Radar loop of Hurricane Wilma making landfall in SW FL.
(graphic courtesy of WUnderground.Com)

 

Category 5 Hurricane Wilma very likely set the record for the lowest sea-level pressure ever measured in the Atlantic basin, at 884 millibars. This was measured by one of our Air Force Reserve Hurricane Hunter crews. If this pressure is verified in post-analysis, it breaks the record set by Hurricane Gilbert in 1988 (888 millibars).

 

The picture to the right shows the first inbound leg towards Wilma. They were flying at 5000feet towards what they knew was a rapidly intensifying hurricane... but just how intense, they were about to find out! The "pinhole eye" is a clue (as an eye shrinks, the storm gets stronger, and an eye less than 5 miles wide is rare). This picture was taken at 0430Z on 19 October, and the eye was estimated around 4 miles wide. After they found the storm had intensified to a strong Cat 4, they began a climb to 10,000 feet while still in the eye, to give more room between the plane and the sea (safety always comes first).

 

 

This picture of the radar shows the aircraft when it was only 22 miles from the eye. The eye is to the southwest of the plane, and the white line shows the track the plane will fly to get into the eye. But the strong, counterclockwise winds are blowing the plane sideways towards the right (towards the west). So the plane is "crabbed" into the wind to use to plane's forward speed to counter this drift, and is actually pointing due south (heading 181 degrees). This is called a "drift-kill heading", a skill any pilot must be able to do; it is fascinating to watch as the winds build up and the plane turns more and more at an angle to our true course.

 

 

 

 

For the next pass at 0611Z (02:11 a.m.), the Hurricane Hunter crew reported an eye just 2 miles wide, incredibly tiny, and flight-level winds of 168 knots (193 mph; winds at the surface are typically lower, due to friction). They extrapolated the sea-level pressure as 892 millibars, close to a record pressure.

The inner eyewall is the colorful donut in the middle of this picture. This radar shot shows the tiny eye surrounded by a band of intense rain (red and purple), surrounded by a rain-free moat (the black ring surrounding the inner eyewall). They had just enough fuel to make one more pass, and a chance to look for a record pressure.

 

 

At 0800Z (4 a.m EDT), they reported a 4 mile eye, and lower pressures yet. The new sea-level pressure was 884 millibars (26.11 in-Hg), measured by a dropsonde instrument. And realize, this was only a Cat 1 (80-mph) hurricane just nine hours earlier! The National Hurricane Center discussions had predicted the storm would intensify, but such "explosive deepening" is still phenomenal to witness.

This likely moves the rankings of this years storms as follows: Strongest Atlantic basin hurricane on record: Wilma (884 mb); Number 4: Rita (897 mb); Number 6: Katrina (902 mb). These are the strengths of the hurricanes at their peak, not landfall pressures. You can see a list of the strongest landfalling U.S. hurricanes at NHC website.

I haven't spoken to the crew yet, but the experience would certainly have been remarkable. With a pressure field so "tight", the crew would have felt the aircraft descending as they crossed the eyewall into the eye. Looking at the data from the aircraft, they lost 2680 feet of altitude during the eyewall penetration, in the five minutes it took to cross the eyewall into the eye. This is expected, and happens because the aircraft autopilot flies off an altimeter set to "standard" pressures (and this is anything BUT standard); we also have other altimeters that tell us how high we REALLY are, and using both types of altimeters is how we determine the pressures ("height of standard surface") at the flight level. In a "standard" atmopshere, the airplane would be at 10,000 feet, but in Wilma, they bottomed out at 6830 feet.

Wilma is also the last name in the hurricane list. This tied the record for the most named storms in a single season, set in 1933 (before they named storms). If another named storm occurs this year, they will name the storms according to letters in the Greek alphabet (Alpha, Beta, Gamma, etc.). If you'd like to learn more about hurricane names, go to the NHC website at http://www.nhc.noaa.gov/aboutnames.shtml.