Dorian: A Potentially Historic Labor Day Hurricane for Florida

Hurricane Dorian continued to organize at upper levels while holding firm to its Category 1 surface winds well east of The Bahamas on Thursday evening. Dorian is expected to grow larger and more powerful over the next three to four days as it carries out a gradual arc toward the northwest and then west. It will most likely reach the Florida coast on Monday or Tuesday as a potentially devastating major hurricane, although the near- and post-landfall outlook is still cloaked in uncertainty.

Update: The National Hurricane Center upgraded Dorian to Category 2 at 11 pm EDT Thursday, with top sustained winds now up to 105 mph. "Unfortunately, I don't see any large-scale factors that would prevent Dorian from becoming an extremely dangerous Category 4 hurricane during the next few days," said NHC hurricane specialist Eric Blake in an 11 pm forecast discussion.

Intensity outlook for Dorian

Dorian’s inner core remained compact late Thursday, with an intermittent eye coming and going on satellite imagery. The Hurricane Hunters found a double-eyewall structure on Thursday morning, which no doubt impeded any rapid strengthening. Meanwhile, the hurricane’s overall envelope was steadily broadening, though, with outflow especially prominent toward the northwest. This broad-scale organization will give Dorian a boost as soon the storm carves out a solid eyewall and clears out an eye.

Figure 1. GeoColor satellite image of Hurricane Dorian as of 0010Z Friday, August 30, 2019 (8:10 pm Thursday). Image credit: RAMMB/CIRA/CSU.

A stream of dry air invading Dorian from the south will likely get pinched off by Friday, as the storm enters a progressively moister air mass. Mid-level relative humidity along Dorian’s path rose from the 40-50% range in the Caribbean to the 55-60% range on Thursday, and it is predicted by the 18Z Thursday SHIPS model to reach 60-65% by late Saturday.

The National Hurricane Center is calling for Dorian to intensify fairly gradually, reaching Category 3 strength on Friday and Category 4 strength on Sunday. Such outlooks are not designed to capture short-term bouts of rapid intensification. Once an eye clears out, Dorian could embark on a round of rapid intensification at any point, and that process could add one or two Saffir-Simpson categories of strength in less than a day. The storm will be passing over very warm sea surface temperatures (29-30°C or 84-86°F), with a moderate amount of oceanic heat content (50-75 kilojoules per square centimeters) along most of its path through the weekend. Wind shear will be light to  (5-10 knots) into Saturday, then possibly increase to a moderate 15 knots.

Bottom line: expect Dorian to undergo a round of rapid intensification by Saturday, which will bring it to at least Category 3 strength as it approaches the Southeast U.S. coast on this weekend. Another round of rapid strengthening is possible as Dorian nears the coast. However, if Dorian slows to a forward speed of 5 mph as it near the coast, as is currently predicted, the slower movement of the hurricane is likely to mix up colder water from below, weakening the storm.

Similar to how Hurricane Michael underwent rapid intensification last year, I suspect the anomalously-cold, upshear IR brightness temperatures now seen in the inner core of Hurricane #Dorian are signaling an upcoming intensification period. https://t.co/Bi6b6ehlty pic.twitter.com/IbzGqVASmQ

— Michael Fischer (@MikeFischerWx) August 29, 2019

Track forecast for Dorian

The main question right now isn’t whether Dorian will be a formidable hurricane—it almost certainly will be—but where and when it might reach land. Models still agree that Dorian will embark on Friday on a well-predicted leftward swing toward the west or west-northwest, as high pressure builds to its north. Various track forecasts bring Dorian near or just north of the northwestern and central Bahamas around Sunday, whereas the southern Bahamas are more likely to miss the storm’s most intense wind and waves. Hurricane watches could be required for northern and central parts of The Bahamas as soon as Friday.

After Dorian embarks on its westward bend, computer models may be able to provide us with a more consistent idea of where Dorian will strike Florida (assuming it does). The NHC forecast and the model consensus remains in the vicinity of central Florida, with the NHC’s “cone” centered near Vero Beach, in line with the 18Z Thursday HWRF model. A number of GFS ensemble solutions show landfall in northern Florida. At the same time, several model solutions, including the 12Z Thursday European and 18Z HMON model runs, suggest landfalls as far south as Miami and Key West, respectively. Update (11 pm EDT Thursday): The latest track forecast from NHC has been nudged about 30-35 miles southward, with the new cone centered near Port St. Lucie.

