VIIRS image of three hurricanes in the Atlantic on Thursday, 7 September 2017: Cat 1 Katia (left), Cat 5 Irma (center), and Cat 3 Jose (right.) Photo: NASA

Dr. Jeff Masters
7 September 2017

(Weather Underground) – 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 addition, 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.

GOES-16 visible image of Hurricane Irma taken at 5:27 pm EDT 7 September 2017, with lightning strikes from the Global Lightning Mapper (GLM) instrument on GOES-16 overlaid. Photo: U-Madison / CIMSS

No way, Jose: Atlantic gets its third major Atlantic hurricane of 2017

The Atlantic hurricane season has now produced more than a year’s worth of major hurricanes before the season is even half over. Hurricane Jose reached Category 3 strength at 5:00 pm EDT, the third storm in a row to do so, after Harvey and Irma. Jose’s winds were up to 120 mph in the 8:00 pm EDT advisory, which placed the center about 540 miles east of the Leeward Islands. Tracking west-northwest, Jose was located only about 80 miles south of where Irma prowled just three days ago. Typically, such a path would encounter cold water churned up by the preceding hurricane, but it appears that Jose’s small core is staying just south of the cold wake left by Irma. Jose has also shrugged off moderate wind shear of around 15 knots, with the help of warm SSTs of around 29°C (84°F) and a fairly moist atmosphere (mid-level relative humidity close to 60%).

Jose’s rapid deepening should come to an end shortly, as its west-northwest path will soon intersect Irma’s west-southwest path. By late Friday, Jose will be directly over the cold wake left by Irma, where SSTs over a small area have dropped below the 26°C threshold for sustaining tropical cyclones. This wake may not be reflected yet in all forecast models. Jose will also be ingesting some dry air by late Friday into Saturday, so it is likely to peak at its current Category 3 strength. Still, in a one-two punch horrifying to ponder, Jose may spend Saturday rolling across some of the northern Leeward Islands hardest hit by Irma earlier this week. Hurricane watches are in effect for Antigua, Barbuda, and St. Martin/Sint Maarten. We can hope that the 12Z Thursday European model ensemble runs are correct, as they generally keep Jose northeast of the islands, whereas a good fraction of GFS ensemble runs bring the hurricane over or very near the islands. The official NHC track, midway between the operational Euro and GFS models, brings Jose within about 50 miles of Barbuda, which would probably be far enough to avoid major impacts from this compact hurricane. [more]

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



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