| Above: Visible satellite image of Dorian at 8:50 am EDT September 2, 2019. At the time, Dorian was a Category 5 hurricane with 165 mph winds over Grand Bahama Island. Image credit: NOAA/RAMMB. |
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.
The only official weather station on Grand Bahama Island is at Settlement Point on the extreme northwestern end. At 9 am EDT Monday, Settlement Point reported sustained winds of 51 mph, gusting to 59 mph, with a pressure of 993 mb. Bahamas Press reported that nearby Grand Bahama International Airport in Freeport was under 5 feet of water early Monday morning.
Storm chaser Jim Edds was on Great Abaco Island for Dorian’s landfall yesterday. According to a Tweet from storm chaser James Reynolds: “Relaying a message from a trusted 3rd party--everyone staying at the Hope Town Inn in Hope Town #Bahamas is safe. They got the eye and Jim measured 911.2hPa minimum pressure.”
The eye wall of #Dorian has been sitting over some of the same places of Grand Bahama Island for 6 HOURS.
— Matt Reagan (@ReaganMatt) September 2, 2019
This is just ridiculous at this point. pic.twitter.com/uycAuyElf7
An eyewall replacement cycle underway
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.
#Dorian's interaction w/ Grand Bahama probably expedited its eyewall replacement cycle (EWRC) w/ the outer moat more favorably juxtaposed to adjacent ocean water. Increasing eccentricity & trochoidal wobbles of the old inner eyewall are signs that this EWRC is pretty far along pic.twitter.com/QBGXAWEU8w
— Eric Webb (@webberweather) September 2, 2019
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. Dorian’s current wind field had hurricane-force winds that were extending out about 30 miles to west, towards Florida, and up to 45 miles to the east, away from Florida. We might expect Dorian’s radius of hurricane-force winds to increase to 40 miles to west, towards Florida, and up to 55 miles to the east by Tuesday afternoon and evening, when the hurricane will be making its closest approach to Florida. This makes it more likely that Cape Canaveral will experience hurricane-force winds, since Dorian may pass within 50 miles of there on Tuesday night.
If we are lucky, the combination of reduced heat energy due to cold water upwelling and an ERC may be capable of causing a significant disruption to Dorian and not allow re-intensification. For example, when Category 4 Hurricane Matthew of 2016 approached Florida, the storm underwent an ERC and was never able to fully close off the new eyewall and re-intensify.
We’ll have another update early this afternoon. The latest set of 0Z and 6Z Monday model runs do not show any major shifts, and the official NHC forecast looks good.
