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Danny Becomes First Atlantic Hurricane of 2015; Invest 93C Moves Toward Hawaii

By: Bob Henson 4:42 PM GMT on August 20, 2015

The little storm that could, Danny, surged from weak tropical-storm status on Wednesday night to become the Atlantic’s first hurricane of the year on Thursday morning. At 11:00 am EDT Thursday, the top sustained winds in Hurricane Danny were estimated at 75 mph, or near minimal hurricane strength. Located in the remote central tropical Atlantic, near 12.5°N and 44.8°W, Danny remains far from land areas, roughly 1100 miles east of the Leeward Islands. Danny is moving west-northwest at about 12 mph, a fairly modest pace for tropical cyclones in this region.

Danny was largely stripped of its core convection (showers and thunderstorms) on Wednesday, as dry air filtered into its center, but the storm retained its overall structure and was able to rebuild central convection on Wednesday night, even managing to produce an eyewall formation and a visible eye over the last few hours. Danny’s quick intensification was facilitated by its small size (see Figure 3 below). Hurricane-force winds only spanned a region 20 miles in diameter on Thursday morning, with tropical-storm-force winds extending out up to 60 miles from the center. Smaller tropical cyclones are able to both intensify and weaken more rapidly, which makes intensity prediction especially challenging. That said, Danny’s rise to hurricane status was well forecast several days in advance by the recently upgraded HWRF model, and since Tuesday by the GFDL model. The leading statistics-based models, which are the most accurate guidance for intensity beyond about three days, also gave a solid heads-up that Danny could attain hurricane status by today.

Figure 1. A 1-km-resolution visible image of Tropical Storm Danny, collected by NOAA’s GOES-East floater satellite at 1315 GMT (11:15 am EDT) on Thursday, August 20. Image credit: University of Wisconsin/CIMSS.

Figure 2. Hurricane Danny as viewed from the International Space Station and tweeted on Thursday morning, August 20, by astronaut Scott Kelly. Image credit: NASA.

Figure 3. A side-by-side comparison of visible satellite imagery from Thursday, August 20, for Super Typhoon Atsani in the Pacific (left) and Hurricane Danny in the Atlantic (right), both at the same scale, shows how much tropical cyclones can vary in size. Even smaller systems like Danny can be devastating if they grow intense enough and strike a highly populated area. Composite image credit: Mark Lander, University of Guam.

The outlook for Danny
Despite the overnight growth spurt, Danny’s future as a hurricane remains highly uncertain. One of the two main obstacles it faces is a huge zone of relatively dry, dusty air that extends across most of the Atlantic around 20°N latitude (see Figure 4). Any large-scale intrusion of this air into Danny’s circulation would tamp down the instability that fuels convection. Thus far, Danny has been able to generate and consolidate enough shower and thunderstorm activity to fight off injections of dry air--another benefit of its small size. As it slowly gains latitude, Danny will be at increasing risk of falling victim to this zone of dry air, provided it does not shift to the northeast in tandem with Danny’s northwestward motion.

Figure 4. Hurricane Danny (small white area near the center of the Atlantic tropics) is tucked just south of a large belt of dry, dusty air, generated in part by the Saharan air layer (SAL) moving west from Africa. Danny’s projected west-northwest motion will bring it closer to this dry air mass. Image credit: University of Wisconsin/CIMSS and NOAA Hurricane Research Division.

Danny’s other nemesis is vertical wind shear, a perennial feature in the North Atlantic tropics during El Niño years. The widespread rising of warm air over the eastern tropical Pacific during El Niño helps foster unusually strong west winds at upper levels, pushing away from the Niño region into the Caribbean and western Atlantic. Such shear can easily disrupt the chimney-like vertical structure that helps to maintain a tropical cyclone’s strength. Currently, wind shear over the Caribbean is below the record values observed earlier this summer, but values of 30 - 60 mph remain widespread. (Anything above 20 knots, or about 23 mph, is problematic for tropical cyclone development.) For now, shear over Danny remains low, in part because its potent convection has produced a small zone of high pressure overhead. This weekend, Danny will be hard-pressed to avoid encountering a belt of higher shear, which would not only jeopardize its structure but also help push dry, dusty air into its circulation. If Danny survives that passage as a well-structured tropical cyclone, then it may encounter somewhat lower shear values ahead of its path early next week (see Figure 5), as a weak but persistent upper-level low is predicted to move northward from the Bahamas and a ridge builds in to replace it.

Figure 5. Predicted vertical wind shear between the 200 mb and 850 mb altitudes at at 0600 GMT on Tuesday, August 24, near the end of the five-day outlook shown below in Figure 6. Shear values above 20 knots (23 mph) are generally destructive to tropical cyclone circulation. Image credit: Levi Cowan, tropicaltidbits.com.

