Unexpected Tropical Storm Julia Pops Up in Northern Florida; TD 12 Forms

In a rare surprise, the Atlantic experienced the formation of a tropical storm with its center located over land on Tuesday evening, when Tropical Storm Julia emerged at 11 pm EDT, centered about five miles west of Jacksonville, Florida. Tropical cyclones (comprising all tropical depressions, tropical storms, and hurricanes) derive their energy from the warm waters of the ocean, so it is very difficult—but not unheard of—for a storm to get its start while centered over land. Julia was helped out by its very large circulation, which pulled a tremendous amount of moisture-laden air from a wide area of ocean. The waters off the coast of Florida that fed Julia featured the exceptional warmth of the Gulf Stream Current, at the time of year when ocean temperatures are at their peak. The Atlantic has had at least one other case of tropical cyclone formation over land: Tropical Storm Beryl in August 1988, which was declared a TD and then a TS while over southeastern Louisiana (thanks go to Boris Konon for this example.)
 

Figure 1. Latest satellite image of Julia.


Figure 2. Latest radar-estimated rainfall accumulation image for Julia.

Radar out of Charleston, South Carolina on Wednesday morning showed that Julia was bringing heavy rains to the coast of Georgia and South Carolina, but these rains were mostly staying offshore. There was no increase in organization or intensity of the echoes apparent, due to the closeness of the storm’s center to land and the presence to a moderately high 15 - 20 knots of wind shear. The main threat of the storm is the heavy rain of 3 - 6” that it is likely to bring to the coast over the next two days. The top winds observed at any coastal sites on Wednesday morning as of noon EDT were sustained winds of 29 mph, gusting to 40 mph, at Fort Pulaski, Georgia at 9:48 am EDT. The strongest offshore buoy winds were 33 mph, gusting to 38 mph, at buoy 41008, 46 miles southeast of Savannah, Georgia, at 10:50 am EDT. Storm surge levels along the Southeast U.S. coast at noon EDT Wednesday were 1.3’ or less.


Figure 3. MODIS image of TD 12 on Wednesday morning, September 14, 2016. Image credit: NASA.

Tropical Depression 12 forms over the Cabo Verde Islands
Tropical Depression Twelve formed in the Cabo Verde Islands off the coast of Africa on Wednesday morning, and was bringing heavy rain showers and gusty winds to the islands as the storm headed west-northwest at 13 mph. Satellite images on Wednesday morning showed that TD 12 was well-organized, with plenty of spin and heavy thunderstorm activity. With wind shear a moderate 10 - 20 knots, a moist atmosphere and warm SSTs near 27 - 27.5°C (81°F), TD 12 is likely to develop into Tropical Storm Karl by Thursday. Ocean temperatures will cool, the atmosphere will get drier and wind shear will increase over TD 12 on Friday, which should weaken the storm—or even dissipate it, as predicted by the GFS model. TD 12 will mostly track to the west-northwest or west over the next five days; it is too early to assume that the storm will recurve to the north and northeast without ever affecting any land areas.

Another African tropical wave may develop next week
The 0Z Wednesday runs of our three reliable models for predicting tropical cyclone genesis—the GFS, European, and UKMET models—agreed that a new tropical wave expected to come off the coast of Africa on Friday will develop into a tropical depression early next week. This storm is expected to take a track more to the northwest than TD 12, and does not appear to be a long-range threat to the Lesser Antilles Islands or North America.

Tropical Storm Ian churning the central Atlantic
Tropical Storm Ian continued its unimposing presence in the central Atlantic late Wednesday morning, about 700 miles east of Bermuda. Ian was headed north at about 19 mph into a region with slightly lower wind shear, which may allow the storm to intensify from its current 50 mph winds to 65 mph winds by Thursday. On Friday, Ian will become entangled with a cold front and and upper level low pressure system, and transition to an extratropical storm. Ian is not a threat to any land areas.

For an update on imposing Typhoon Meranti, which is just hours from landfall in China, see our Wednesday afternoon post, which followed this one. Sorry for the site issues and lack of blog access this morning; we had a disk issue that was causing us trouble.

Jeff Masters