Fearsome Five: Florence, Helene, Isaac, Olivia, and Mangkhut

All by itself, Tropical Storm Florence is plenty to contend with, but there is much more going on during this turbo-charged peak of the Northern Hemisphere hurricane season. See our Saturday morning post for full details on Florence, which is intensifying in the Atlantic and poised to reach the U.S. East Coast as a major hurricane around Thursday. We will have another complete update on Florence on Sunday.

The Atlantic and Pacific are ginning up at least four other systems that are already—or soon predicted to be—at hurricane strength. Each of these is likely to affect land, with an unusual cluster of simultaneous U.S.-affiliated targets. It’s possible that by Tuesday, we will see:

            •  Guam under a typhoon warning
            •  Hawaii under a hurricane or tropical storm warning
            •  parts of the U.S. East Coast under a hurricane watch

Here’s a look at each of the non-Florence threats setting up for the next week.

Figure 1. Visible-wavelength image of Tropical Storm Helene at 1605Z (12:05 pm EDT) Saturday, September 8, 2018. The southern Cabo Verde Islands are outlined at top left. Image credit: tropicaltidbits.com.

Helene strengthens en route to a pass near the Cabo Verde islands

A Hurricane Watch and Tropical Storm Warning were in effect on Saturday afternoon for the three southernmost Cabo Verde islands: Santiago, Fogo, and Brava. As of 5 pm EDT Saturday, Tropical Storm Helene was located about 155 miles southeast of the island of Praia, heading west at 13 mph with top sustained winds of 60 mph.

Helene is expected to make a gentle turn toward the west-northwest that will likely bring it within 100 miles of Brava and Fogo on Sunday morning local time. Tropical storm winds extend out to about 60 miles north of Helene, so minimal tropical-storm-force winds and some squally rounds of heavy rain are the most likely outcome on Brava and Fogo. As it traverses seasonally warm water, with sea-surface temperatures (SSTs) around 27°C (81°F), Helene will be in a low-wind-shear environment (5 – 10 knots) for the next 2-3 days, so at least some intensification is likely. The 18Z run of the SHIPS model gives a 34% chance of gaining 25 knots of strength in 24 hours, which would make Helene a Category 1 hurricane, and there is some chance it could reach Cat 2 strength before shear increases on Tuesday. (Only three hurricanes on record have attained Category 3 strength east of 40°W.) Models agree that Helene will then be arcing northwest on a recurvature through the Central Atlantic, far away from any land areas.

Figure 2. Enhanced infrared-wavelength image of Tropical Storm Isaac at 2130Z (5:30 pm EDT) Saturday, September 8, 2018. Image credit: NASA/MSFC Earth Science Branch.

Isaac likely to reach the Caribbean by Thursday

Tropical Storm Isaac is the newest member of the Atlantic pack. Christened at 5 pm EDT Saturday with top sustained winds of 40 mph, Isaac was located west of Helene, about 1640 miles east of the Windward Islands, moving west at 7 mph.

Isaac will need to traverse the length of the Atlantic tropics before it poses any threat to land. Recurvature is still possible, as indicated consistently by the UKMET and Canadian models, but the bulk of model guidance keeps Isaac heading very steadily westward on a path that would reach the Lesser Antilles around Thursday (as depicted in the NHC forecast). Conditions will favor strengthening over the next several days: wind shear will be low (5 – 10 knots) and SSTs around 27°C (81°F). Later in the week, Isaac will encounter even warmer water (around 28°C) with greater oceanic heat content, but wind shear will also be increasing--perhaps due, in part, to the outflow from Hurricane Florence. The HWRF model, our top performer on intensity in the 2017 season, makes Isaac a solid Category 3 storm by Wednesday: this is considerably above other guidance but is certainly possible.

It’s too soon to have a good handle on Isaac’s fate beyond the Lesser Antilles should it get that far. The GFS and European models suggest that steering currents would keep Isaac churning west toward the central Caribbean, with extremely warm water at hand but periods of strong wind shear possible. We may be dealing with Isaac for some time to come.

Figure 3. Natural-color visible-wavelength image of Hurricane Olivia at 2035Z (4:35 pm EDT) Saturday, September 8, 2018. Image credit: tropicaltidbits.com.

Olivia on track to reach Hawaii by Wednesday—but which island(s)?

Hurricane Olivia remains on a course that will likely bring it across Hawaii, and make weather history, on Wednesday. The islands have seen just two hurricane and two tropical storms make landfall in modern records, although several others have passed close enough for major impacts. Landfall is defined by the National Hurricane Center as the center of a tropical cyclone passing over land. See our Thursday post for more on Hawaii’s hurricane history.

As of 5 pm EDT Wednesday, Olivia was about 1190 miles east of Honolulu, heading west at 15 mph. Top sustained winds were holding steady at around 85 mph, as a weakening trend has halted for the time being. Olivia’s structure has degraded markedly and showers and thunderstorms (convection) have greatly diminished since the hurricane peaked at Category 4 strength on Thursday. Convection was trying to rebuild on Saturday afternoon, though, and it’s possible Olivia will remain a Category 1 hurricane for the next couple of days. Wind shear—currently very low, around 5 knots—will increase dramatically on Tuesday, which will probably kick in a new round of weakening.

There is fairly high confidence that a strong upper-level high will shunt Olivia toward Hawaii from the east-northeast, an unprecedented track for any tropical storm or hurricane known to reach the islands. This angle of approach means that just a small shift in direction could make a big difference in which islands are affected, most likely on Wednesday. Track models at 12Z Saturday morning varied from landfall on the Big Island (GFS) to Maui (HWRF) to Oahu (European, Canadian, and HMON). The models agree strongly that Olivia will most likely be a tropical storm rather than a hurricane when it reaches the islands. Very heavy localized rainfall is a good bet if Olivia passes over or very near any of the islands; top amounts should be considerably lower than during slow-moving Tropical Storm Lane several weeks ago, which is being blamed for at least $200 million in flood damage, according to insurance broker Aon Benfield.

Figure 4. Enhanced infrared image of Typhoon Mangkhut at 2050Z (4:50 pm EDT) Saturday, September 8, 2018. Image credit: RAMMB/CSU/CIRA.

Time for Marianas to prepare for Mangkhut

Fast-growing Tropical Storm Mangkhut will be a fierce typhoon throughout most of the next week in the Northwest Pacific. Mangkhut’s top sustained winds were up to 65 mph at 5 pm EDT Saturday according to the Joint Typhoon Warning Center. The JTWC projects Mangkhut to become a Category 3 equivalent storm by Tuesday local time and a Category 4-strength super typhoon just a day later. With exceptionally favorable conditions for strengthening at hand, and strong model agreement on steering currents, Mangkhut could end up making a devastating strike on Taiwan and/or the east coast of China around next weekend.

Before then, residents of the Mariana Islands, including Guam, need to prepare for Mangkhut’s arrival on Monday night local time (Monday morning EDT). A Typhoon Watch is in effect for Guam, Rota, Tinian, and Saipan. Mangkhut is moving quickly westward at about 23 mph amid strong steering currents. The latest JTWC forecast brings Mangkhut between the islands of Rota and Tinian, only about 50 miles north of Guam. A direct hit on Guam remains possible, and impacts on the Marianas will be widespread in any event. Mangkhut will likely be a Category 3 or 4 equivalent when it arrives, and hurricane-force winds may span 100 miles, with gales spanning 300 miles. Seas could reach 40 feet near Mangkhut’s center.