Ernesto Goes Tropical; Lane Gathers Strength

With little time to spare, Subtropical Storm Ernesto became Tropical Storm Ernesto on Thursday afternoon as it headed toward the chilly waters of the far North Atlantic. As of 3 PM EDT, the newly reclassified Ernesto was located midway between far eastern Canada and the Azores, or about 650 miles east-southeast of Cape Race, Newfoundland. The broad storm was heading northeast at 18 mph across the open North Atlantic.

Christened as a subtropical storm on Wednesday, Ernesto developed a more symmetric field of convection (showers and thunderstorms) and more pronounced banding near its center on Thursday, which prompted the NOAA/NWS National Hurricane Center (NHC) to reclassify the system. Ernesto remained a large but relatively weak system on Thursday afternoon: its top sustained winds were up to only 45 mph, though winds of 40 mph extended out as far as 160 miles to the east and southeast. Ernesto was located less than 200 miles from the track of last week’s short-lived Tropical Storm Debby, another relatively weak cyclone that was first named as a subtropical storm.

Ernesto is already traveling over waters of around 25°C (77°F), which is just below the traditional 26°C threshold for tropical development. The storm will encounter progressively cooler sea-surface temperatures as it continues northeast, and Ernesto will become a post-tropical cyclone as soon as Friday morning as it begins to merge with a frontal zone. It could remain a potent gale-producer, though, as it accelerates toward the east-northeast. Ernesto’s remnants will sweep across Ireland and Great Britain over the weekend, bringing some heavy rain and gusty winds.

Figure 1. Infrared image of Invest 99L at 1915Z (3:15 pm EDT) Thursday, August 16, 2018. Image credit: NASA/MSFC Earth Science Branch.

A new disturbance heads for Lesser Antilles

A tropical wave in the central tropical Atlantic, dubbed Invest 99L and located about 750 miles east of the Lesser Antilles Islands at 8 am EDT Thursday morning, bears watching as it heads west-northwest at 15 - 20 mph. Sea surface temperatures (SSTs) in this region were near 28.5°C (83°F), and wind shear was a moderate 10 – 20 knots, conditions favorable for development. There is a large area of very dry air, associated with the Saharan Air Layer (SAL), just to the north of the wave, and this dry air will retard development. Satellite images Thursday morning showed that 99L had a moderate amount of spin, but heavy thunderstorm activity was sparse and disorganized.

The 18Z Thursday run of the SHIPS model predicted that 99L would be moving into a steadily drier environment, with the relative humidity at mid-levels of the atmosphere falling from 65% on Thursday to around 50% by late Saturday. The 0Z Thursday runs of our top three models for predicting tropical cyclone genesis, the European, GFS, and UKMET models, did not develop 99L. About 30% of the 50 members of the 0Z Thursday European model ensemble and 5% of the 20 members of the 0Z Thursday GFS model ensemble supported development, but all of these forecasts showed the system dying by Sunday in the eastern Caribbean—a region nicknamed the “hurricane graveyard” for its well-documented effects on incipient systems, especially early in the season. In its 2 pm EDT Tropical Weather Outlook, the National Hurricane Center gave 2-day and 5-day odds of development of 20% and 20%, respectively, which implies there is very little chance of 99L becoming a depression after Day 2 (Saturday).

Figure 2. Enhanced infrared image of Tropical Storm Lane at 1900Z (3 pm EDT) Thursday, August 16, 2018. Image credit: RAMMB/CSU/CIRA.

Lane continues to intensify in East Pacific

Tropical Storm Lane was approaching hurricane strength on Thursday in the remote Northeast Pacific, about 1500 miles west-southwest of Cabo San Lucas, Mexico, and about 1900 miles east-southeast of Hawaii’s Big Island. At 3 pm EDT Thursday, Lane’s top sustained winds were up to 70 mph.

Lane's convection blossomed through the day on Thursday, with hints of an eye trying to form. Satellite imagery showed ample outflow at upper levels helping to ventilate the storm. Lane is enveloped in a broad field of rich moisture (mid-level relative humidity around 70%) that will nurture development. Wind shear is expected to remain light to moderate (around 10 knots) for the next several days, and SSTs along Lane’s path will be quite warm, around 28°C (82°F).

Overall, Lane has plenty of potential to grow into a robust hurricane, as indicated by both statistical and dynamical models. The NHC forecast as of Thursday morning brings Lane to hurricane strength on Thursday night and to Category 3 strength by Saturday. Lane has a distinct chance of undergoing a cycle of rapid intensification, a tough-to-predict process that could make Lane a major hurricane even sooner than predicted. The 12Z Thursday run of the SHIPS model gives Lane a 21% chance of gaining 55 knots of strength within 48 hours.

Just as Ernesto is echoing Debby’s behavior in the North Atlantic, Lane appears to be following in the footsteps of Hurricane Hector from a few days ago. Hector bolted to Category 4 strength and passed just south of Hawaii’s Big Island last week, finally dissipating in the Northwest Pacific. Lane is predicted to angle slightly west-northwest over the weekend, in the general direction of Hawaii, as a deep upper low develops in the North Pacific. However, strong upper ridging in the wake of that low is predicted to return Lane to a more westerly track by early next week. Both the Euro and GFS models project that Lane will pass south of the Big Island around Tuesday or Wednesday, with the GFS bringing it a bit closer than the Euro. This is too far out to take any track forecast as gospel, so we’ll continue to keep an eye on Lane, especially if it strengthens as much as the raw ingredients suggest it might.

Figure 3. Infrared photo of Tropical Storms Bebinca, Rumbia, and Soulik in the Northwest Pacific as of 1830Z (2:30 pm EDT) Thursday, August 16, 2018. Image credit: CIMMS/SSEC/UW–Madison.

A cluster of storms in the Northwest Pacific

The Northwest Pacific, Earth’s most prolific producer of tropical cyclones, is living up to its reputation this week. Tropical Storm Rumbia will be sweeping across the coast of eastern China near Shanghai early Friday local time, most likely as a weak tropical storm or tropical depression. Xinhua reported that some 50,000 people were evacuated ahead of Rumbia. Also on Friday morning local time, another tropical storm, Bebinca, will be coming ashore in northern Vietnam. The news site vnexpress.com reported that the nation is mobilizing more than 500,000 soldiers, police, and volunteers to stand ready to assist as Bebinca strikes. Heavy rains of 5”-10” are possible well inland from the landfall locations of both Rumbia and Bebinca.

The biggest potential threat early next week is from Tropical Storm Soulik, which was fast approaching typhoon strength several hundred miles northwest of Guam. Soulik is predicted by the Joint Typhoon Warning Center to become a Category 4 typhoon this weekend as it arcs toward southern Japan, hard hit by deadly floods earlier this summer. At least some weakening would be likely before any landfall.

Dr. Jeff Masters wrote the section of this post on Invest 99L.