Hurricane Irma made landfall on Sunday morning along the Florida Keys, with sustained winds of 130 mph. Although Irma has weakened some, this is still a rare and powerful storm that will have devastating effects on parts of the Florida peninsula, and will disrupt the entire state for days.
Irma’s forecast track is pretty well locked in, as the storm should essentially cruise up the western coast of the state—with the center remaining just on or off shore—before moving into the Florida panhandle, Georgia, and Alabama before dying over the southeastern United States. Winds, storm surge, and inland rain are all major threats for Florida.
When you watch television coverage of hurricanes, wind speeds get all of the attention. But for scientists, the key determinant of hurricane intensity is central pressure—the extent to which a storm’s center is lower than the Earth’s normal sea-level pressure of 1013.25 millibars.
By developing a low central pressure, a storm is better able to pull in warm air and moisture to its center, and this rising air creates swirling thunderstorms. Like a vacuum cleaner creates lower pressure to draw in dirt and debris, a strong tropical system also can “suck” water to its center.
On Saturday afternoon a video posted on Twitter from Long Island, in the Bahamas, demonstrated the power of the easterly winds on the back side of Irma. The viral video shows the absence of water at the beach, and this was caused both by winds blowing the water away, and the low pressure center of Irma drawing it elsewhere. (They have already returned).
— #ForeverFlourish (@Kaydi_K) September 9, 2017
As of Sunday morning, Irma’s central pressure has been hovering just below 930 millibars. Although this is far from a record, only six US hurricanes have made landfall with pressures below this—The Indianola storm, the 1919 Florida Keys hurricane, the 1935 Labor Day storm, Camille, Andrew, and Katrina. All are legendary storms.
Already, even before reaching the United States, Irma has set or tied some impressive records for hurricane intensity in the Caribbean Islands. According to Colorado State University hurricane scientist Phil Klotzbach, these records include:
Leeward Islands: Strongest storm on record to impact, with maximum winds of 185 mph. Okeechobee Hurricane (1928) and David (1979) were previous strongest at 160 mph.
Turks and Caicos: Closest approach of a Category 5 hurricane on record.
The Bahamas: First Category 5 hurricane to make landfall since Andrew (1992).
Cuba: First Category 5 hurricane to make landfall since the Cuba Hurricane of of 1924.
That interaction with Cuba weakened Irma some, bringing the storm down from 160mph winds to 120mph. While it has restrengthened a bit on Sunday morning (to 130 mph as of 9am ET), the window for intensification is short. Later today, Irma’s center will begin to interact with the southwestern part of Florida, and this will disrupt the storm’s organization. Another critical factor is wind shear; after having a very smooth go of it for days, Irma will now face greater wind shear as it moves north.
All the same, Irma is likely to be an historic storm in terms of its damage throughout the Caribbean Sea and Florida. Everyone should be ready to help those in need in the days ahead.
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