Tropical storm Dorian strengthens slightly, several days away from land

As forecast, Tropical Storm Dorian has entered an environment that limits how quickly it can strengthen. The maximum sustained winds have increased only slightly in the past 24 hours, due to a balancing act between favorable and unfavorable influences.

As of 11 a.m., Dorian is packing 60 mph winds and is trekking westward at 17mph. It is still days away from any island or land.

Enhanced satellite image of Dorian from 9:15am EDT today. The low clouds show up as yellow, while the high clouds (mostly tops of thunderstorms) show up as white. (NOAA)

Enhanced satellite image of Dorian from 9:15am EDT today. The low clouds show up as yellow, while the high clouds (mostly tops of thunderstorms) show up as white. (NOAA)

Dorian’s cloud pattern has visibly improved, which is the main reason that the peak wind speed assigned to it has increased. Dorian is presently about 2,900 miles east of Miami (or about 7 days). When a storm is this far east, forecasters rely on satellite data for intensity estimates, but as it tracks further west, it moves into range of reconnaissance aircraft that fly directly into the tropical cyclone to gather invaluable data on its position, intensity, central pressure, and structure (see:“The sequester’s worrisome impact on Hurricane Hunters”).

As far as the large-scale environment goes, the ocean temperature under the storm will increase by the end of the weekend, but the wind shear is expected to increase as well — though still not too strong — and the mid-level moisture is forecast to remain relatively low. These factors should combine to curb the intensification rate through the end of the weekend… beyond that, it becomes less certain.

Dorian's current position, official forecast track, and cone of uncertainty overlaid on the sea surface temperature – where red is warmer and yellow is cooler. (wunderground.com)

Dorian’s current position, official forecast track, and cone of uncertainty overlaid on the sea surface temperature – where red is warmer and yellow is cooler. (wunderground.com)

(For a graphical though somewhat technical view of these parameters that affect intensity, see this plot that shows past, current, and forecast values of shear, SST, mid-level relative humidity, track, and intensity from a handful of models.)

Computer models are still in strong agreement on the track over the next 4-5 days. Every reliable model places the center of Dorian just north of Puerto Rico on Tuesday. After that, there is divergence, with some hints of a continued WNW trajectory into the Bahamas and southern Florida, and some hints of curvature out to sea before even reaching the Bahamas.

Track probabilities from the latest GFS deterministic run and ensemble run.  The deterministic (single model run, higher resolution) run is shown by the line labeled "AVNO", the 20 members in the ensemble run are shown by the thin gray lines and their mean is the line labeled "AEMN", and the track densities are shaded in color.  (Brian Tang, Univ of Albany).

Track probabilities from the latest GFS deterministic run and ensemble run. The deterministic (single model run, higher resolution) run is shown by the line labeled “AVNO”, the 20 members in the ensemble run are shown by the thin gray lines and their mean is the line labeled “AEMN”, and the track densities are shaded in color. (Brian Tang, Univ of Albany).

The intensity forecasts are in close agreement: no dynamical model is forecasting this storm to become a hurricane within the next five days; one statistical-dynamical model (LGEM) does, but just barely.

Climatology

In the past 162 years, just three other storms were named (tropical storm intensity or greater) this far east during the entire month of July:  Anna 1969, Alex 1998, and Bertha 2008.  Dorian has joined that list, and for reference, its current location is marked with a red “D”… basically right on top of Bertha’s track; however, Dorian is not expected to curve out to sea so quickly.

The only three storms that were at least tropical storms at or near Dorian's longitude during July going back to 1851.  (NOAA)
The only three storms that were at least tropical storms at or near Dorian’s longitude during July going back to 1851. (NOAA)

* Brian McNoldy is a senior researcher at the University of Miami’s Rosenstiel School of Marine and Atmospheric Science

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