Hurricane Iselle continued to intensify overnight, reaching Category 4 strength with 140 mph winds at 11 am 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 pm EDT August 3, 2014. At the time, Iselle was a Category 3 hurricane with 115 mph winds. Iselle was showing an annual 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.