Figure 6. NHC’s outlook for Danny as of 11:00 am EDT Thursday.

Computer forecast models remain in general agreement on Danny’s path over the next five days, a west-northwest trek that would bring it close to the northernmost Leeward Islands, and perhaps eventually Puerto Rico, by early next week. NHC predicts that Danny will maintain Category 1 hurricane strength throughout the next four days, then weaken to strong tropical-storm levels by Day 5 (Tuesday, August 24). Assuming that Danny follows the projected track, its intensity would be modulated not only by the factors above but also by any interaction with islands. Danny should gradually grow in size, but if it remains a compact cyclone, any interaction with mountainous islands could have an unusually large detrimental effect on its intensity. Danny’s intensity forecast is more uncertain than usual because of its very small size: day-to-day conditions could produce dramatic spikes in strength, both upward and downward. Even so, the statistical and dynamical model guidance suggests that Danny is unlikely to strengthen beyond Category 1 levels over the next five days.

Figure 7. This infrared image from the GOES-East satellite, from 1445 GMT (10:45 am EDT) shows Invest 97L (top center) dwarfing Hurricane Danny (bottom center) in size, if not in organization. Image credit: NOAA/NHC.

Another disturbance in the North Atlantic
A disorganized cluster of convection--actually much larger than the entire circulation of Danny--has been designated Invest 97L. The storminess is associated with the upper-level low mentioned above, intersecting with a weak surface front. Sea-surface temperatures near 97L are 28-29°C (82-84°C), about 1-2°C above average, which is more than adequate for tropical development. Statistical guidance indicates that 97L could evolve into a subtropical or tropical storm as it drifts northward, but neither of the most reliable dynamical models for short-term intensity trends--the HWRF and GFDL--developed 97L in their 0000Z Thursday model runs. NHC gives 97L a 20% chance of development in the next two days and a 60% chance in the next five days.

Hawaiians need to keep a close watch on Invest 93C
Dynamical models are increasingly bullish on the odds that Invest 93C could become a historical rarity: a hurricane threatening the Hawaiian Islands. Now located about 900 miles south-southeast of Hilo, 93C is slowly organizing and has a large area of showers and thunderstorms associated with it. The Central Pacific Hurricane Center gives Invest 93C an 80% chance of development over the next 48 hours. The 0000Z and 0600Z Thursday runs of the GFDL model both strengthen 93C into a major hurricane by Tuesday, when the system could be within striking distance of the islands. While less extreme than the GFDL forecasts, the 0600Z Thursday run of the HWRF model also projected a solid Category 1 hurricane approaching the islands by Tuesday, and long-range statistical intensity models (the more reliable guidance beyond Day 3) agree that 93C has a good chance of reaching hurricane strength. Track models agree that 93C, now moving slowly west-northwest, will soon begin moving more briskly along a northwestward track that should continue through the weekend, putting it southwest of the islands by Sunday. The major uncertainty is how soon 93C will recurve toward the northeast, which in turn will determine whether 93C’s track intersects the northwest-southeast chain of Hawaii’s islands. As we noted yesterday, northward-moving hurricanes are very unusual near Hawaii, but 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.

Further west, the Central Pacific is also playing host to Invest 94C, far at sea, which shows no immediate signs of development.

Figure 8. Forecasts from various track models at 1200 GMT Thursday, August 20, bring Invest 93C northwestward over the next several days, putting it in position for a potential run at Hawaii early next week. These track forecasts are subject to large change over the next several days and should not be used as official guidance. A list of models and definitions can be found at the NHC website.

Figure 9. Typhoon Goni (left) and Super Typhoon Atsani (right) are unmissable in this enhanced infrared image from Japan’s Himiwari-8 satellite, collected at 1550Z (11:50 am EDT) on Thursday, August 20. Image credit: Japan Meteorological Agency and CIRA/RAMMB.

Atsani, Goni continue to rage across Northwest Pacific
Long-lived Super Typhoon Atsani and Typhoon Goni continue to make waves far out to sea in the Northwest Pacific. Atsani’s peak sustained 1-minute winds are 150 mph, down slightly from a peak yesterday of 160 mph, and a slow weakening trend should begin soon as Atsani starts to recurve and encounters cooler waters. Atsani could threaten southern Japan as a relatively weak typhoon early next week. Meanwhile, Goni is uncomfortably close to the northern Philippines and Taiwan, but models insist it will sharply recurve over the next several days, perhaps moving near Okinawa at Category 3 strength and approaching Kyushu, the southernmost of Japan’s large islands, early next week. Goni’s top winds are holding steady at 135 mph, making it a Category 4 storm, and it could briefly grow stronger over deep warm water before weakening as it approaches Japan.

Jeff Masters and I will have an update on the planet’s sizzling July later today. The next tropical update will be Friday morning at the latest.

Bob Henson


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