approaches Florida, if wind shear, land interaction, and eyewall replacement cycles do not interfere.
Irma’s storm surge
Irma is a medium-large hurricane, and is expected to grow in size as it progresses west-northwest over the next four days. As of 5 pm EDT Wednesday, the diameter of hurricane-force winds surrounding Irma was up to 105 miles wide, and the diameter of tropical storm-force winds was up to 310 miles. The official NHC forecast predicted that these diameters would grow to 115 miles and 345 miles, respectively, by Friday, when Irma will be pounding the central Bahamas. This increase in size will be due to eyewall replacement cycles, which spread out the wind field over a larger area, and due to the fact that storms moving towards the pole get more spin from the Earth’s spin.
To check out your neighborhood using the National Hurricane Center's Potential Storm Surge Flooding Map (Inundation) tool click here.
Irma’s large wind field is putting in motion a vast amount of water, which is spiraling into the center of Irma and creating a large mound. In the open ocean, that water is forced downward, pushing deeper water outward, and the sea surface is not elevated more than a few feet. However, once the hurricane drives that mound of water into a shallow area near land, the water cannot flow downwards, and instead piles up and is forced on land, creating a storm surge. In the Turks and Caicos Island and in the southeastern and central Bahamas, a highly destructive storm surge of 15 – 20 feet above ground is expected near the coast to the right of where the eyewall hits.
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Note: Sanibel, St. James City, Iona, much of Cape Coral, and Fort Myers Beach are in the current mandatory evacuation area
Everglades City, Goodland, and Chokoloskee are under mandatory evacuation orders.
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