Atlantic/2019/05L/Archive/25

As expected, Dorian underwent a period of rapid intensification this afternoon. A reconnaissance aircraft investigating the hurricane found that the central pressure had plummeted 20 millibars over 6 hours, with the most recent dropsonde supporting a minimum value of 948mb. Maximum flight-level winds were 129kt, with reliable SFMR values of 119kt. These numbers support upping the initial intensity to 120kt, making Dorian an intense Category 4 hurricane.
Dorian is moving west-northwest this evening as high pressure builds offshore the Southeast United States. A curve toward the west is expected tomorrow, and that general trajectory should continue through 72 hours, accompanied by a slowing speed during that time. On days 4 and 5, an upper-level trough is expected to erode the aforementioned high pressure system, forcing Dorian to curve toward the north. Given the multi-day trend of the hurricane slowing down on approach, it is becoming increasingly feasible that Dorian remains offshore the eastern coastline of Florida, although subtleties in the evolution of the 500mb pattern could easily negate that statement. The 18z ECMWF was similar to its previous run and the 18z GFS, and the 18z HWRF shifted northward too, albeit while still showing a landfall. Dynamical guidance is almost squarely offshore, with the consensus track paralleling nearly every state of the Southeast United States. Still, a majority of the GFS and UKMET ensembles continue to point toward a landfall. For the purpose of this advisory, I have decided to keep Dorian offshore, albeit barely. It cannot be overstated how tenuous this change is though—we are dealing with a matter of miles between landfall and a close shave.
There are a few factors to discuss with regard to Dorian's intensity in the short term. For one, while the hurricane as a whole is very well organized, the central dense overcast remains slightly asymmetric—the deepest convection is weighted to the east of the eye. This is indicative of some slight southwesterly shear that may be injecting environmental dry air into the western semicircle of the hurricane. This shear is courtesy of the same upper-level trough that has been well covered in advisories past; the same one, ironically, that has so effectively evacuated mass from the hurricane's core through upper-level outflow and allowed for rapid intensification to occur. Analysis of 200mb wind forecasts from the global models indicate that this shear should decrease overnight, and so upper-level winds should be a non-factor in the future.
Observations from the NOAA plane indicate that the eye of Dorian has shrunk over 10 nautical miles over the past few hours. While there are no signs of a secondary wind maxima in flight-level wind data, a 2257z SSMIS microwave pass depicted several spiral bands wrapping into the core, beginning to form a large outer ring. This evolution is likely to lead to an eyewall replacement cycle within the next 24 hours, a process that should in theory lead to a leveling off, or slight weakening, of Dorian during that time.
Data from the SHIPS text file indicates that oceanic heat content has been decreasing over the past day. This trend is expected to continue overnight into tomorrow morning. The significance of these lower values is that the hurricane will be afforded less energy to produce deeper convection, which is necessary for tropical cyclones to reach the upper echelons of the intensity database. That being said, oceanic heat content is expected to begin rising again tomorrow, exceeding 60 joules per kilogram by Monday. This will also be accompanied by a rise in 700-500mb relative humidity. So, the general forecast theme remains the same: Dorian is expected to remain an exceptionally powerful storm for the foreseeable future, with the potential to near Category 5 intensity while it traverses the northwestern Bahamas. The last remaining question as the forecast timeline heads toward days 3 and 4 is the forward speed of Dorian. Given expectations of a slowing hurricane, it seems plausible to expect upwelling may eventually become a problem for the cyclone, even in the Bahamas where waters and exceptionally warm and deep. Increasing shear and dry air entrainment from the Southeast U.S. should further weakening on day 5.
FORECAST MAX WINDS
INIT 31/0300Z 120 KT 140 MPH
12H 31/1200Z 120 KT 140 MPH
24H 01/0000Z 120 KT 140 MPH
36H 01/1200Z 125 KT 145 MPH
48H 02/0000Z 130 KT 150 MPH
72H 03/0000Z 130 KT 150 MPH
96H 04/0000Z 110 KT 125 MPH