Florence a Cat 3, But Wind Field Expands; Olivia Hits Hawaii

September 12, 2018, 6:04 PM EDT

Above: Eye of Hurricane Florence as seen on Wednesday morning, September 12, 2018, from the International Space Station. Image credit: Alexander Gerst.

Hurricane Florence’s peak winds have fallen to Category 3 strength, but the storm remains a catastrophic rainfall threat and significant storm surge and wind threat to the Southeast U.S. Florence is expected to stall on Friday and move slowly west-southwestward along or just inland of the coast for several days, bringing a catastrophic rainfall well inland, and a destructive storm surge and wind event along a considerable swath of the North and South Carolina coasts.

The Hurricane Hunters and microwave satellite imagery this afternoon found that Florence was not able to recover from this morning’s eyewall replacement cycle (ERC). During this phenomenon, common in intense hurricanes, the eye shrinks to such a small diameter that the eyewall becomes unstable and collapses. The hurricane then creates a new larger-diameter eyewall out of a spiral band. During this process, the peak winds typically fall by 10 – 15 mph, and the central pressure can rise 10 – 15 mb. Florence was never able to rebuild its inner core after this morning’s ERC, though, and data from the Hurricane Hunters and satellites have shown large gaps in the eyewall this afternoon.

As a result, Florence has not been able to concentrate a major portion of its wind energy into an intense eyewall, and the hurricane’s wind energy has spread out over a larger area. This will reduce the potential wind damage from the storm at landfall, but will allow a larger storm surge to build. At 5 pm EDT Wednesday, Florence’s tropical storm-force winds had expanded, and extended out up to 195 miles from the center; hurricane-force winds extended out up to 70 miles. For comparison, at landfall, Hurricane Katrina’s tropical storm-force and hurricane-force winds extended out up to 230 miles and 125 miles from the center, respectively.

Satellite images on Wednesday evening showed that the hurricane had a less impressive unsymmetric appearance, with the eye filled with low-level clouds. The south side of the storm had little upper-level outflow, and there was likely some wind shear affecting the storm there.

Intensity forecast for Florence: Category 3 strength through Thursday, then weakening

Florence’s environment is conducive for intensification. The SHIPS model predicts wind shear will remain low through Thursday afternoon, SSTs will be a warm 29°C (84°F), and ocean heat content will be moderate, near 30 kilojoules per square centimeter. The presence of two upper-level outflow channels will allow Florence to suck in large quantities of moisture-laden air near the surface and hurl it aloft, helping it maintain its intensity. However, given the current structure of the storm, the odds favor at best a slight strengthening over the next day, and more likely, a slow weakening trend.

The 12Z and 18Z Wednesday forecasts by our top three intensity models all predicted that Florence would be a high-end Category 3 storm with 125 mph winds through Thursday afternoon, then weaken to a borderline Category 2/Category 3 storm with 110 - 115 mph winds by the time of its closest approach to the North Carolina coast on Friday morning. This weakening is to be expected, since wind shear is expected to increase to a moderate 10 – 15 knots and the hurricane will be moving slowly over shallower waters with low heat content, which will provide less energy to the storm. Florence will also have a portion of its circulation over land, which will cut down the amount of moisture available to the storm; the increased friction of the winds blowing over the land will also slow down the storm’s winds.

Track forecast: a stall near the North Carolina coast, then west-southwest to west motion

The latest 12Z Wednesday runs of our top 5 track forecasting models were similar to their previous 0Z runs, agreeing that Florence would cruise towards the North Carolina coast through Thursday afternoon, as the clockwise flow around the Bermuda High pushes the storm northwest. But on Thursday night, Florence will put on the brakes and screech to a halt as the steering currents collapse. Given that the steering currents will be very weak Friday and beyond, slight changes in the balance between these forces guiding the hurricane will make a huge difference in where the hurricane makes landfall. This makes the exact landfall location difficult to predict accurately.

The most concerning forecast continued to be from our top model for forecasting hurricanes, the European model, whose 12Z Friday run predicted that Florence’s stall would occur just offshore of the NC/SC border, with the hurricane then traversing the northern half of the coast of South Carolina just offshore, until making landfall near Charleston. This would allow Florence to keep its eye over water, greatly increasing the amount of rain it can generate, and would subject a very long stretch of coast to high winds and a destructive storm surge. Our other top models--the GFS, HWRF, UKMET, and HMON--all predicted Florence would move ashore near the NC/SC border, then turn to the west-southwest over land. On this track, Florence would still keep a large part of its circulation over water and dump extreme rains, but would not bring a devastating storm surge and hurricane-force winds to a long stretch of coast. By Monday, Florence is expected to be headed due west.

