Triple Trouble: Cat 5 Irma, Cat 3 Jose, Cat 1 Katia

Hurricane Irma plowed through the Turks and Caicos Islands (population 31,000) on Thursday evening, as it headed west-northwest at 16 mph towards The Bahamas and Florida. Irma maintained Category 5 intensity with 175 mph winds as its eye crossed the small privately-owned Big Ambergris Cay, but the core of hurricane missed the capital of Cockburn Town, and may pass just south of Providenciales, the most heavily populated island in the country.

Hurricane hunter observations on Thursday evening showed lower winds in Irma than earlier in the day, and found a distinct double wind maximum, caused by concentric eyewalls. By my count, this may be Irma’s 7th eyewall replacement cycle, and appears to be the first one that has notably degraded the hurricane’s maximum winds. The Air Force hurricane hunter plane in the storm Thursday night found top surface winds near 165 mph on their first pass through the eye near 7:35 pm EDT, but only 140 mph on their second pass through near 9 pm EDT. The pressure stayed nearly constant in the two passes, at 920 and 921 mb, respectively. The reduction in the peak winds is not necessarily a good thing, since the hurricane hunter data showed that the hurricane-force winds of the storm had spread out over a larger area, which will increase the storm surge. In addtion, the fact that Irma maintained its very low pressure may mean the aircraft may have missed sampling the strongest winds of the hurricane. Total precipitable water loops showed that part of Irma’s apparent weakening may be due to the island of Hispaniola blocking the inflow of moisture from the south, which was reducing the amount of moisture to the storm. This effect will diminish on Friday morning, when Irma moves to the west of Hispaniola. The hurricane will then be drawing air from the south over high terrain in Cuba, but the blocking effect of Cuba is much less than that of Hispaniola. I don't see any reason for Irma to weaken further through Saturday, and it may intensify again to 185 mph winds.


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