|Above: The NOAA-20 polar-orbiting satellite captured this close up view of the wide eye of Hurricane Dorian as it passed over in the early morning hours of Aug. 31. Image credit: NOAA.|
There is increasing hope that Florida will escape a direct hit from extremely dangerous Hurricane Dorian, but South Carolina, North Carolina, and Georgia are at increased risk. The entire Southeast U.S. coast from central Florida to northern North Carolina is in the NHC cone of uncertainty for Dorian, and is at risk of a devastating storm surge, damaging winds, and extreme flooding rains. The northwest Bahamas are at the highest risk of seeing the worst of Dorian, however. Update (5 pm EDT): A tropical storm watch has been issued for the east coast of Florida from Deerfield Beach to Sebastian Inlet, based on uncertainty in the track and on the possibility that Dorian's shield of tropical-storm-force winds will expand.
Dorian put on an impressive show of rapid intensification overnight, and at 2 pm EDT Saturday was an upper-end Category 4 hurricane with 150 mph winds—just 7 mph away from Cat 5 strength—and had a central pressure of 945 mb. The hurricane had completed its westward turn and was headed west at 8 mph towards the northwest Bahamas. These islands will begin seeing the hurricane’s impacts on Sunday morning.
Hurricane hunter data and satellite images on Saturday afternoon showed an extremely impressive mature hurricane with a prominent eye surrounded by an intense ring of eyewall thunderstorms with very cold cloud tops. Dorian had established two powerful upper-level outflow channels, one to the east and one to the west, which enabled the hurricane to put on its burst of rapid intensification from Friday night into Saturday morning. Dorian was also passing over some very rich oceanic heat content, with values of greater than 75 kilojoules per square centimeter, which helped set the stage for rapid strengthening.
|Figure 1. Visible satellite image of Dorian at 1 pm EDT August 31, 2019. Image credit: NOAA/RAMMB.|
The intense thunderstorms along the western side of Dorian were relatively thin at midday Saturday, probably due to some moderate wind shear created by upper-level northerly winds from the ridge of high pressure to Dorian’s north. Though a bit larger than Friday, Dorian was a medium-small hurricane, with hurricane-force winds that extended out 30 miles from the center and tropical storm-force winds that extended out 115 miles. This wind field is predicted to expand; NHC predicted that on Monday, hurricane-force winds would extend out 45 miles from the center and tropical storm-force winds would extend out 125 miles.
|Figure 2. Predicted wind speeds (colors) and pressure (black lines) for Dorian at 5 pm EDT Wednesday, September 4, 2019, from the 12Z Saturday, August 31, 2019 run of the HWRF model. This model, one of our top three performing intensity models at long ranges last year, predicted that Dorian would approach landfall in South Carolina as a Category 2 hurricane with 105 mph winds. Image credit: tropicaltidbits.com.|
Intensity forecast for Dorian
Over the next three days, conditions will be favorable for Dorian to maintain its intensity. Wind shear is expected to mostly be light, less than 10 knots, the atmosphere will be moist, with a mid-level relative humidity near 65%, and Dorian will be passing over very warm water, with sea surface temperatures of around 29 - 30°C (84 - 86°F)--about 1.0°C (1.8°F) warmer than average. Oceanic heat content (OHC) will be 50 – 90 kilojoules per square centimeter along Dorian’s path for the next three days; values above 75 kJ/sq cm are closely associated with higher odds of rapid intensification.
Expect Dorian to experience at least one more eyewall replacement cycle (ERC). These cycles can produce a dip in strength lasting a day or so, as the inner eyewall collapses and is replaced by the outer eyewall, but they also tend to enlarge a hurricane.
Dorian is expected to slow to a forward speed of just 5 mph by Sunday afternoon. Hurricanes moving this slowly tend to weaken, since they are unable to “outrun” the large amount of cooler water from the depths that their winds stir to the surface. The weakening from this effect for Dorian may be less than one usually sees for a slow-moving storm, since the warm waters of the Gulf Stream lie along its path. This powerful current will be able to supply a continuous stream of warm water from the south to offset the cooling that upwelling from Dorian’s winds cause.
Weakening can also be expected when it comes within about 100 miles of the U.S. coast, since the hurricane will have a large part of its circulation over land, limiting the amount of moisture that feeds into it. The 6Z Saturday runs of our top intensity models all predicted Dorian would be at Category 3 or 4 status through Tuesday.
Bottom line: expect Dorian to affect the northwest Bahamas as an extremely dangerous Category 4 hurricane and approach the Southeast U.S. coast on Tuesday as a powerful Category 3 or 4 hurricane.
A devastating pounding coming for Abaco Island in The Bahamas
Dorian will continue to the west through Sunday, as it feels the steering effects of a westward extension of the Bermuda high to its north. As the hurricane approaches the northwest Bahamas on Sunday morning, it will slow down to a forward speed of around 5 mph, subjecting the northernmost large island in The Bahamas, Abaco Island (population 17,000), to an extended two-day pounding.
The fact that Abaco Island is likely to be on the weaker (left) side of the approaching hurricane won’t help it much, since Dorian will be moving so slowly. In a fast-moving landfalling storm coming ashore at 15 mph, the peak winds on the right-front side of the hurricane can be expected to be about 30 mph greater than those on the left-front side. If Dorian is making landfall at just 5 mph, this difference will be more like 10 mph. Thus, the “weak” southern side of Dorian’s eyewall will not be all that weak; the hurricane’s destructive winds will be spread out over a larger area, and not focused in a relatively narrow region in the right-front quadrant.
A turn to the north before reaching Florida now expected
The 0z, 6Z, and 12Z suite of model runs continued their trend from the previous day, advertising an earlier turn to the north for Dorian. This is because of a weaker-than-expected edge of the Bermuda High, which is steering Dorian clockwise around its western flank. None of the operational versions of the top five tracking models used by NHC have showed with their 0Z, 6Z, or 12Z Saturday runs that Dorian might make a direct hit on Florida. However, many of the 70 ensemble members of the GFS and European model did take Dorian over Florida, and most of the east coast of Florida remains in the NHC cone of uncertainty. If you are in the cone of uncertainty, you are at risk of a direct strike!
If Dorian’s track does keep it offshore from Florida, that will result in limited wind and flood damage from heavy rains. Storm surge damage will still be a big concern, though—Hurricane Matthew of 2016 never made landfall in Florida, as it followed a path hugging the coast northwards, but still did approximately $3 billion in damage to Florida--mostly from storm surge.
A dangerous threat for South Carolina, North Carolina, and possibly Georgia
The latest set of 6Z and 12Z model runs have been relatively stable in their depiction of a significant threat to the coasts of South Carolina, North Carolina, and Georgia by the middle of next week. I estimate that there is a 40% chance of Dorian making a direct hit on South Carolina, North Carolina, or Georgia, and a 30% chance it will miss the Southeast U.S. coast entirely. The hurricane is expected to make its closest approach to South Carolina on Wednesday, and North Carolina on Thursday. We’ll expand upon the threat to these states in future posts.
Bob Henson contributed to this post.