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Danny Strengthens in Atlantic; Goni, Atsani Rev Up in Pacific

By: Bob Henson and Jeff Masters 6:28 PM GMT on August 19, 2015

Tropical activity is on the upswing today, with two intensifying cyclones in the Northwest Pacific (one a super typhoon), a Central Pacific disturbance that could make a run at Hawaii, and a tropical storm in the Atlantic threatening to become the region’s first hurricane of the year. At 11:00 am EDT Wednesday, Tropical Storm Danny was located near 11.2°N, 41.1°W, or about 1400 miles east of the Lesser Antilles, moving just north of due west at about 12 mph. Danny was struggling this morning, in part due to having ingested dry air from its northwest. Showers and thunderstorms were more scattered than last night, and much of Danny’s central convective core had dissipated. However, in its 11:00 am EDT discussion, the National Hurricane Center notes that the structure of the inner core has improved somewhat since Tuesday, and core convection was beginning to rebuild on Wednesday afternoon. Apart from the vast swath of Saharan dust and dry air that lurks just to the north, conditions remain favorable for Danny to intensify, with sea-surface temperatures (SSTs) around 28°C (82°F) and vertical wind shear quite low (less than 10 mph) for at least the next couple of days.

Figure 1. Infrared satellite image of Tropical Storm Danny, collected by the GOES-East floater satellite at 1715 GMT (1:15 pm EDT) on Wednesday, August 19. Image credit: NOAA/NESDIS.

Figure 2. Dry air dominates the northeast Caribbean and north Atlantic around latitude 20°N, as Tropical Storm Danny (lower right of image) continues on a west-northwest track. Image credit: National Hurricane Center.

Figure 3. Various track models at 1200 GMT Wednesday largely agreed on a west-northwest track for Tropical Storm Danny over the next five days (120 hours). A list of models and definitions can be found at the NHC website. Image credit: NCAR/RAL Tropical Cyclone Guidance Project.

This morning’s models (1200 GMT Wednesday) have converged more strongly on a steady west-northwest bearing for Danny, predicted to continue throughout the next five days. Such a track would bring Danny close to the northernmost Leeward Islands by Sunday or Monday, as projected by the official NHC outlook (see Figure 4 below). There’s less model agreement on Danny’s future intensity, although it appears that Danny may peak in the next 2 or 3 days before entering a period of greater struggle. The 1200 GMT run of the recently upgraded HWRF model, which has been bullish on Danny from the start, make Danny a strong Category 1 hurricane by the end of this week, then weakens it back to tropical-storm strength. The GFDL model, the other of our two most reliable dynamical models for intensity, had retreated from its initial skepticism about Danny by Tuesday. The 1200 GMT GFDL run brings Danny to a similar peak intensity as the HWRF, with a minimal central pressure close to 970 millibars. Statistical intensity models, which are the most reliable guidance beyond Day 3, have begun to pull back from earlier forecasts that Danny would reach solid Category 2 strength by the weekend.

Figure 4. NHC’s outlook for Danny as of 11:00 am EDT Wednesday.

The official NHC outlook from 11:00 am EDT Wednesday projects Danny to reach Category 1 strength by Friday, with a slight decrease in strength over the weekend. Beyond the five-day outlook period, questions multiply as to how strong Danny will be (if it survives) and whether it might continue tracking toward the Bahamas or recurve out to sea. It remains far too soon to know how much of a threat Danny might pose to the United States next week, assuming it holds together. NOAA is scheduled to begin research flights around Danny on Friday, using its Gulfstream V and P-3 aircraft, while Air Force hurricane-hunter flights into the storm are currently slated to begin on Saturday afternoon.

Figure 5. Infrared satellite image of the central tropical Pacific, collected by the GOES-West satellite at 1630 GMT (12:30 pm EDT) Wednesday, August 19, showing Hawaii (center) along with Invest 93C, the large, poorly organized system to its southeast. Image credit: CIMMS Tropical Cyclones.

Invest 93C: a potential threat for Hawaii
Forecasters in Hawaii are already casting a wary eye toward Invest 93C, which was located about 915 miles south-southeast of Hilo, Hawaii, at 2:00 am HST (8:00 am EDT) Wednesday. Convection is disorganized but widespread in the vicinity of 93C. Now moving slowly toward the north, 93C is predicted to turn toward the northwest over the next several days before recurving northeastward. The particulars of that track—still to be determined--are crucial to whether 93C might impact Hawaii. The 0600 GMT Wednesday run of the GFDL model suggests the possibility that 93C could affect the Big Island as a strong tropical storm or minimal hurricane late in the weekend, while the 0600 GMT run of the HWRF model similarly develops 93C but recurves it well to the east of Hawaii. The 1200 GMT runs of both models paint a similar picture, and other models agree that 93C is likely to become Tropical Storm Kilo over the next several days. This year has been active in the Central Pacific, with an assist from unusually warm SSTs (currently running about 1-2°C above average south of Hawaii) partially associated with El Niño. SSTs south of Hawaii are well above the 26°C threshold for supporting tropical cyclones, and the high wind shear now over Hawaii (40 – 60 knots) is predicted to relax significantly over the next several days, as a weak upper trough is replaced by a building ridge. 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. The Air Force is currently scheduled to begin reconnaissance flights into 93C starting on Friday afternoon.

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.

Figure 7. Full-disk visible image from Japan’s Himiwari-8 satellite, collected at 1100 GMT on August 19, showing twin typhoons Goni (left) and Atsani (right). Image credit: Japan Meteorological Agency, courtesy Tom Niziol, The Weather Channel.

Goni and Atsani roil the Northwest Pacific
Like two ghostly eyes peering out from the sea, the centers of Typhoon Goni and Super Typhoon Atsani are gazing up at satellites monitoring their powerhouse development. Now a Category 3 storm, with peak sustained 1-minute winds of 125 mph as of 1200 GMT Wednesday, Goni is maintaining a steadily westerly course over very warm waters. Models now agree that Goni will sharply recurve this weekend, and the model trend has been for the recurvature to happen before reaching Taiwan—good news for that populated island, still picking up the pieces from Typhoon Soudelor a few days ago. However, the recurvature will put Okinawa and Japan’s southernmost islands more at risk from a weakening Goni. As it recurves, Goni is projected by the Joint Typhoon Warning Center (JTWC) to peak as Category 4 strength, with winds topping out around 145 mph.

Well to the east, Category 5 Atsani was packing sustained winds close to 160 mph at 1200 GMT Wednesday as it continued its northwesterly course. Favorable SSTs and light wind shear should allow Atsani to continue racking up accumulated cyclone energy (ACE). The JTWC keeps Atsani at super-typhoon strength (1-minute sustained winds of at least 150 mph) for at least another day before gradual weakening begins. Atsani is expected to recurve long before reaching Japan.

If Goni intensifies a bit more than predicted, it’s possible we will have two super typhoons at the same time. According to Phil Klotzbach (Colorado State University), this has happened only five times since 1950, most recently in 1997 (the last year we had a strong and intensifying El Niño):

Pamela and Nancy (9/11/1961)
Mary and Lucy (8/17-8/18/1965)
Alice and Cora (9-2-9/3/1966)
Owen and Page (11/27/1990)
Ivan and Joan (10/17-10/19/1997)

We’ll have a post later today on how a subtle but important atmospheric feature called a convectively coupled Kelvin wave (CCKW) may be influencing Tropical Storm Danny. For more on Danny, see today’s update from WU blogger Steve Gregory.

Bob Henson and Jeff Masters


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