Danny Vaults to Category 3 Status; Tropical Storm Kilo Aims for Hawaii

Going against the grain of a hurricane-snuffing El Niño event, a tiny tropical cyclone has become the strongest hurricane in years over the deep Atlantic tropics. Hurricane Danny intensified dramatically on Thursday night and Friday morning, strengthening to Category 3 status on the Saffir-Simpson Hurricane Wind Scale. At 11:00 am EDT, Danny was located near 14.0°N, 48.2°W, about 930 miles east of the Leeward Islands, and still moving west-northwest at a modest clip of around 10 mph. At 11:00 am EDT, Danny’s top sustained winds were estimated at close to 105 mph. Indicative of Danny’s unusually compact size, hurricane-force winds extended only 15 miles from the storm’s center, and tropical-storm-force winds extended out up to 70 miles. It can be difficult for satellite-based instruments to estimate the intensity of very small hurricanes like Danny due to limited sensor resolution, but it’s clear that Danny is a surprisingly well-organized hurricane. A NOAA P-3 hurricane-hunter aircraft originally scheduled to sample the air west of Danny was instead approaching the storm early Friday afternoon, so we may soon have a stronger estimate of Danny’s actual intensity. Regular NOAA and Air Force reconnaissance flights into Danny are slated to begin on Saturday afternoon. Update: Based on reconnaissance data from the NOAA flight mentioned above, the National Hurricane Center has upgraded Danny to Category 3 status as of 2:00 pm EDT Friday, with top sustained winds at 115 mph. Danny is the first major Atlantic hurricane since Hurricane Gonzalo in 2014.


Figure 1. Visible imagery of Danny at 1-km resolution from the GOES-East satellite. Image credit: CIMMS/University of Wisconsin.


Figure 2. A comparison of imagery collected aboard polar-orbiting satellite by the NASA Visible Imaging Infrared Radiometer Suite (VIIRS) at around the same time (close to 0400 GMT Friday, August 20) for Typhoon Atsani (left) and Hurricane (Danny). The two images are at the same scale, revealing how compact Danny is, although Danny has enlarged somewhat since this image was taken. Image credit: Dan Lindsey, RAMMB/CIRA/University of Colorado.

In satellite imagery, Danny appears almost like a scale model of a well-developed hurricane, with quite symmetric structure, a clearly defined eye, and a minuscule but dense central core of strong thunderstorms (convection). Spiral banding is focused toward the east and south sides of Danny, where the atmosphere is comparatively moist. Danny is the strongest hurricane observed in the open North Atlantic tropics between the Lesser Antilles and Africa since Hurricane Julia in 2010, which became a Category 4 further east (longitude 32°W) than any other Atlantic hurricane since regular satellite observations began in the 1970s.

Danny is also one of the smaller hurricanes on record in the Atlantic. The smallest tropical cyclone in Atlantic history is 2008’s Tropical Storm Marco, whose brief life played out in the southern Bay of Campeche. Marco’s central core of convection was only about 10 miles in diameter--smaller than many supercell thunderstorms! The Pacific basin tends to produce a wider variety of tropical cyclone sizes, from tiny typhoons to the world’s largest and strongest on record: Typhoon Tip, which boasted a global record low for sea-level barometric pressure (870 mililbars, or 25.69”) and gale-force winds that at one point spanned some 1,380 miles in diameter.

The outlook for Danny: becoming more complex
Track models remain in fairly good agreement on a continued west-northwest track for Danny. The NHC forecast track (see Figure 3) now brings Danny to the vicinity of Puerto Rico by Tuesday and Hispanola by Wednesday. Assuming the west-northwest bearing remains solid, only a slight deviation could play a big role in Danny’s future, as interaction with the mountainous terrain of these islands could quickly disrupt weaken a storm as small as Danny. If the model trend further toward the north continues, Danny has a better chance of escaping landfall on the islands; in this case, its small size could actually result in less disruption from the islands than for a larger hurricane.


Figure 3. The outlook for Hurricane Danny issued at 11:00 am EDT on Friday, August 21.

Danny’s recent surge in strength has drawn on seasonably warm warm sea-surface temperatures (SSTs) of around 27-28°C, together with light to moderate wind shear (now running between 10 and 20 knots). Danny will be encountering higher wind shear (20 – 40 knots) over the next couple of days, as it nears a belt of upper-level westerlies extending across the Caribbean into the North Atlantic. Even though the shear within this zone has been lessening over the last couple of days, it will remain a formidable impediment to Danny. The same west-to-east belt also features a large zone of dry, dusty air with roots in the Saharan Desert. Thus far, Danny has managed to wall off a central core of convection intense enough to keep dry-air intrusions at bay, but this will become an increasing challenge for Danny over the next several days. Both dynamical and statistical models are in strong agreement that Danny will begin weakening by Saturday, as it encounters the increasing shear and dry air, and Danny will most likely be a strong tropical storm or weak Category 1 hurricane by Monday as it approaches Puerto Rico. An important caveat: tropical cyclones like Danny can strengthen or weaken very quickly, so there is more uncertainty than usual in this intensity forecast, especially at longer range.

