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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


The views of the author are his/her own and do not necessarily represent the position of The Weather Company or its parent, IBM.

Reader Comments

Quoting 3457. DavidHOUTX:
Not sure how reliable this model is but it has it Gulf Bound (it seems)


Thats the Navy NOGAPS Model....not in the top 5 I would think.
12z GFS run is starting. This is the first run on a new supercomputer so we'll see. Many of us are concern about this not showing a hurricane because of the switch during storm.

Atlantic High strengthen now 1033 and becoming 1034 moving W and maybe a little faster than Dorian as now it is NW of Dorian before it was inline with Dorian

new steering maps also indicate the high has gotten stronger and pushing further W and is now starting to make that weakness a bit slimmer
06 GFS Run shows it crossing the northern FL peninsula and entering the GOMEX for a brief period of time. It seems that the GFS is starting to hint at the trof not being intense enough to grab Dorian and take it to the north. I belive future runs will demonstrate westerly trend this is one worth watching. It reminds me the "K" storm back in 05. I hope I'm wrong. Rather eat crow than having to rebuild again.
3506. ncstorm
Joe Bastardi ‏@BigJoeBastardi 1h

Twisting starting sse hatteras. Rapid feedback development on way off east coast on way nne http://www.goes.noaa.gov/HURRLOOPS/huecvs.html …
3507. Msdrown
Quoting 3492. Patrap:
The Range of the C-130 Hunter Aircraft is the Limiting ,as per fuel...from St. Croix.

En route, on scene, and return all have constraints.

Seems to me if they wanted they could use a KC10 or just go to the Cape Verdes. Having said that is there even a population the cape?
Quoting 3503. wunderkidcayman:
Atlantic High strengthen now 1033 and becoming 1034 moving W and maybe a little faster than Dorian as now it is NW of Dorian before it was inline with Dorian

new steering maps also indicate the high has gotten stronger and pushing further W and is now starting to make that weakness a bit slimmer

99 hours out on super computer

The anticipation is killing me:)
3511. GatorWX
Looks quite disrupted.

GFS 12Z takes Dorian into Hispaniola.
Gfs takes Dorian into Puerto Rico hour 117

He dead
Quoting 3512. WPBHurricane05:
GFS 12Z takes Dorian into Hispaniola.

That's the Western trend I started noticing during the 06Z run. If that's the case we could see a significantly weakened system after crossing the Hispaniola. Perhaps even a dissipating sytem.
Cariboy and Wunderkidcayman will be so happy.
Quoting 3513. weatherh98:
Gfs takes Dorian into Puerto Rico hour 117
dorain into the caribbean? haha nice
Dorian still popping up some convection but it has remained at its size:

Responding to little increase in SST. Conditions should improve as it transverse in increasingly warmer water which should off set any dry air and like what JM said upper level winds will stay under 20kts in a low shear environment with a ULL ventilating it along with the ULAC over heading fanning it out as well. I expect gradual intensification to a hurricane late Friday through Sunday. After 60W dry air is a nonfactor and possibly the MJO in Octant 1 and 2. I'm interested to see how that unfolds as well and beyond after Dorian.

MJO forecasts


New Blog
Quoting 3483. CaneHunter031472:
Polling time:

Dorian will...

A. Hit FL

B. Turn North

C. Enter the GOMEX

As a

A. Hurricane


C. Remnant low

Me first...
B, A

A, A
3521. centex
It moving more west. This was expected. Every advisory mentioned wnw or w.
3522. K8eCane