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Tuesday, August 5, 2014

Rare annular Hurricane Iselle headed towards Hawaii; Bertha becomes a hurricane

by Jeff Masters, PhD, wunderblog, wunderground, August 4, 2014
Hurricane Iselle continued to intensify overnight, reaching Category 4 strength with 140 mph winds at 11 a.m. EDT on Monday. Iselle is likely at peak intensity, since ocean temperatures beneath the storm are now 26 °C, which is marginal for maintaining a hurricane. Interestingly, plots of Maximum Potential Intensity from the Center for Ocean-Land-Atmosphere Studies show that the Iselle should only be able to maintain Category 2 strength with these ocean temperatures and the current atmospheric background conditions, so the storm is definitely over-achieving. Iselle is headed westwards at 10 mph towards Hawaii, and could affect the Hawaiian Islands as a tropical storm by Thursday night. Satellite images show an impressive storm with a large eye, good symmetry, and plenty of upper-level outflow. The relative lack of spiral bands and large, thick eyewall qualify Iselle to be a rare breed of hurricanes known as "annular." Annular hurricanes are a subset of intense tropical cyclones that are significantly stronger, maintain their peak intensities longer, and weaken more slowly than average tropical cyclones. The latest SHIPS model output indicates that Iselle has passed the initial screening step to be considered an annular hurricane, and the model's "Annular Hurricane Index" shows a high level of annularity for the hurricane. Only 4% of all hurricanes are annular hurricanes. The most recent annular hurricane in the Eastern Pacific that I am aware of was Category 4 Hurricane Kenneth of November 2011.

Figure 1. True-color MODIS image of Hurricane Iselle from approximately 6 p.m. EDT August 3, 2014. At the time, Iselle was a Category 3 hurricane with 115 mph winds. Iselle was showing an annular structure -- a lack of spiral bands and large, thick eyewall. Image credit: NASA.

Figure 2. Category 4 Hurricane Kenneth of November 22, 2011 -- the most recent annular hurricane to appear in the Eastern Pacific.

Forecast for Iselle
Wind shear is expected to stay light to moderate for the next four days, and ocean temperatures will remain near 26 °C. However, the atmosphere surrounding Iselle will begin to dry considerably beginning on Tuesday, which should induce a steady weakening trend Tuesday through Thursday. By the time Iselle reaches the Hawaiian Islands on Thursday night, rapid weakening may be occurring, but Iselle could still be a strong tropical storm, capable of generating dangerous heavy rains. Hurricanes approaching from the east typically fall apart before they reach Hawaii, though, due to the cool waters and dry air that lie to the east of the islands. It is hurricanes approaching from the south that 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. 

The NOAA Hurricane Hunters' jet is scheduled to fly a dropsonde mission on Tuesday evening out of Honolulu, and an Air Force hurricane hunter aircraft is scheduled to fly a low-level mission into the hurricane early Wednesday morning. 

After Iselle comes Julio
After Iselle finishes its close encounter with the Hawaiian Islands late this week, the islands need be concerned with yet another tropical cyclone: Tropical Storm Julio, which formed in the Eastern Pacific south of Baja, Mexico, this morning. Satellite loops show that Julio is headed westwards towards Hawaii on a path very similar to Iselle's, and the storm should be able to take advantage of moderate wind shear and warm ocean temperatures to become a hurricane by Tuesday. Long range forecasts from the GFS and European models have been consistently predicting that Julio will pass very close to Hawaii on Sunday night and be stronger than Iselle. It's been a very active hurricane season in the Eastern Pacific, which has seen 10 named storms, 4 hurricanes, and 3 intense hurricanes so far in 2014. On average, we expect to see 6 named storms, 3 hurricanes, and 1 intense hurricane by August 4 in the Eastern Pacific.

Figure 3. Latest satellite image of Bertha.

Bertha a hurricane
In the Atlantic, Hurricane Bertha took advantage of decreasing wind shear and a moister atmosphere to intensify into a Category 1 hurricane with 80 mph winds as of 11 a.m. EDT Monday. Visible satellite loops on Monday morning showed a Central Dense Overcast (CDO) of high cirrus clouds over Berth's core, which is a typical feature of intensifying tropical storms about to reach hurricane strength. However, Bertha's satellite presentation was probably the lamest I've even seen for a hurricane, with only a small, misshapen area of heavy thunderstorms, and little in the way of spiral bands. Bertha is headed northwards, and will pass midway between the U.S. East Coast and Bermuda. After a short stint as a hurricane later today and on Tuesday, high wind shear and very cool waters of 20 °C will convert Bertha into a powerful extratropical storm on Wednesday, halting the intensification process. Bertha will not be a threat to any more land areas, though its remnants could bring some heavy rain showers and tropical storm-force winds gusts to Southeast Newfoundland on Thursday. Along with Hurricane Arthur, Hurricane Bertha gives us two Atlantic hurricanes so far this year, matching the total number of hurricanes during the entire 2013 Atlantic hurricane season. The second (and final) hurricane of the 2013 season (Ingrid) did not arrive until September 14. On average, the second hurricane of the Atlantic season arrives on August 28. The last time the first two named storms in the Atlantic became hurricanes was in 1983, when Alicia, Barry and Chantal all became hurricanes (kudos to TWC's Stu Ostro for this stat).

Weakening Typhoon Halong headed towards Japan
In the Western Pacific, Typhoon Halong, formerly a mighty Category 5 super typhoon with 160 mph winds, has weakened significantly to a Category 3 storm with 120 mph winds on Monday morning. Satellite loops show that Halong's eye is no longer distinct and the eyewall has collapsed, but the typhoon still has a large area of very intense eyewall thunderstorms. Halong is expected to affect Southern Japan as a Category 1 typhoon late this week.

Figure 4. The NOAA P-3 Orion hurricane hunter aircraft, N42RF and N43RF. Image credit: Alan Goldstein/Terry Schricker.

A dangerous flight through Hurricane Hugo, remembered 25 years later on The Weather Channel [Readers, I have read Jeff's story, and it is well worth the read!]
Twenty five years ago, on September 15, 1989, the fifteen members of the crew of NOAA hurricane hunter aircraft N42RF very nearly became Hurricane Hugo's first victims. Expecting to encounter a powerful yet manageable Category 3 hurricane east of the Lesser Antilles Islands, the plane instead hit extreme turbulence in an intensifying Category 5 storm, and very nearly did not make it out. I was the Flight Meteorologist on that mission, and photographed the wild events of that unforgettable flight. My story of that remarkable flight into Hurricane Hugo is a must-read for all who follow these great storms. On Monday, August 4, 2014, The Weather Channel will be showing a 3-minute piece on that flight, which will be airing at 3:40 p.m., 4:40 p.m., and times later in the day. I flew to Tampa in June to help film the piece, which interviews myself and two members of that mission who still work for the NOAA Hurricane Hunters -- Hurricane Project Manager Jim McFadden, and the Science and Engineering Division chief Alan Goldstein. The piece will also play again on the actual 25th anniversary of the flight, on September 15, 2014.

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