Tropical storm Dorian forms in eastern Atlantic

Large-scale satellite image of Dorian from 7:30am EDT today. The Lesser Antilles is the island chain on the left side of the image, the Cape Verdes is the island cluster just northeast of the storm, and then the west coast of Africa is on the far right. For scale, this image spans 3300 miles… or the distance between Washington D.C. and Anchorage, Alaska. (NRLMRY)

Dorian, the fourth named storm of the 2013 Atlantic hurricane season, has formed in the far eastern Atlantic, about 2,400 miles east of Puerto Rico.
The average formation date of the fourth named storm over the past fifty years is August 23rd, so Dorian was born about a month ahead of average. But in 2012, the ‘D’ storm, Debby, formed two months ahead of average (on June 23).
This storm is also notable for how far east it formed (29.9 W longitude). Since 1851, I could only find one other storm  that got named so far east so early in the season:  Bertha, in 2008, became a tropical storm at 24 W longitude on July 3.
As I mentioned yesterday when it was still an unnumbered disturbance (see “New Disturbance Exits Africa”), this storm’s time in favorable environmental conditions is limited.
Although the vertical wind shear (turning of the winds with altitude) is forecast to remain low (low shear is favorable for storm development), a hostile combination of mid-level dry air and cooler (sub 79°F) water temperatures will affect the development of the storm beginning later today.
The ocean temperatures should warm up some by Friday, but Dorian is not forecast to escape the choking dry air for several days. This does not mean that it won’t intensify, it just means that if it does, it will do so at a slower rate than it otherwise could.
Early morning satellite images of the storm reveal what appears to be an eye forming. This is most decidedly a signal of organization and intensification. As of the 10:00 am EDT, maximum sustained winds have increased to 50mph. But, before the day is out, I suspect we’ll see a brake on this trend and it intensity should level off for at least a couple of days.

Visible satellite of Tropical Storm Dorian as of 10 a.m. EDT this morning.

Models are in excellent agreement on the track over the next five days, with the primary differences found only in forward speed. On average, and this is what the National Hurricane Center’s official forecast closely follows, Dorian is expected to keep a steady west-northwest trajectory, which will likely bring it just north of the Lesser Antilles on Monday. Beyond that, there is still too much uncertainty to say if it will maintain that track towards the Bahamas and Florida, or if it will begin to turn northward and recurve out to sea.

Plot of model forecast tracks over the next 5-7 days. The green lines, the orange line, and the dashed black line represent very basic models, climatology, and extrapolation and can generally be dismissed. The thin gray lines are different ensemble members (possible realizations) of the “AVNO” (GFS) model, and the mean of those gray lines is the purple “AEMN” line. (SFWMD)

I will be following the progress of this storm very closely, as it has the potential for a U.S. interaction by late next week.
* Brian McNoldy is a senior researcher at the University of Miami’s Rosenstiel School of Marine and Atmospheric Science