The Hurricane Hunters supplied the first in-storm measurements of Hurricane Iselle following a Tuesday night flight through the eyewall. The team found stronger wind speeds than expected and an airfield on north side of the system that was larger than anticipated.
Wednesday morning, the National Weather Service upgraded the Big Island’s status to a hurricane warning, with an urgent 10:55 a.m. bulletin saying Iselle was continuing its forward march and had not weakened. The hurricane was 615 miles east-southeast of Hilo and was moving west-northwest at 18 mph with sustained winds near 90 mph.
The hurricane was registering 123 mph winds at 10,000 feet and 98 to 101 mph at the sea surface Tuesday night when the Hurricane Hunters took their measurements.
The findings add a new dynamic to forecasts, which up to then had been based on satellite imagery and computer models to determine hurricane strength, said Maj. Jon Brady, weather officer with the 53rd Weather Reconnaissance Squadron, based out of Biloxi, Mississippi.
“Forecasters had been thinking speeds were 10-15 miles less than we found and that they didn’t go out quite so far,” said Brady.
Given that the forecast track takes the system directly over Hawaii Island and through waters just south of the other islands, the larger-than-expected airfield on the north could deliver impacts from Maui to Kauai that hadn’t been expected earlier, Brady said in a phone interview with West Hawaii Today on Wednesday morning.
The Hurricane Hunters used dropsondes to measure wind speeds from 10,000 feet down to sea level, and instruments mounted on the nose of a WC-130J plane took measurements at flight level. A wing-mounted radiometer capable of peering through clouds scanned the wave action on the ocean to gauge surface wind speeds.
A main objective of the mission was to pinpoint the center of the storm to reduce the uncertainty in the track model.
Iselle’s annular characteristics have made it resistant to weakening, Brady said. A thick eyewall tends to keep out the drier air that has been expected to weaken the system.
“Only 1 percent of hurricanes are annular,” Brady said. “They look like a big, round truck tire. They have a huge eye. They are resistant to dry air once they form, and they are resistant to shear, both of which we have in Hawaii. I think that’s what has helped Iselle get this far….That’s why we think Iselle will continue to weaken only slowly.”
The Hunters will stick around and continue to fly the hurricane every six hours using a three-plane rotation. Hurricane Julio is a Category 1 following on the heals of Iselle, and it’s next in line for the flights.
“We will continue flights until Iselle is no longer a threat,” Brady said. “At some point, we’re also going to have to start dealing with Julio. We’re going to be really busy with two storms on our hands. This is very rare.”