Danny Heading West, Remaining a Hurricane; Rain, Wind Threat to Leeward Islands, Virgin Islands, Puerto Rico

August 21,2015

Highlights

• Danny became the first major hurricane (Category 3 on the Saffir-Simpson hurricane wind scale) of the 2015 Atlantic hurricane season Friday afternoon.
• Peak maximum sustained winds were estimated at 110 mph as of 11 p.m. EDT Friday.
• Hurricane Danny was located about 810 miles east of the Leeward Islands in the eastern Atlantic Ocean,
• Danny peaked as a category 3 hurricane with maximum sustained winds up to 115 mph Friday afternoon.
• Danny will track west-northwest, reaching the Leeward Islands Monday.
• A weakened Danny may bring drought-relieving rainfall to Puerto Rico and the Virgin Islands early next week.

Storm Stats and Satellite

Danny has started to move into an environment with more wind shear, or change in wind speed with height either in speed and/or direction, and this has caused the hurricane to be downgraded from Category 3 to Category 2 intensity Friday evening.

(MORE: Expert Analysis | Hurricane Central)

Friday afternoon, a NOAA research plane not only sent back near real-time radar images from a hurricane for the first time, but also found maximum winds had increased to Category 3 intensity for a time. 

(INTERACTIVE: Satellite Loop of Danny)

Danny's tropical storm-force winds extended up to only about 60 miles from the center of circulation as of the 11 p.m. Friday advisory from the National Hurricane Center, a tiny wind field for a hurricane. 

Comparisons were made Thursday to the much larger west Pacific typhoons Goni and Atsani.

Water Vapor Image: Danny

Current Wind Shear Analysis

Small circulations such as Danny are prone to sudden changes in intensity, which can be difficult to forecast.

(MORE: Track Danny with our Interactive Storm Tracker)

The large-scale atmosphere Danny is moving into will induce further weakening through this weekend. Satellite imagery as of 5 p.m. Friday indicated that the eyewall of Danny has begun to collapse and convection around the main circulation has become less robust. 

Danny is now encountering an ample reservoir of dry air extending westward into the Caribbean Sea.

Dry air hampers tropical cyclones by encouraging the development of stronger thunderstorm downdrafts, which then either squelch nearby thunderstorms from forming or push them away.

This dry air is also stable, meaning it suppresses upward vertical columns of air needed to maintain or form new thunderstorms.

Danny is also approaching what has been so far this season a figurative Caribbean "wall of wind shear". According to tropical meteorologist Phil Klotzbach, wind shear over the Caribbean Sea had been at record levels from mid-July through mid-August.

Wind shear can blow convection away from the center from a tropical cyclone. If strong enough, it can rip apart existing tropical cyclones. In addition, those increasing west to southwest winds aloft will transport more dry air into the circulation of Danny.

(MORE: Satellite Maps)

Therefore, Danny is expected to continue weakening this weekend upon encountering these twin atmospheric nemeses.

Danny will move generally toward the west-northwest the next several days. Our latest forecast indicates the center of Danny would approach the Leeward Islands sometime Monday, most likely as a tropical storm.

Projected Path

A weakening Danny would then continue toward the Virgin Islands, Puerto Rico and Hispanola.
It remains to be seen how quickly Danny weakens in the hostile environment described above next week.

Latest Drought Monitor analysis for the Virgin Islands and Puerto Rico as of August 20, 2015.
(NCEI/NESDIS/NOAA/NDMC)

That said, it seems probable at least some bands of locally heavy rain will wrap into parts of the Virgin Islands and Puerto Rico. This could end up being too much of a good thing in the drought-stricken region.
NBC News reported six more municipalities in Puerto Rico were declared federal disaster areas Wednesday, bringing the total number of municipalities declared a disaster area to 36.
The U.S. Geological Survey says roughly 45 percent of the public water supply in Puerto Rico is supplied by reservoirs.
The eastern half of the island territory, as well as the U.S. Virgin Islands, are classified as in either severe or extreme drought, according to the U.S. Drought Monitor analysis.
San Juan, Puerto Rico, (-10.70 inches) and St. Croix (-11.59 inches) have significant year-to-date rain deficits through Thursday, a large chunk of those since June 1. Dating to 1951, it has been the driest year-to-date on record at Henry E. Rohlsen Airport on St. Croix, picking up only 7.31 inches of rain through August 20.
Stay tuned to The Weather Channel and check back with weather.com for updates on this system.
(MORE: Heart of the Hurricane Season is Here)
Incidentally, while small, Danny isn't the smallest tropical cyclone on record. According to the NOAA's Hurricane Research Division, Tropical Storm Marco in 2008 had gale-force winds extending only 12 miles from the center.  Danny's tropical storm-force winds extended up to an estimated 60 miles from the center, as of Friday morning.
Danny became only the fourth named storm to become a hurricane in the strip of the Atlantic Ocean from off west Africa to the Lesser Antilles since the start of the 2011 season. Gonzalo (2014), Humberto (2013) and Katia (2011) were the only others to do so in that stretch of over four seasons.
Friday, Danny was the first hurricane to strengthen to Category 2 intensity in that Lesser Antilles to west Africa strip of ocean since 2010's Tomas and Julia.
Senior meteorologists Nick Wiltgen and Jonathan Erdman and meteorologists Chris Dolce and Quincy Vagell contributed to this report.

MORE: Hurricanes by the Numbers