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Tropical Storm Dorian Forms

By: Dr. Jeff Masters, 3:48 PM GMT on July 24, 2013

The season's fourth named storm, Tropical Storm Dorian, is here. Born from a strong tropical wave that moved off the coast of Africa on Monday, Dorian formed unusually far east for so early in the season, at longitude 29.9°W. Only Hurricane Bertha of 2008, which became a tropical storm at 22.9°W longitude on July 3, formed farther to the east so early in the year. Satellite images show that Dorian is a small but well-organized system with a moderate amount of heavy thunderstorms. A large area of dry air lies to Dorian's west, as seen on water vapor satellite images, but Dorian has moistened its environment enough that this dry air should not interfere with development for the next day. 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 26.5°C.

Figure 1. MODIS satellite image of Tropical Storm Dorian taken at approximately 8 am EDT July 24, 2013. At the time, Dorian had top winds near 50 mph. Image credit: NASA.

Forecast for Dorian
The SHIPS model predicts that wind shear will stay in the low range through Thursday, then rise to the moderate range Friday through Monday. Ocean temperatures will fall to 25 - 26°C Wednesday night through Thursday night, which may induce some weakening of Dorian. Thereafter, ocean temperatures will rise again, but wind shear will rise. This increase in wind shear will be capable of causing weakening, since there will still be a large area of dry air to Dorian's west that the shear may be able to bring into Dorian's core. 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. However, 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 6% chance of being a hurricane at that time. Dorian should maintain a west-northwest track through the week, and spread heavy rains and gusty winds to the northern Lesser Antilles Islands beginning on Sunday. The usually reliable European model (ECMWF) has Dorian passing several hundred miles to the north of the Lesser Antilles Islands, while the other models show Dorian passing closer, within 100 miles. 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 this trough is currently depicted as being fairly weak, reducing the chances of Dorian missing the Bahamas and U.S. East Coast.

Figure 2. Tracks of all Atlantic tropical depressions, tropical storms, and hurricanes (tropical cyclones) occurring in the months of June and July off the coast of Africa. Only Bertha of 2008 became a named storm farther east so early in the year, compared to Tropical Storm Dorian. Reliable satellite records of Eastern Atlantic tropical cyclones go back to 1966. Image credit: NOAA/CSC.

Jeff Masters


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