The first Cat 5 storm on Earth this year, Typhoon Hinnamnor, peaked with 160 mph winds at 8 a.m. EDT Tuesday, August 30, according to the Joint Typhoon Warning Center (JTWC). This is quite late in the year for the planet’s first category 5 storm – the 1990-2021 average is 5.3 Cat 5s per year, according to ratings by the National Hurricane Center and Joint Typhoon Warning Center.
Forecast for Hinnamnor
After maintaining category 5 strength for two 6-hourly advisories on August 30, Hinnamnor underwent an eyewall replacement cycle, with the inner eyewall collapsing and being replaced with a new eyewall with a larger diameter. This process weakened the typhoon, despite the presence of very favorable conditions for intensification – sea surface temperatures of 30-31 degrees Celsius (86-88°F) and light wind shear of 5-10 knots. At 8 a.m. EDT Wednesday, August 31, Hinnamnor was a category 4 storm with 145 mph winds, headed west-southwest at 15 mph – a somewhat unusual trajectory for a strong typhoon in this area, about 115 miles south of Okinawa, Japan.
Hinnamnor is less than 500 miles to the north-northwest of newly-formed Tropical Depression 13, and the two storms are rotating around a common center of motion, a complex interaction known as the Fujiwhara effect. As a result, Hinnamnor is expected to slow down and stall Thursday and Friday. The typhoon’s very slow motion will allow it to upwell a considerable amount of cool water near the storm’s core, which should weaken it. Wind shear is also expected to rise, with the result that Hinnamnor may be reduced to category 2 or 3 strength when it begins an expected northerly track between China and Japan on Saturday. Reintensification should occur at that time, when the typhoon finally leaves behind the pool of cool upwelled water.
Hinnamnor could become a dangerous landfalling major typhoon early next week, but it is too early to pin down the location most at risk. Sea surface temperatures are unusually warm throughout the far northwest Pacific – about 30-31 degrees Celsius (86-88 degrees Fahrenheit) east of central China, which is as much as 2 degrees Celsius (3.6 degrees Fahrenheit) above average – providing unusually strong support for any typhoon passing through the area. Hinnanmor at that point would be large and associated with a broad field of typhoon-force winds, making it capable of driving an unusually large storm surge.
91L in the central Atlantic struggling with dry air
A tropical wave located a few hundred miles east of the Leeward Islands in the Lesser Antilles could organize into a tropical depression later this week, but it continues to struggle with dry air. At 2 p.m. EDT Wednesday, the disturbance, designated 91L by the National Hurricane Center, was headed west-northwest to northwest at 5-10 mph. 91L had warm sea surface temperatures of 28 degrees Celsius (82°F), with light wind shear of 5-10 knots favorable for development. A Wednesday morning hurricane hunter mission into 91L found a closed circulation at mid-levels of the atmosphere, but satellite images showed the system had an elongated surface circulation and only a moderate amount of heavy thunderstorms. 91L was surrounded on three sides by a large area of dry air associated with the Saharan Air Layer (SAL).
Forecast for 91L
The clockwise flow of air around the Bermuda-Azores High to the north of 91L will impart a west-northwesterly to northwesterly track for the next five days, and the latest forecasts from the GFS and European models show 91L passing a few hundred miles north of the Leeward Islands on Saturday, then recurving to the northeast early next week. On this track, 91L will encounter the shearing winds of a tropical upper-tropospheric trough (TUTT) by Sunday, with high wind shear expected early next week. This high wind shear will likely squelch development of 91L, limiting its chances of becoming a hurricane. So far, it appears 91L will not be a direct threat to any islands, but residents of the Turks and Caicos Islands and Bermuda should watch 91L carefully. In a 2 p.m. EDT Wednesday Tropical Weather Outlook, NHC gave 91L two-day and five-day odds of formation of 60% and 80%, respectively.
The next name on the Atlantic list of storms is Danielle, one of the most recycled names on the rotating list of storms: Seven previous incarnations of Danielle have appeared, beginning in 1980.
93L in central subtropical Atlantic may be most likely to become a hurricane
In this upside-down Atlantic hurricane season, it would be appropriate if the first hurricane of the year ended up forming in an upside-down location – far to the north, near 40°N. That’s quite possible, given the forecast for newly-designated Invest 93L, located at 8 a.m. EDT Wednesday near 37.5°N, 46.6°W, about 850 miles west-southwest of the Azores. Some of the models develop 93L into a strong tropical storm or subtropical storm by Saturday, as it meanders at less than 5 mph far from any land areas. It’s not far-fetched to speculate that 93L could become a hurricane early next week (see Tweet by Andy Hazelton above).
93L developed along the boundary of an old front, and satellite images on Wednesday afternoon showed that 93L had developed a well-defined surface circulation and a moderate amount of heavy thunderstorms. Wind shear is moderate at 10-20 knots, but sea surface temperatures (SSTs) are exceptionally warm for so far north across a huge area. At 27.5 degrees Celsius (82°F) SSTs are about 1 degree Celsius (1.8°F) above average for this time of year, which in this case makes the difference between marginal and favorable for development. The main impediment to development was dry air, with a mid-level relative humidity of 50%. In a 2 p.m. EDT Wednesday Tropical Weather Outlook, NHC gave 93L two-day and five-day odds of formation of 70% and 80%, respectively.
Tropical wave in the far eastern Atlantic
Satellite images on Wednesday afternoon showed a large tropical wave located in the far eastern tropical Atlantic, between the coast of Africa and the Cabo Verde Islands, with heavy thunderstorm activity to the Cabo Verdes. The wave had a modest amount of spin at mid-levels of the atmosphere, but no well-defined surface circulation. Conditions were marginal for development, with sea surface temperatures of 27 degrees Celsius (81°F) and high wind shear of 20 knots. The wave, predicted to move west to west-northwest over the next five days, may become a short-lived tropical depression by Friday. This weekend, the wave will move into a region with drier air and higher wind shear, likely ending chances for further development. In a 2 p.m. EDT Wednesday Tropical Weather Outlook, NHC gave the wave two-day and five-day odds of formation of 40% and 50%, respectively.
Editor’s note: this post was corrected on Sept. 1 to correct the lat/lon for 93L, whose numbers had been transposed.
Bob Henson contributed to this post. Website visitors can comment on “Eye on the Storm” posts (see comments policy below). Sign up to receive notices of new postings here.