At some point next week, Dorian will likely be shunted northeast, roughly parallel to the East Coast. One big question is when this northeastward turn will kick in. Of the 12Z Thursday ensembles of our leading track models, about a third of the European members and about 60% of the GFS members took Dorian across the Florida Peninsula and into the Gulf of Mexico before any north and northeastward turn. Such a track would lead to a second hurricane landfall on the northern Gulf Coast around midweek. Other solutions, including a number of Euro ensemble members as well as the 12Z operational GFS run, pull Dorian along or near the U.S. East Coast from Florida northeastward, which could lead to several days of torrential rain and coastal impacts. It’s also possible Dorian will slow or stall just shy of this steering current, perhaps over Florida or Georgia, which would greatly increase the risk of inland flooding.

I'm struck by how similar this ensemble track evolution has been compared to Hurricane #Florence last year.

Initially was fcast w/ much more rapid track, but EPS caught on to Florence stalling along the Carolina coastline w/ a WSW component of motion.https://t.co/pESrNkBGQ2

— Philippe Papin (@pppapin) August 29, 2019

It is frustrating that the models have not given us much more clarity on where and when Dorian will make landfall, but that clarity may arrive soon; the NOAA jet was in the storm Thursday evening on a dropsonde mission, and this data should improve the 0Z Friday set of models runs. For now, the entire Southeast U.S. coast—especially the Florida coast—needs to stay abreast of Dorian’s evolution. People along the northern Gulf Coast should also be aware that there is a chance of a Gulf Coast landfall from Dorian next week.

It’s worth noting that in NOAA records going back to 1851, no major hurricane (Category 3 or stronger) has made landfall on Florida’s entire Atlantic coast north of Stuart, where Frances arrived in 2004. This weather.com article has more on why major hurricanes seem to avoid the central and northern Florida coast. Needless to say, any major hurricane striking this stretch of coastline head on would be unprecedented.

Figure 1. Major hurricane landfalls on Florida's Atlantic coast from the Keys northward to the Georgia/Florida border from storms originating in the Atlantic, 1851-2018.

Coastal flooding and storm surge will be major threats from Dorian

As we discussed in our Thursday morning post, the extension of the Bermuda high that will build, then steer Dorian westward this weekend is already pushing onshore flow against the Southeast coast. At Savannah, Georgia, and Charleston, South Carolina, major flooding is expected with the evening high tide on both Thursday and Friday night. The predicted peaks for Thursday night are 8.2’ at the Charleston Harbor tidal gauge and 10.1’ at the Fort Pulaski, GA, tidal gauge--enough to cause downtown flooding in both Charleston and Savannah. The new-moon-driven king tides will only exacerbate coastal flooding from multiple days of onshore flow ahead of Dorian.

Tidal flooding setting in. Be aware of impassable roads tonight. #chswx pic.twitter.com/HNTIRhisKE

— Glenn Smith (@glennsmith5) August 29, 2019

A Dorian landfall could produce very dangerous storm surge, especially from around Palm Beach northward. “In general, surge potential increases as we move north along the Florida coast,” storm surge expert Hal Needham told Category 6. Needham added:

“I am especially concerned about the trend that slows the forward motion before and during landfall process. That process would prolong the amount of time strong winds are blowing over coastal waters and enable storm surge to build to higher levels at the coast and push farther inland. Coupled with increased rainfall, slow moving hurricanes tracking near coastlines often generate tremendous ‘compound flooding’ from the combination of storm surge and heavy rain. Prolonged surge and waves act as a dam to inhibit freshwater runoff.”

LAKELAND, FL - #NOAA49 prepares for a Hurricane #Dorian reconnaissance mission with the first all female three-pilot flight crew, featuring Capt. Kristie Twining, Cmdr. Rebecca Waddington, and Lt. Lindsey Norman. Get the latest forecast at https://t.co/3phpgKvnMi.#FlyNOAA pic.twitter.com/DMn1wOxBUA

— NOAA Aircraft Operations Center (@NOAA_HurrHunter) August 29, 2019

Jeff Masters contributed to this post.