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Katrina: Category 5

By: Dr. Jeff Masters, 12:24 PM GMT on August 28, 2005

Katrina is in the midst of a truly historic rapid deepening phase--the pressure has dropped 34 mb in the 11 hours ending at 7am EDT, and now stands at 908 mb. Katrina is now the sixth strongest hurricane ever measured in the Atlantic. At the rate Katrina is deepening, she could easily be the third or fourth most intense hurricane ever, later today. The list of strongest hurricanes of all time includes:

Hurricane Gilbert (888 mb, 1988)

The Great Labor Day Hurricane (892 mb, 1935)

Hurricane Allen (899 mb, 1980)

Hurricane Camille (905 mb, 1969)

Hurricane Mitch (905 mb, 1998)

Hurricane Ivan (910 mb, 2004)

Katrina's winds and storm surge
Maximum sustained winds at flight level during the 7am Hurricane Hunter mission into Katrina were 153 knots, which translates to 160 mph at the surface, making Katrina a minimal Category 5 hurricane. The winds are likely to increase to "catch up" to the rapidly falling pressure, and could approach the all-time record of 190 mph set in Camille and Allen. Winds of this level will create maximum storm surge heights over 25 feet, and this storm surge will affect an area at least double the area wiped clean by Camille, which was roughly half the size of Katrina. Katrina has continued to expand in size, and is now a huge hurricane like Ivan. Damage will be very widespread and extreme if Katrina can maintain Category 5 strength at landfall.

Landfall projections
The computer models are very tightly clustered and have been so for almost a day. The data used to initialize the models is excellent, since all available hurricane hunter aircraft have been in the air continuously making measurements for several days. Katrina has already made her turn northward, which makes the task of landfall prediction for the models much easier. The offical NHC forecast of a landfall in SE Louisiana, on the western edge of New Orleans, is thus a high-confidence forecast. The spread in the landfall location is just 90 miles, meaning the eye of Katrina is very likely to hit somewhere between New Orleans and a point just east of the Mississippi-Louisiana border.

Intensity forecast
Katrina's intensity at landfall is likely to be Category 4, but could easily be Category 3 or 5. She will undergo another eyewall replacement cycle before landfall, and this will weaken her maximum winds by 20 - 30 mph for a 12-hour period. Additionally, some increase in shear is possible in the 12 hours prior to landfall, which could weaken Katrina's winds another 10 - 20 mph. If we are extrememly lucky, both factors will conspire to knock Katrina down to a Category 3 and she will hit at low tide. Given that the storm is so large and is already pushing up a huge storm surge wave in front of it, even a weakened Category 3 Katrina hitting at low tide will cause an incredible amount of damage. A stretch of coast 170 miles long will experience hurricane force winds, given the current radius of hurricane force winds around the storm. A direct hit on New Orleans in this best-case scenario may still be enough to flood the city, resulting in heavy loss of life and $30 billion or more in damage.

Dr. Jeff Masters

The views of the author are his/her own and do not necessarily represent the position of The Weather Company or its parent, IBM.