Olivia and Mangkhut Set Their Sights on Hawaii and Guam

It’s possible that Tuesday will see a very unlucky alignment of the stars: typhoon warnings up for Guam at the same time that hurricane warning flags are flying in Hawaii. This rare potential juxtaposition is the result of Tropical Storm Mangkhut, which is expected to be a formadible typhoon moving across or near Guam on Tuesday, and Hurricane Olivia, which is weakening but on track to reach the Hawaiian Islands on Wednesday.

Typhoons are a relatively frequent visitor to Guam, but hurricanes seldom hit Hawaii. Some close passes have been quite damaging—as in 1982’s Hurricane Iwa, whose right-hand eyewall passed over Kauai—but only two hurricanes are known to have made landfall on the Hawaiian Islands, both of them in Kauai:  Dot (1959) and Iniki (1992). 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.

Outlook for Olivia

As of 5 pm EDT Friday, Olivia was still a long way from Hawaii: about 1550 miles east of Honolulu, according to the National Hurricane Center. Olivia’s top sustained winds were down to 110 mph, making it a Category 2 storm. Classified as a Category 4 storm as recently as Thursday night, Olivia has seen its strength dented by less-than-optimal sea surface temperatures (SSTs) of around 25 – 26°C (77 – 79°F) and a large swath of dry air surrounding the hurricane (mid-level relative humidities around 40%).

The ace up Olivia’s sleeve is very low wind shear. The shear is predicted by the 12Z Friday run of the SHIPS model to stay around 5 knots through at least Monday. If shear were higher, it would be much easier for dry air to be injected into the hurricane’s circulation. With the shear so light, though, it will give Olivia a better chance of maintaining a protective pocket of higher humidity in its central core of showers and thunderstorms (convection).

Figure 1. Wind shear around Olivia was quite low (less than 5 knots) on Friday, September 7, 2018. As the hurricane moves closer to Hawaii (west side of image), it will eventually encounter stronger wind shear. Image credit: UW/SSEC/CIMMS.

Satellite images show that Olivia is an annular hurricane—one that consists of a large eye surrounded by a tire-like core of convection, without the spiral bands of a more classically structured hurricane. Annular hurricanes tend to be more resistant to weakening, and this trait may help Olivia remain stronger on approach to Hawaii than a different hurricane might be. Also working in Olivia’s favor are unusually warm SSTs between the hurricane and Hawaii, about 0.5 – 1°C (1 – 2°F) above average. As Olivia moves toward the islands, the water temperature along its path will gradually climb by Monday to near the benchmark 26°C (79°F) temperature typically needed to sustain a hurricane. SSTs along Olivia’s path will be approaching 27°C (81°F) by Wednesday. However, wind shear will be increasing by this point, perhaps pushing dry air into the storm.

Computer models agree that Olivia will carry out a westward track that gradually bends toward the west-southwest, as a very strong zone of high pressure builds to the north of Hawaii and cuts off the typical “escape route” for hurricanes approaching the islands from this direction. The NHC forecast as of 5 pm EDT Friday calls for Olivia to become a tropical storm with 60 mph winds by the time it reaches the islands on Wednesday.

It’s too soon to know which of Hawaii’s island(s) are most at risk, as a slight deviation north or south in Olivia’s track could make a big difference and could even send the hurricane north or south of Hawaii entirely. There is no record of any hurricane or tropical storm reaching Hawaii from the east-northeast, so we have no past experience to draw on. Given the rarity of the situation, it behooves all residents of Hawaii to carry out any long-range hurricane preparations this weekend.

Figure 2. Enhanced infrared satellite image of Mangkhut as of 1930Z (3:30 pm EDT) Friday, September 7, 2018. Image credit: UW/SSEC/CIMSS.

Mangkhut a serious threat for Guam and East Asia

Though it was just a minimum-strength tropical storm with 40 mph winds on Friday, Mangkhut promises to become a ferocious typhoon over the next few days. The system is surrounded by a large envelope of very moist air (mid-level relative humidity around 80%), and it will be traveling over very warm SSTs of 29 - 30°C (84 - 86°F). Moreover, oceanic heat content will be very high along Mangkhut’s path for the next several days (see Figure 3). This will give a marked boost to Mangkhut, helping it to strengthen quite rapidly. Wind shear will be light to moderate, in the range of 10 – 15 knots.

Figure 3. Oceanic heat content below the path of Mangkhut will be close to 100 kilojoules per square centimeter, a very high value supportive of rapid strengthening. Image credit: UW/SSEC/CIMSS.

As of 21Z Friday, the Joint Typhoon Warning Center predicts that Manghut will rocket to typhoon strength by Sunday local time and will be a Category 4 typhoon by the time it nears Guam on Tuesday. There is still time for the track to angle just north or south of the island, though, so a direct hit is not a certainty. Mangkhut may intensify even more rapidly before it reaches Guam, noted the JTWC in a forecast discussion Friday morning.

Longer-range models give every indication that Mangkhut could become a Category 5 super typhoon on its way toward East Asia. Residents of Taiwan and the east-central China coast need to pay especially close attention to Mangkhut, as steering currents are predicted to keep the typhoon heading steadily in that direction until a potential landfall in about 8 or 9 days.

We'll have a new post on Florence Saturday morning.

Jeff Masters contributed to this post.