Little Change to Dorian as it Heads West-Northwest

Tropical Storm Dorian continues steaming west-northwest at 17 mph across the middle of the Atlantic Ocean with little change in appearance this morning. Satellite images show that Dorian is a small but well-organized system with a moderate amount of heavy thunderstorms. Although Dorian is a small storm, it has tapped into a large area of moisture to its southwest associated with the Intertropical Convergence Zone (ITCZ), and has significantly moisturized its environment. A large area of dry air still lies to Dorian's west, as seen on water vapor satellite images. Dorian is under a low 5 - 10 knots of wind shear, which will tend to allow slow development. Ocean temperatures are barely adequate for maintaining strength of a tropical storm, about 25.5°C, but are now on the rise.


Figure 1. This morning's 00Z GFS model was run 20 different times at low resolution with slight perturbations to the initial fields of temperature, moisture, pressure, and winds. This ensemble of forecasts for Dorian is presented here in the pink lines, with the regular high-resolution forecast in white. The ensemble shows that the long-range track of Dorian is likely to follow one of two paths: a sharp recurvature to the north, missing the U.S., or a continued west-northwest path into the Bahamas, Florida, or Cuba.

Forecast for Dorian
The band of moist, unstable air to its southwest that has been feeding Dorian the past two days is forecast to get cut off on Friday, which will leave the storm isolated in a region with relatively dry, stable air. However, the SHIPS model predicts that wind shear will stay in the low range through Sunday, so there may be no mechanism to drive dry air into the storm to weaken it significantly. Ocean temperatures will begin to rise significantly next two days, reaching 27°C by Friday night and 28°C by Sunday. This increase in water temperature may counteract the more stable air Dorian will be in, allowing the storm to maintain its strength. Given its small size, Dorian is capable of relatively large changes in intensity in a short amount of time, and it would not surprise me if the storm dissipated by the end of the week--or became a Category 1 hurricane. The official NHC forecast of a tropical storm passing just north of the Lesser Antilles on Sunday is the most likely outcome; the 11 am wind probability forecast from NHC gave Dorian a 25 - 32% chance of being a hurricane at that time. Since Dorian is a small storm, the impacts to the northern Lesser Antilles Islands may be minor, if the core of the storm passes more than 50 miles to the north of the islands, as the official NHC forecast currently anticipates. It currently appears that Dorian will be a potential threat to the Bahama Islands, Bermuda, and the U.S. East Coast next week. There will be a trough of low pressure capable of recurving Dorian out to sea before the storm reaches the Bahamas and U.S., but it is uncertain if this trough will be strong enough to do the trick.

The Friday and Saturday updates on Dorian will be later than usual this week, as I am traveling on the West Coast.

Jeff Masters