The Hurricane Hunters took the first in-storm measurements of Hurricane Iselle during a Tuesday night flight through its eyewall. The team found stronger wind speeds than expected and a wind field on the north side of the system that was larger than anticipated.
Partly in response to the findings, the National Weather Service upgraded a tropical storm warning to a hurricane warning for the Big Island on Wednesday morning.
The real-time data from the flights 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.
Hurricane Iselle was registering 123 mph winds at 10,000 feet and 98 to 101 mph at the sea surface Tuesday night when the team of Air Force reservists measured the storm.
“Forecasters had been thinking speeds were 10 to 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 wind field on the north of the system 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 and barometric pressure from 10,000 feet down to sea level, and instruments mounted on the nose of a WC-130J plane took readings 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 had 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.”
Navy personnel Wednesday were slated to drop buoys to measure sea temperatures in the wake of Iselle. As a hurricane passes, its winds tend to shove warmer surface water to the side, allowing colder water to circulate to the top. The measurements should shed light on how sea surface temperatures affect Hurricane Julio, which is following in Iselle’s tracks, said Master Sgt. Brian Lamar, a spokesman for the squadron.
“Does it take the fuel away? Does it not bother it?” will be among the questions the research tries to answer, Lamar said.
The Hurricane Hunters will continue to fly through the hurricane every six hours using a three-plane rotation. Hurricane Julio is a Category 1 following on the heels 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.”