Confidence is increasing that Dorian will be a dangerous major hurricane that will bring devastating winds, storm surge, and rains to the northwest Bahamas on Sunday and to Florida on Monday and Tuesday. The hurricane is expected to slow at landfall in Florida to a forward speed near 5 mph, resulting in punishing long-duration hurricane conditions.
At 11 am EDT Friday, Dorian was an upper-end Category 2 hurricane with 110 mph winds and a central pressure of 972 mb, headed northwest at 10 mph. Hurricane hunter data and satellite images have shown relatively little change to the storm this morning, with the winds ranging from 105 – 110 mph and the central pressure ranging from 972 – 979 mb.
Dorian is now a well-organized, medium-sized hurricane that has increased in size over the past day. Low-level spiral banding and upper-level outflow have both improved, though dry air and an area of moderate wind shear impinging on the southwest side of Dorian have been helping limit intensification.
A hurricane hunter eye report from 12:14 pm EDT Friday found that Dorian’s central pressure had dropped to 966 mb, and the hurricane had completed yesterday’s eyewall replacement cycle (ERC). Dorian now has a single eyewall with a diameter of 30 miles, instead of two concentric eyewalls. The new eyewall structure is more conducive for strengthening than the old one, since the two eyewalls were competing for resources. The Hurricane Hunters noted that the eyewall did not form a full ring around the calm eye—there was a gap in the southwest side. Unless this gap closes off, Dorian is likely to only undergo slow intensification, not rapid intensification. You can see the gap in the eyewall on the radar display in the tweet below, taken from the NOAA hurricane hunter mission this morning.
Recent radar images of Dorian's eye taken from the NOAA P-3 aircraft. pic.twitter.com/vPSGkS1Sqf
— National Hurricane Center (@NHC_Atlantic) August 30, 2019
Intensity forecast for Dorian
Dorian is likely to strengthen in the coming days, and perhaps undergo a round of rapid intensification. For the remainder of today, intensification will likely be slow, due to moderate wind shear impinging on the hurricane’s southwest flank from an upper-level low. This low is expected to weaken and move away by Saturday, and we may see more significant strengthening into a Category 4 hurricane then.
Over the next three days, wind shear is expected to mostly be light, less than 10 knots, and mid-level relative humidity will increase from around 55% to around 65%. Dorian will be passing over very warm water, with sea surface temperatures of around 29.5 – 30.5°C (85 – 86°F)–about 1.0°C (1.8°F) warmer than average.
Oceanic heat content (OHC) will be above 50 kilojoules per square centimeter along much of Dorian’s path for the next three days. Values above 75 kJ/sq cm are most closely associated with higher odds of rapid intensification, but we often see rapid intensification when OHC is between 50 – 75 kJ/sq cm.
The Rapid Intensification Index in the 12Z Friday run of the SHIPS and DTOPS models gave a 28% and 19% chance, respectively, of Dorian’s sustained winds increasing by 30 mph by Saturday morning, which would make it a Category 4 hurricane with 135 mph winds.
Expect Dorian to experience at least one more eyewall replacement cycle (ERC) before any U.S. landfall. 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. Between this possible effect and the tendency of hurricanes to expand over time and with latitude, we can expect Dorian to be larger at landfall than it is now, with a larger wind field.
Among our top intensity models, their 6Z Friday runs predicted Dorian would peak off the coast of Florida on Monday somewhere between Category 3 (GFS model) and Category 5 (HMON model).