Rain, storm surge, and wind

The greatest damage from Florence is likely to come from its rains, which may cause catastrophic flooding along a long swath of its track. The storm’s second greatest threat is storm surge; Florence’s wind field has put a large region of ocean into motion, which is already increasing water levels along the coast of North and South Carolina. As of 6 pm EDT Wednesday, the highest surge along the coast of the Carolinas that had occurred was 1.8 feet at Oyster Landing, SC (see the SURGEDAT storm surge update page for live storm surge values along the path of Florence). Florence’s winds, though likely to be at Category 2 or Category 1 strength at landfall, will be plenty strong enough to cause a billion or more dollars in damage. All told, Florence is likely to be at least a $5 billion storm, and probably much greater.

See our previous post for more details on Florence’s rain and storm surge threat.

Olivia radar
Figure 1. Wundermap depiction of NWS/NEXRAD radar for Hawaii at 11:43 am HST (5:43 pm EDT) Wednesday, September 12, 2018, showing rainbands associated with Tropical Storm Olivia.

Olivia makes landfall—twice—in Hawaii

Not content to make just one historic landfall in Hawaii, Tropical Storm Olivia went for a double. Olivia first made landfall at 9:10 am HST (3:10 pm EDT) Wednesday as a 45-mph tropical storm on the northeast coast of Maui. Radar showed very heavy rains falling along the West Maui Mountains in the flow north of Olivia’s center, where the storm’s strongest winds were concentrated. After zipping across Maui, Olivia and made a second landfall at 9:54 am HST on the northeast coast of Lanai, just northeast of Lanai City. Rainbands and some of Olivia’s stronger winds were pushing toward Molokai, again north of Olivia’s center.

Olivia reached Hawaii as a disorganized, weakening storm, and impacts appeared to be modest as of late morning Wednesday. Officials reported about 2000 power outages across the Upcountry region of central Maui, according to Hawaii News Now. Just before 7 am HST, winds were sustained at 31 mph gusting to 43 mph at the Lanai City Airport, and a gust to 47 mph was reported just before 2 am. At 11:10 am HST, winds were gusting to 39 mph at the Daniel K. Inouye International Airport in Honolulu. The highest 24-hour rainfall reported in Hawaii by CoCoRaHS volunteers as of early Wednesady morning was 3.65”, about 3 miles southeast of Haiku, Maui.

Simply making landfall in Hawaii is no small achievement for a tropical cyclone. Prior to Olivia, only four named systems had made landfall in Hawaii.

--Hurricane Darby made landfall along the southeast shore of Hawaii’s Big Island on July 23, 2016, as a minimal tropical storm (top sustained winds of 40 mph). Damage was minimal and there were no deaths from Darby.
--Tropical Storm Iselle, which, like Darby, made landfall along the southeast shore of the Big Island, arrived as a 60-mph tropical storm on August 8, 2014. Iselle killed one person and did $79 million in damage.
--Hurricane Iniki made landfall in Kauai as a Category 4 hurricane on September 11, 1992, killing 6 and causing $1.8 billion in damage (1992 dollars.)
--Hurricane Dot made landfall on Kauai as a Category 1 hurricane on August 6, 1959, about two weeks before Hawaii gained statehood. Dot caused 6 indirect deaths and $6 million in damage (1959 dollars).

As defined by NHC, landfall requires the center of a tropical cyclone to pass over land. Other hurricanes and tropical storms, most notably Iwa in 1992, have caused significant damage without making landfall. It’s worth noting that four of the six landfalls now on the books for Hawaii (counting the two from Olivia) have occurred in just the last five years! (A 2016 modeling study found that we could expect to see a significant increase in hurricanes near Hawaii in coming decades due to climate change; see also Jeff Masters' August 2014 post, Climate Change May Increase the Number of Hawaiian Hurricanes.)

Bob Henson wrote the Olivia portion of this post.

The Weather Company’s primary journalistic mission is to report on breaking weather news, the environment and the importance of science to our lives. This story does not necessarily represent the position of our parent company, IBM.

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Dr. Jeff Masters

Dr. Jeff Masters co-founded Weather Underground in 1995, and flew with the NOAA Hurricane Hunters from 1986-1990.

jeff.masters@weather.com

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