Hurricane Dorian exits the Caribbean; Expected to be a Powerful Hurricane near the Southeast U.S.

Tropical Storm Dorian intensified into a hurricane as it continued deviating east of the expected track yesterday. Instead of crossing over Puerto Rico, it completely missed the island, passing over the U.S. Virgins Islands to the east as a minimal hurricane. Damage reports so far have been spotty, and one death has been confirmed so far. Dorian is now well north of the Greater Antilles as a category 1 hurricane with winds of 85mph and a minimum pressure of 986mb, moving northwest at about 13mph. In other news, Tropical Storm Erin, previously called Tropical Depression Six, has dissipated off the coast of the Mid-Atlantic, and the remnants will cross Atlantic Canada tomorrow.

GOES-EAST True-color image of Hurricane Dorian north of the Greater Antilles (SSEC RealEarth)

Yesterday, Dorian was undergoing a steady intensification trend from a moderate tropical storm to a moderate category 1 hurricane. So far today, the intensification has leveled off within the storm for a few reasons. The upper-level trough to the west of the storm, which was discussed in previous articles, is causing some slight, but noticeable southerly shearing in the mid-levels. In addition, some drier mid-level air has been advected into the storm on the south side as well in tandem with the upper-level low. As a result, most of the storm’s convective influence extends towards the north of the core. Infrared satellite and microwave imagery suggest that the core is oscillating in definition, as evidence of a shallow eye has shown up every now and then before another convective burst occurs. The core as a whole, however, is quite small, so it is susceptible to drastic intensity swings. There is also strong banding on the eastern side of the storm, which has been caused by high levels of moisture and instability as well as orthographic lifting from Puerto Rico. While the eyewall remains incomplete due to the dry air advection, the storm as whole should begin intensifying once again soon.

Infrared imagery of Hurricane Dorian, revealing the small core and strong eastern banding. (MSFC)

From this point forward, Dorian should continue its northwestward motion for at least the next 24 hours before a westward turn occurs. The upper-level trough responsible for the shear and dry air entrainment will be continuing to shift west towards the northwestern Bahamas and South Florida. A ridge will start to build to the north of the hurricane and extend from Bermuda to the Carolinas. This cause Dorian to halt its northward motion and transition more westerly. At this point, the track has been trending farther south, as the ridge has been forecast to become steadily more intense as new runs progress. Environmental conditions will also be more favorable for intensification at this point because of upper-level jets around the storm allowing for enhanced outflow, naturally higher instability on the northeastern side of the upper-level low, more ample moisture, and increasing sea surface temperatures and oceanic heat content. It is expected that Dorian becomes a major hurricane, the first of the season, by tomorrow, and will then probably become a category 4 hurricane by the weekend northeast of the Bahamas.

Intensity Guidance Models on Dorian, with the consensus showing an increase to a major hurricane by the weekend. (UWM Model Guidance)

The longer-range threats to both The Bahamas and the southeastern U.S. has increased. Because the ridge is expected to be stronger and slightly farther south, the forecast track has shifted farther south in response. As a result, the threat of a major hurricane to both the northwestern Bahamas and southern Florida has gone up. Most of the guidance, including the GFS, suggests that the hurricane passes through or just north of the islands of Great Abaco and Grand Bahama before striking the central Florida Peninsula by Monday while gradually slowing down in forward speed. In general, the stronger models are showing a further south track, as the storm would be strong enough to force itself away from the parent ridge. A northward turn is now expected, as it has become a common trend recently, and it revolves around Dorian approaching a weakness between ridges, the one steering it west and another over the Great Plains. While most models show that it curves north over the peninsula, some models, such as the EURO, which is both slower and stronger than most models, have even shown the storm taking a slight west-southwest jog into the Bahamas before turning north along the Florida coast before riding through Georgia and the Carolinas. The storm would then curve northeast as a trough travels through the Great Lakes and Northeast, which could cause the storm to ride up along or just off the coast of the remainder of the U.S., but that would occur past a week from now, so take that portion with a grain of salt.

12z GFS solution, which lies along the general model consensus track. (Tropical Tidbits)

Track confidence with Dorian, as has been the case throughout its existence, is still quite low, so make sure that, if you have interests in the area, you keep a very close eye on this system. If these expected solutions generally verify, there could be lots of threats associated with the storm. At this point, regardless of how strong the storm is, heavy rainfall looks to be the primary concern. Because the storm is approaching weakening steering currents as it approaches land, it is expected to slow down, meaning that there will be prolonged periods of heavy rainfall. As of now, some models are showing over a foot of rain over portions of central and northern Florida and points north of landfall, but a stronger storm is more likely to make landfall further south, potentially as far south as the Miami area, so the exact extent of these rains is unknown. Rainfall forecasts are usually quite uncertain this far out, as the usually underestimate the true rainfall totals. For now, expect widespread heavy rains that can be capable of producing flash flooding, but expect these forecast totals to go up as forecast confidence increases.

Day 9 image not available
WPC 1-7 Day Quantitative Precipitation Forecast, showing a large area of heavy rains over the northwestern Bahamas, Florida, and portions of bordering states.

With any hurricane, there is a threat for high winds, the exact strength of which will be dependent on how strong the storm is at landfall, but any major hurricane will bring winds high enough to cause considerable damage to even well-built structures. In addition, storm surge will be another huge problem, but if the storm trends further south, then the highest surge will go into the Bahamian islands, which could serve as some protection against Florida, but if it remains on its expected path, then surge values of up to 1o’ are certainly a possibility. Isolated tornadoes are also possible northeast of the landfall location, as is the case with any landfalling hurricane, but anything that does spawn will be weak and short-lived.

NHC 5-day forecast cone, showing Dorian approaching the Bahamas and Florida as a major hurricane.

Preparations for Dorian across these regions should begin as soon as possible, and if you are in an evacuation zone, please listen to the advice given by your authorities. There is still about 3 days before impacts will start to be felt in Florida, but don’t procrastinate on your preparations because of that. Remember that if you have interests in this area, you need to pay very close attention to this storm because confidence is low and the storm could be particularly intense. Even though variability is still there, treat the storm as if it is coming for you and do not compare it to previous storms for guidance. Please stay tuned to the National Hurricane Center, your local officials, news outlets, and any other emergency management agency for the most pertinent information to you. That’s all for today, I’ll be back with a follow-up article either tomorrow or Saturday.

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