Hurricane Danny Weakens; Tropical Depression Kilo Struggles

It’s been a challenging 24 hours for the two named tropical cyclones closest to North America, one in the Atlantic and one in the Central Pacific. After surging to Category 3 strength on Friday, Hurricane Danny has weakened almost as quickly, as wind shear and dry air chip away at its integrity. As of 5:00 pm EDT, Danny was located near 15.8°N, 53.3°W, or about 570 miles east of the northern Leeward Islands. Danny remains a very small hurricane: hurricane-force winds extend up to 10 miles and tropical-storm-force winds up to 60 miles from Danny’s center, according to the National Hurricane Center (NHC).

WIth top sustained winds now down to 75 mph (minimal Category 1 strength), Danny was continuing its steady west-northwest track at a slightly speedier pace of about 14 mph. Danny’s current bearing will take it into the northernmost Antilles by Monday, and tropical storm watches have been hoisted for Antigua, Barbuda, Montserrat, St. Kitts, Nevis, Anguilla, Saba, St. Eustatius, St. Maarten, Guadeloupe, St. Barthelemy, and St. Martin. Although no watches are yet in effect, Puerto Rico could experience tropical storm conditions by Tuesday. (A watch means that tropical storm conditions are possibe within the next 48 hours.)



Figure 1. A visible image of Hurricane Danny from NOAA’s GOES-Floater satellite, collected at 2045 GMT (4:45 pm EDT) on Saturday, August 22. Image credit: NOAA/NESDIS.



Figure 2. Vertical wind shear over Danny is now around 20 knots (23 mph), and even larger value above 30 knots lie ahead in Danny’s path over the next day or two. Image credit: NOAA/NESDIS


Danny has encountered increasing vertical wind shear and dry, dusty air since Friday, and these factors are taking an obvious toll on the hurricane. The well-defined eye and eyewall that Danny boasted for a time on Friday are long gone. Spiral banding has largely dissipated around Danny, and the central core of convection is much more fragmented and asymmetric, as upper-level westerlies push showers and thunderstorms east from Danny’s center. Data from an Air Force hurricane-hunter aircraft that surveyed Danny on Saturday afternoon provided further evidence that Danny is going downhill, with maximum flight-level winds peaking at only 61 knots (70 mph), just below hurricane strength, at 1821 GMT (2:21 pm EDT). Danny’s decline is echoed in the 5:00 pm EDT outlook from the NHC (see Figure 3), which brings Danny down to tropical-storm strength on Saturday night. Both upward and downward swings in intensity can be especially large and rapid with small hurricanes like Danny.




Figure 3. The NHC’s outlook for Hurricane Danny as of 5:00 pm EDT Saturday.

Could Danny get a new lease on life?
Danny’s weakening is a very confident forecast for the next couple of days: both statistical and dynamical models agree that Danny should be a mid-range tropical storm at best by the time it approaches Puerto Rico on Tuesday. The 1200 GMT run of the HWRF model, one of the best for short-term intensity change, weakens Danny to a tropical depression by Monday. Given Danny’s small size, it’s entirely possible Danny could be little more than a tropical wave by then.

The longer-range outlook for Danny has a bit more uncertainty, as long-range outlooks so often do. Track models continue to bring Danny or its remnants very close to Puerto Rico and Hispanola, though perhaps staying just north of the islands. Rains from Danny would be more than welcome over these drought-stricken islands, but only a small difference in track angle at this point could make a big difference in the outcome. Given Danny’s small size, it might be able to avoid the detrimental effect of the islands’ mountainous terrain if its track is on the north side of the model solutions, consistent with the 1200 GMT run of the GFDL model. There still might not be much left of Danny at that point, but if a reasonable circulation does persist, large-scale conditions could support some restrengthening. Sea-surface temperatures will be very warm (around 29°C or 84°F), and an upper-level ridge is forecast to strengthen over the western North Atlantic, keeping Danny or its remnants rolling westward with the potential for relatively low wind shear. Further down the line, any revitalizated version of Danny would face an upper-level trough predicted to dig into the southeast United States. If the models are correct, the resulting southwesterly flow across Florida would most likely force a recurvature of Danny around the middle of next week. Given the very warm waters and deep oceanic heat content over the Bahamas, any potential tropical cyclone bears watching, but at present it does not appear that Danny will pose a major threat to the U.S. East Coast.



Figure 4. The 1200 GMT run of the GFS model on Saturday, August 22, valid at 1200 GMT Thursday, August 27, depicts an upper-level trough across the southeast United States at the 200-mb level (about 40,000 feet high). If this trough intensifies and shifts east as forecast, it would likely steer Danny or its remnants away from the U.S. East Coast. Image credit: Levi Cowan, tropicaltidbits.com.



Figure 5. Large zones of dry, dusty Saharan air (yellow and orange areas) ontinue to prevail across the Atlantic subtropics, although Hurricane Danny and a large tropical wave to its east have made inroads into the dry air. Image credit: University of Wisconsin/CIMMS and NOAA Hurricane Research Division.

