After a highly unusual August without a named storm – for the first time since 1997 – the Atlantic has finally woken up. On September 1, Tropical Storm Danielle formed at 11 a.m. EDT about 960 miles west of the Azores Islands. Danielle formed unusually far to the north – near 38°N, one of the most northerly genesis locations on record for a September named storm. Danielle took advantage of sea surface temperatures (SSTs) that were exceptionally warm for so far north: about 27.5 degrees Celsius (82°F), about 1 degree Celsius (1.8°F) above average for this time of year.
Satellite images on Thursday afternoon showed that Danielle with a symmetric area of heavy thunderstorms steadily growing in intensity and areal coverage. A microwave satellite image taken in the 37 GHz band Thursday morning showed a complete cyan-colored ring at Danielle’s center, not uncommon when a tropical cyclone is about to undergo rapid intensification (see Tweet by Andy Hazelton above).
Forecast for Danielle
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 38°N. That’s quite possible, given the forecast for Danielle. Danielle is predicted to have favorable conditions for strengthening through Sunday, with light-to-moderate wind shear of 5-15 knots and SSTs holding steady near 27.5 degrees. The National Hurricane Center predicts that Danielle will become a hurricane on Saturday, as it meanders to the northeast at less than 5 mph, far from any land areas. By Monday, Danielle is predicted to cross into cooler waters, causing a weakening trend.
Danielle’s formation on September 1 comes more than two weeks later than the usual appearance of the season’s fourth named storm, according to the 1991-2020 climatology. Typically, by September 1, the Atlantic has spawned six named storms, two hurricanes, and one intense hurricane. To this point, the year has had three named storms and no hurricanes, with an accumulated cyclone energy (ACE) index just 9% of average. Danielle is the first tropical cyclone in the Atlantic since Tropical Storm Colin dissipated over North Carolina 59 days ago, on July 3.
Danielle is one of the most recycled names on the rotating list of storms: Seven previous incarnations of Danielle have appeared, beginning in 1980. Given that the storm is not expected to impact any land areas, the name Danielle will likely return for its ninth tour of the Atlantic in 2028.
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 8 a.m. EDT Thursday, the disturbance, designated 91L by NHC, was headed west-northwest at roughly 8 mph. 91L had warm sea surface temperatures of 28 degrees Celsius (82°F), with light wind shear of 5-10 knots favorable for development. 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 limit development of 91L, though a number of the members of the latest GFS and European models ensemble forecasts show 91L eventually becoming a hurricane (Figure 2).
So far, it appears 91L will not be a direct threat to any islands, but residents of the Turks and Caicos Islands should watch 91L carefully. In an 8 a.m. EDT Thursday Tropical Weather Outlook, NHC gave 91L two-day and five-day odds of formation of 60% and 80%, respectively.
Tropical wave 94L in the far eastern Atlantic
NHC on Thursday morning designated a tropical wave near and to the northeast of the Cabo Verde Islands off the coast of Africa as Invest 94L. Satellite images showed the disturbance was not well organized, with two surface swirls competing to be the center, and very few heavy thunderstorms. Conditions were marginal for development, with sea surface temperatures of 26 degrees Celsius (79°F) and moderate wind shear near 10 knots. The wave, predicted to move west to west-northwest over the next five days, may become a short-lived tropical depression on Friday. On Saturday, the wave will move into a region with drier air and higher wind shear, likely ending its prospects for further development. In an 8 a.m. EDT Thursday Tropical Weather Outlook, NHC gave the wave two-day and five-day odds of formation of 30%.
A weakening Typhoon Hinnamnor may rebound to threaten Korea, Japan
Former Category 5 Typhoon Hinnamnor was stalling and struggling just east of Taiwan on Thursday night local time, but it is still expected to race northward by Sunday and Monday as a large and powerful storm, potentially striking South Korea by Monday local time. One death has already been attributed to Hinnamnor’s heavy rains in the northern Philippines.
As of 12Z (8 a.m. EDT) Thursday, Hinnamnor’s top sustained winds had dropped from 140 knots (160 mph) to a still-potent 120 knots (140 mph), according to the Joint Typhoon Warning Center (JTWC), making Hinnamnor the equivalent of a mid-range Category 4 hurricane. Hinnamnor was drifting south at just 5 knots, about 350 miles east of the southern tip of Taiwan.
Hinnamnor’s unusual southerly drift is partly the result of its interaction with former Tropical Depression 13, which has been absorbed into Hinnamnor’s broadening circulation. Competing steering currents around Hinnamnor will cause the storm to move little over the next couple of days: By 12Z Friday, TWC predicts Hinnamnor will be less than 100 miles from its 12Z Thursday position. This foot-dragging motion will allow ample quantities of cooler water to be upwelled by Hinnamnor’s strong, expansive winds, gradually weakening the typhoon. Moreover, wind shear has risen to 25 knots (30 mph) near the storm, and pockets of strong convection at some distance from Hinnamnor’s center are competing with convection around the center, disrupting the typhoon’s overall integrity.
JTWC predicts Hinnamnor to weaken to category 2 strength (105 mph maximum sustained winds) by 0Z Saturday. By that point, an upper-level trough to the northwest will begin to haul Hinnamnor northward at an accelerating clip. While recurving tropical cyclones sometimes weaken, in this case the trough will provide a favorable outflow channel for Hinnamnor at upper levels, and the accelerating motion will take the typhoon over exceptionally warm untrammeled waters (30 degrees Celsius or 86°F, about 1-2°C above average).
The high-resolution HWRF model, which performs well at intensity forecasts once a tropical cyclone has been established, suggests that Hinnamnor could regain Category 3 and perhaps Category 4 strength by Sunday, as it moves through Japan’s Ryukyu Islands. By this point, Hinnamnor should be moving quickly toward South Korea and the northwest tip of southern Japan. Some models show timing differences among models in speed of motion, but model ensembles are now in close agreement on a track that could bring Hinnanmor onto the southeast coast of South Korea around Monday, potentially affecting the city of Busan.
Hinnamnor has already enlarged substantially since its first rapid strengthening, and it should grow even larger as it weakens and restrengthens while moving toward higher latitudes. As a result, Hinnamnor could pack an unusually strong punch, with an expansive field of strong winds and torrential rains affecting South Korea and also southwest Japan.
As shown in Figure 4, only five typhoons of major-hurricane strength (Category 3 or stronger) are on record as passing within 100 miles of Busan.
Typhoon Maemi (2003) – which made landfall with top sustained winds of 105 mph, according to JTWC, and 120 mph – was the strongest cyclone to strike South Korea since Typhoon Sarah of 1959 (top sustained winds at landfall 115 mph) in records dating back to 1904.
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