Dorian Update: Florida will Get Glancing Blow, Carolinas in Crosshairs

August 31, 2019

Dorian “stationary” over Bahamas.

National Hurricane Center briefing for monday morning Sept. 2
0850 EST September 2

Jeff Masters in Weather Underground:

Hurricane Dorian slowed to a crawl overnight, stalling over Grand Bahama Island and giving that island an unfathomable multi-hour Category 5 beating. Dorian’s stall location increases the probability that the hurricane will turn north in time to miss a direct landfall in Florida, but a developing eyewall replacement cycle (ERC) may broaden Dorian’s wind field, increasing the wind and storm surge threat to Florida.

Dorian has expanded in size since Sunday, with hurricane-force winds at 8 am EDT Monday extending outward up to 45 miles from the center and tropical-storm-force winds extending outward up to 140 miles. These radii are likely to increase today, since radar observations from the Miami radar on Monday morning showed the hurricane forming a second eyewall with a diameter of about 45 miles that was concentric with the inner 23-mile diameter eyewall. This development is likely the start of an eyewall replacement cycle (ERC), a common occurrence in intense hurricanes where the inner eyewall collapses and is replaced by a larger-diameter outer eyewall formed from what used to be an outer spiral band. Dorian’s current ERC may have been helped along due to the long period of time the current eyewall has had to spend over Grand Bahama Island, which has created friction that could have helped disrupt the eyewall.
During an ERC, the peak winds of the hurricane typically drop by 10 – 25 mph for a period of a day or so, after which the storm will re-intensify. ERCs typically take 1 – 2 day to complete, and this one been going on since Sunday night. Thus, I give 50% odds that Dorian will be reduced to a Category 4 storm by Tuesday afternoon, assuming that the ERC will progress in a typical fashion. Additional weakening is likely due to the more unfavorable oceanic conditions now being created by Dorian’s stall. The hurricane’s winds are mixing to the surface cooler waters from below that are limiting the hurricane’s heat and moisture supply. Dorian’s lack of motion means that it cannot move itself over to a new area of warm water to feed off of. The combined effect of the ERC and the reduced heat energy being supplied to Dorian could reduce the hurricane to a Category 4 storm with 130 – 140 mph winds by Tuesday afternoon.Hurricane Dorian slowed to a crawl overnight, stalling over Grand Bahama Island and giving that island an unfathomable multi-hour Category 5 beating. Dorian’s stall location increases the probability that the hurricane will turn north in time to miss a direct landfall in Florida, but a developing eyewall replacement cycle (ERC) may broaden Dorian’s wind field, increasing the wind and storm surge threat to Florida.

Dorian has expanded in size since Sunday, with hurricane-force winds at 8 am EDT Monday extending outward up to 45 miles from the center and tropical-storm-force winds extending outward up to 140 miles. These radii are likely to increase today, since radar observations from the Miami radar on Monday morning showed the hurricane forming a second eyewall with a diameter of about 45 miles that was concentric with the inner 23-mile diameter eyewall. This development is likely the start of an eyewall replacement cycle (ERC), a common occurrence in intense hurricanes where the inner eyewall collapses and is replaced by a larger-diameter outer eyewall formed from what used to be an outer spiral band. Dorian’s current ERC may have been helped along due to the long period of time the current eyewall has had to spend over Grand Bahama Island, which has created friction that could have helped disrupt the eyewall.
During an ERC, the peak winds of the hurricane typically drop by 10 – 25 mph for a period of a day or so, after which the storm will re-intensify. ERCs typically take 1 – 2 day to complete, and this one been going on since Sunday night. Thus, I give 50% odds that Dorian will be reduced to a Category 4 storm by Tuesday afternoon, assuming that the ERC will progress in a typical fashion. Additional weakening is likely due to the more unfavorable oceanic conditions now being created by Dorian’s stall. The hurricane’s winds are mixing to the surface cooler waters from below that are limiting the hurricane’s heat and moisture supply. Dorian’s lack of motion means that it cannot move itself over to a new area of warm water to feed off of. The combined effect of the ERC and the reduced heat energy being supplied to Dorian could reduce the hurricane to a Category 4 storm with 130 – 140 mph winds by Tuesday afternoon.

The downside of an ERC is that it spreads hurricane-force winds over a larger area, since the total energy of the storm remains unchanged. Thus, the ERC is potentially good news for Grand Bahama but bad news for Florida, since it will likely cause hurricane-force winds along a larger swath of the state and increase the storm surge flooding along the coast.


Updated Sunday evening, 09/01/19, Hurricane Center Briefing from 1700

Dorian at 175 mph sustained winds Sunday morning, and in a period of rapid intensification.
I’ll be following the National Hurricane Center and Levi Cowan’s Tropical Tidbits.

Sunday evening: Rising concern about the Storms continued western track.

Destruction in the Bahamas.

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One Response to “Dorian Update: Florida will Get Glancing Blow, Carolinas in Crosshairs”


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