Elsewhere in the Atlantic
The Cape Verde wave train--which normally peaks from late August to late September--is now in high gear, with multiple large easterly waves developing over Africa and rolling into the eastern tropical Atlantic. One such wave, called out in the 4:00 pm EDT tropical discussion from NHC, is located near 18°N, 28°W, moving west at about 20 mph. Convection blossomed around this wave on Friday night, then subsided on Friday, a typical sequence for developing tropical cyclones. The wave is embedded in a large swath of moisture, boding well for its development, although SSTs of around 26-27°C (79-81°F) are only modestly supportive. A strengthening upper-level ridge across the subtropical Atlantic should keep this wave scooting westward for several days, at which point models suggest a break in the ridge could allow it to move northward. As of 2:00 pm EDT Saturday, NHC was giving the wave 30% odds of developing into a depression by Monday and 60% odds by Thursday. Another healthy wave is now approaching the African west coast at a somewhat lower latitude, which may give it a better chance of traversing the deep Atlantic tropics. NHC gives this wave 10% odds of development by Monday and 30% odds by Thursday.

In the Northwest Atlantic, Invest 97L, a cluster of storms associated with a weakening upper low and frontal zone, is nearly devoid of convection, and models show little inclination to develop 97L into a tropical or subtropical storm over the next several days.

Slow going for Kilo, but Hawaii still a possible target
Residents of Hawaii should keep their guard up despite the laggard development of Tropical Depression Kilo. Briefly a tropical storm, Kilo was downgraded on Friday night as its large area of westward-moving showers and thunderstorms (convection) outran the low-level circulation. This outcome is somewhat surprisingly, since vertical wind shear has been quite low near Kilo (10 knots or less). Infrared satellite loops over the last few hours show Kilo developing a large core of strong convection, although still strongly sheared from the east, suggesting that more than one low-level center of circulation may be present. An Air Force reconnaissance flight found maximum surface winds of only 26 knots (30 mph) at around 1800 GMT Saturday.



Figure 6. Infrared image of Tropical Depression Kilo, collected from the GOES-Floater satellite at 2000 GMT (4:00 pm EDT) on Saturday, August 22. Image credit: NOAA/NESDIS.



Figure 7. The outlook for Kilo from the Central Pacific Hurricane Center as of 11:00 am HST (5:00 pm EDT) Saturday, August 22.


Forecast models agree that Kilo will eventually organize and begin a northwestward trek. An upper-level ridge is forecast to strengthen over Kilo about 2-3 days from now, which will slow its movement and support its intensification (assuming it organizes as expected). Thereafter, the upper-level steering currents become somewhat chaotic, although the most likely outcome is that westerlies north of Hawaii would steer Kilo in the general direction of the western Hawaiian islands for at least a period of time. The high-resolution HWRF and GFDL models have consistently projected a powerful hurricane within striking distance of Kauai around the middle of next week, although the timeline has been shunted back due to the predicted weak steering currents in the 3-to-5-day period. Longer-range global models are generally pulling Kilo back to the northwest before it might reach the islands, with the exception of the ECMWF model, which is closer to the HWRF and GFDL tracks. Statistical model guidance, which is generally the best with intensity in the 3- to 5-day period, is less aggressive on strengthening Kilo than the HWRF and GFDL models. The 11:00 am EDT outlook from the Central Pacific Hurricane Center (CPHC) calls for Kilo to be an intensifying Category 1 hurricane well southwest of Kauai by Thursday. With the upper-level prognosis cloudy and Kilo itself less organized than expected at this point, the outcome is highly uncertain, but the high potential risk to Hawaii still calls for vigilance in monitoring Kilo.

West of Kilo, Tropical Depression Loke became the fifth named storm in the Central Pacific on Friday. Eric Blake (NHC) noted on Twitter that this is the largest number of tropical storms assigned names in the Central Pacific since modern records began, beating the record of four (Akona, Hana, Ema, and Iwa) set in 1982 (which, like 2015, was the year of the onset of a major El Niño event). These numbers do not reflect the total number of tropical cyclones affecting the basin, since other named storms move into the Central Pacific from the Northeast Pacific. This brings another record to the fore: Loke is the seventh named storm to either form or pass through the Central Pacific this year, well above the previous record of five set in 1982, 1994, and 2013, as tweeted by Phil Klotbach (Colorado State University). Loke should have little impact outside the record books: as it moves slowly northward over progressively cooler waters, it is projected to regain only minimal tropical-storm strength over the next five days.


Goni heading toward Japan, leaving 10 dead in Philippines
After giving the northernmost Phillipines a stronger sideswipe than expected, Typhoon Goni is now heading north-northeast toward Japan. Goni embarked on a very sharp but very slow recurvature on Friday, taking a 90-degree turn from west to north just north of the Philippine island of Luzon. The island was on the weaker left-hand side of Goni, but the typhoon’s large shield of heavy rain extended over the northern reaches of the island for several days, triggering mudslides and flooding. At least 10 deaths were reported, and some locations received more than a foot of rain, according to weather.com’s Nick Wiltgen. More than 5,000 people were reportedly evacuated during the storm.

Now heading north-northeast, Goni is down to Category 1 strength, with top sustained 1-minute winds of 90 mph as of 1800 GMT Saturday. Wind shear will be light for at least the next couple of days over Goni as the typhoon passes over the warm Kuroshio Current (the Pacific’s analogue to the Gulf Stream). The Joint Typhoon Warning Center projects that Goni will reintensify to Category 3 strength by early Monday local time as it passes near Japan’s small southernmost islands. Though weakening as it moves further north, Goni could still strike the western part of Kyushu as a minimal typhoon. Further to the northeast, Category 1 Typhoon Atsani--also down to 90-mph sustained winds--will continue to weaken as it moves northeast over open water.

Jeff Masters will be back with our next update on Sunday.

Bob Henson