(above) A map shows the 5-Day Forecast Cone for Hurricane Iselle courtesy the National Weather Service
HILO, Hawaii – With maximum sustained winds gusting at 115 miles per hour, Iselle maintains its status as a Category Three hurricane on the Saffir-Simpson Scale, making it a major hurricane. The storm is 1,435 miles east of Hilo and heading towards the Hawaiian Islands at 10 miles per hour. It is expected to reach the islands by late Thursday / early Friday, but by then forecasters believe it will have weakened to a Tropical Storm.
The latest discussion (11 a.m. HST) from the National Weather Service explains how Iselle has yet to weaken Sunday:
The cold cloud tops of Iselle’s convection have suddenly begun to expand during the past few hours, suggesting that vertical shear is relaxing. The eye has become a little more distinct, and the hurricane appears to be taking on a more circular, compact shape. Current intensity estimates remain unchanged from earlier this morning, so the initial intensity is held at 100 kt.”National Weather Service, 11 p.m. HST on Sunday August 3, 2014
Meteorologists have been keeping an eye out for characteristics of an annular hurricane, which they said in an earlier update “could delay the weakening during that time.” Here is what the NWS said about that at 11 a.m. HST.
Regarding the compact shape of the hurricane, conventional and microwave images seem to suggest that Iselle is losing its convective banding features. And, for the first time, the Annular Hurricane Index is indicating that Iselle has a marginal annular structure. If it does in fact become annular, then the hurricane is likely to only gradually weaken during the next couple of days in an environment of light easterly shear and slowly decreasing sea surface temperatures. Increasing vertical shear and even colder waters should induce a faster weakening trend after 48 hours, but most of the guidance still suggests that Iselle will maintain tropical storm status as it moves near the Hawaiian Islands on days 4 and 5. The NHC intensity forecast is essentially unchanged from the previous one and lies very close to the LGEM and intensity consensus ICON for the entire forecast period.National Weather Service, 11 p.m. HST on Sunday August 3, 2014
The current track still puts the storm over Hawaii by the end of the week.
The latest center fixes suggest that Iselle jogged west-northwestward earlier today, but the estimated initial motion is now westward, or 280/9 kt. The hurricane is expected to slow down a little during the next 36 hours coincident with a developing break in the subtropical ridge. A mid-level high is then forecast to develop between Hawaii and California by day 3, which should steer Iselle at a faster rate toward the west-northwest through the end of the forecast period. There is very little spread among the track models, but the entire envelope has shifted northward on days 3 through 5. Further shifts could be required if the models continue their recent trends, but at this point Iselle is still a potential threat to the Hawaiian Islands later this week.National Weather Service, 11 p.m. HST on Sunday August 3, 2014
We’ll be following the storm as it develops – and what Hawaii County does in preparation – here on Big Island Video News.