If Danny manages to track well north of Puerto Rico and Hispanola with its structure relatively intact, it could encounter a more favorable environment for some potential restrengthening in the 5-to-6-day window, as suggested by the 0600 and 1200 GMT run of the GFDL model. Should such a scenario occur, the fate of Danny would then hinge on the state of an upper-level low that will scoot across the Midwest and Northeast over the next few days. An extension of that low is forecast to settle into the southeast United States as a weak upper-level trough, and the steering flow on the east side of that low would determine how soon Danny would recurve, assuming that it moves north of the Caribbean early next week. The most reliable long-range track models (GFS and ECMWF) suggest that this weak upper trough may be far enough east to keep Danny or its remnants offshore, but it is still far too soon to know with confidence how the upper trough will evolve.



Figure 4. A NOAA GOES-West infrared satellite image of Tropical Storm Kilo (center), gathering strength south of Hawaii, at 1600Z (noon EDT) on Friday, August 21. Image credit: NOAA National Hurricane Center.

Tropical Storm Kilo gaining strength, threatening Hawaii
The risk of a rare Hawaiian hurricane is growing as Tropical Storm Kilo (pronounced KEE-lo) slowly organizes in the Central Pacific. (“Kilo” is a Hawaiian term meaning meaning “to observe carefully”; the U.S. Navy maintains a research vessel at the University of Hawaii dubbed the R/V Kilo Moana, or “observing the ocean carefully”.)

At 5:00 am HST (11:00 am EDT) Friday, Kilo was located near 12.7°N, 151.7°W, or about 720 miles southeast of Honolulu, according to the Central Pacific Hurricane Center (CPHC). Kilo is moving west-northwest at about 16 mph, and that general track should continue through Sunday before upper-level westerlies force Kilo to undergo a sharp recurvature toward the north and northeast. Tropical cyclones approaching Hawaii from the south can generally maintain their strength more easily than those approaching from the east, as they spend less time over marginally warm water and may experience less wind shear. The two most intense hurricanes to strike Hawaii in modern times--Iniki (1992) and Dot (1959)--both arrived at Kauai from the south (see Figure 6 at bottom).


Figure 5. The outlook for Tropical Storm Kilo issued at 11:00 am EDT on Friday, August 21.

Kilo has a large, consolidated mass of convection within a moist overall environment, and there is a good chance that Kilo will strengthen significantly, with SSTs of 28-29°C (82-84°F) along its path, about 2°C above the seasonal average. Vertical wind shear is quite low over Kilo (less than 10 knots) and it has relaxed significantly over the last several days over Hawaii, further increasing the risk that Kilo will intensify. Although the amount of heat in the upper layer of the ocean is not particularly high by the standards of the Caribbean or tropical Northwest Pacific, Kilo’s brisk motion should minimize any negative effects from upwelled cooler water.

The timing and sharpness of Kilo’s recurvature will be critical to any potential impacts on Hawaii. The 1200 GMT GFDL model portrays a potent hurricane of at least Category 2 strength in the vicinity of Kauai by Monday, while the 1200 GMT Friday run of the HWRF projects a slower approach that could put other Hawaiian islands at risk in the Tuesday/Wednesday time frame. The most recent official outlook from the Central Pacific Hurricane Center places Kilo west-southwest of Kauai as an intensifying Category 2 storm. People throughout Hawaii need to keep very close tabs on Kilo, which could become the most significant hurricane to approach Hawaii since the devastating Hurricane Iniki of 1992. An Air Force C-130 hurricane-hunter aircraft is scheduled to fly into Kilo late Friday night.

Elsewhere in the tropics
Invest 97 could become a subtropical storm south of Bermuda late this weekend or early next week, most likely moving north or northeastward away from the United States. Although Typhoon Goni and Typhoon Atsani continue to rage over the Northwest Pacific, both have embarked on recurvature, and neither of them pose major landfall threats over the next day or so, although Goni will pass near Japan’s southern islands over the weekend and could strike Kyushu as a weakening typhoon early next week. In the Central Pacific, Tropical Depression 4-C could become a named storm but will remain far at sea.

I’ll have a full update by midday Saturday at the latest.

Bob Henson


Figure 6. Tracks of all tropical cyclones (tropical depressions, tropical storms, and hurricanes) to pass within 100 miles of the Hawaiian Islands, 1949 - 2014. Hurricanes approaching from the east typically fall apart before they reach Hawaii due to the cool waters and dry air that lie to the east of the islands. Only two named storms approaching from the east have hit the islands since 1949, an unnamed 1958 tropical storm and Tropical Storm Iselle of 2014, which hit the Big Island. Hurricanes approaching from the south represent the biggest danger to the islands, due to the warmer waters and more unstable air present to the south. The only two major hurricanes to have affected the islands since 1949, Hurricane Iniki of 1992 and Hurricane Dot of 1959, both came from the south. Image credit: NOAA/CSC.