Weak wind shear and deep moisture preserve the storm's threat of direct impact
POSTED: 01:30 a.m. HST, Jul 29, 2013
LAST UPDATED: 09:57 a.m. HST, Jul 29, 2013
Tropical Storm Flossie will likely be the first cyclone in recorded history to hit Hawaii island head-on, a National Weather Service meteorologist said Sunday.
It also on track to be the first tropical cyclone to make landfall in the islands since Hurricane Iniki ravaged Kauai in 1992.
"We've had some close calls that have come by since Iniki … (but) this is the first time in quite a while that we've seen a direct threat from a land-falling tropical cyclone, in this case a tropical storm," NWS warning coordination meteorologist Michael Cantin said.
Forecasters at first thought Flossie might drop in strength to a tropical depression before it hit the isles, but Cantin said Sunday that a combination of deep moisture feeding into the circulation of the storm and a spreading, or ventilation, of air above it unexpectedly helped it overcome cooler waters and gain steam Saturday night.
Weak wind shear, which usually is responsible for breaking up storms before they hit the isles, also helped keep Flossie on course to hit Hawaii as a tropical storm.
"It kind of goes to show you that we typically on average see that shear out there, but in instances when it's not there we can have things (happen) that we haven't seen in quite a while," Cantin said.
Cantin said remnants of systems have occasionally affected the isles since Iniki, such as Tropical Storm Paul in 2000, but that Hawaii's usual buffer of cooler water and wind shear have kept the state relatively safe for more than 20 years.
"It wasn't even a tropical cyclone anymore, but it provided just enough moisture and then gave us some heavy rains," Cantin said of Paul.
Cantin said Hurricane Felicia in 2009, which at its peak strength was a category 4 hurricane, looked similar to this year's Tropical Storm Flossie as it came from the east but that "as it approached it just fizzled right out off to our east."
He added, "It was a stronger system, came right at us, a shear blew it apart and then it weakened very, very rapidly. We didn't get much of an impact out of it."
Hurricane Flossie in 2007 — the most recent to carry this year's moniker for the letter F — came in from more of a southern track but also fizzled out southeast of Hawaii island, he said.
"As they approached the state, the winds — the strong winds at the top of the system — tipped the circulation over, blew off the top, and then the systems (died) rapidly," Cantin said of the two storms. "We haven't seen that shear with this version of Flossie in 2013, so that's been the big difference, allowing it to hold its strength as it gets closer."
Cantin said it's hard to tell how the rest of the 2013 hurricane season will play out but that forecasters still expect it to yield below-average numbers. The season started June 1 and runs through November.
Hurricane Iniki hit in September, and Hurricane Iwa, which struck 10 years earlier in 1982, hit the isles in November. Both made landfall during El Niño years, when sea surface temperatures are warmer than usual.
The Central Pacific continues to exhibit neutral conditions, which means the sea temperatures are near average — not warmer or cooler than normal.
In May the National Weather Service predicted a 70 percent chance of a below-average season in the Central Pacific, a 25 percent chance of a normal season and just a 5 percent chance of an above-average season, due to the ENSO neutral conditions.
The average season sees about four to five cyclones, but Cantin said this year between one and three tropical cyclones could enter the Central Pacific basin. Flossie is the first.