Impressive Hurricane Larry continues to maintain major hurricane status as it rumbles northwest at 9 mph over the open central Atlantic. At 11 a.m. EDT Tuesday, Larry had top sustained winds of 115 mph, and had been a major category 3 hurricane since 3Z Saturday (11 p.m. EDT Friday).
Satellite images showed that Larry had a large eye, thick eyewall, and virtually no outer spiral bands. These qualities are characteristic of a special type of hurricane referred to as “annular.” Such hurricanes tend to form when the surrounding atmosphere is dry, making it difficult for the storm to maintain far-flung spiral bands.
Warm waters and low wind shear will provide a supportive environment for Larry over the next three days, but dry air and upwelling of cool water, having already dented the perfect donut shape that Larry had on Monday, will likely continue to degrade Larry over the next few days. Larry is expected to pass just east of Bermuda on Thursday morning, close enough to bring it heavy rain showers and strong winds. In its 11 a.m. EDT Tuesday wind probability forecast, the National Hurricane Center gave Bermuda a 30% chance of experiencing tropical storm-force winds of 39 mph or more from Larry, but no chance of hurricane-force winds.
Larry’s large size and long duration at category 3 strength are making the hurricane a formidable wave machine, and the Caribbean islands, Bahamas, and Bermuda were experiencing its resulting large waves and dangerous rip currents on Tuesday. These conditions will spread westward and northward during the week, and affect much of the U.S. East Coast and Canadian Maritime Provinces by Wednesday.
Larry will be steered to the northwest by the Azores-Bermuda High through Wednesday, but the hurricane is predicted to turn to the north and northeast later in the week, when the it will feel the pull of a trough of low pressure passing to the north. Larry is expected to pass near southeastern Newfoundland, Canada, on Friday night. In its 11 a.m. EDT Tuesday wind probability forecast, the National Hurricane Center gave Cape Race, Newfoundland, a 63% chance of experiencing tropical storm-force winds of 39 mph or more from Larry, and a 10% chance of hurricane-force winds.
Moisture pulled to the northwest by Larry is expected to affect New England late this week, potentially bringing 1-2” of rain to the coast.
Disturbance 91L likely to bring rains of 1-4” to Southeast U.S
In the south-central Gulf of Mexico, an area of disturbed weather designated 91L was moving northeastward at about 5 mph, bringing disorganized heavy rain showers to the waters north of the Yucatan Peninsula, as seen on satellite imagery. Though waters are warm and the atmosphere moist, 91L was experiencing moderate wind shear of 10-20 knots from an upper-level trough of low pressure, which was interfering with development.
Steering currents are predicted to continue carrying 91L to the northeast at about 5 mph this week, bringing the system near the Florida Panhandle on Wednesday night. Heavy rains of 1-4” from 91L will likely affect the U.S. Gulf Coast from Louisiana to the northern Florida Peninsula, along with southern Georgia, Wednesday through Thursday. On Thursday, 91L is predicted to cross over northern Florida and southern Georgia, emerging into the Atlantic Ocean off the Southeast U.S. coast by Friday morning. 91L will then have the opportunity to develop over the warm waters of the Gulf Stream as it moves to the northeast, parallel to the Southeast U.S. coast. 91L does have some modest model support for development, and in its 2 p.m. EDT Tuesday Tropical Weather Outlook, the NHC gave the system 2-day and 5-day odds of development of 30% and 40%, respectively. The next name on the Atlantic list of storms is Mindy. No hurricane hunter missions are tasked for 91L.
Two other areas to watch this weekend
The southwestern Gulf of Mexico’s Bay of Campeche will be another area for possible tropical cyclone formation this weekend, when a tropical wave may enter the Gulf and begin to develop, according to the 12Z Tuesday run of the GFS model. This weekend may also be a time to watch for development of a new tropical wave expected to move off the coast of Africa.
Oddly quiet in Northwest Pacific: no typhoons in August
For the first time on record, the Northwest Pacific did not record a single hurricane-strength typhoon in August. In records that go back to 1945, databases of both the Joint Typhoon Warning Center (JTWC) and the Japan Meteorological Agency (JMA) before this year had never recorded an August without at least one category 1 or stronger typhoon. This is a remarkable occurrence for the Northwest Pacific, which, on average, sees about 40% of all global tropical cyclone activity.
The basin’s unprecedented streak without a typhoon was finally broken Tuesday morning, when Typhoon Chanthu vaulted to category 1 status with the 5 a.m. EDT advisory from JTWC. Prior to Chanthu, the last typhoon observed in the basin was category 2 In-fa, which weakened below typhoon strength on July 25, more than six weeks ago.
Chanthu gives the basin five typhoons to date, compared to the average of nine. However, the number of named storms this year has been exactly average: 15. There has been only one major typhoon so far this year: Surigae, back in April, which packed top 1-minute winds of 190 mph. In a typical year, we’d already have seen four typhoons equal to category 3 strength.
There will likely be another major typhoon this week, as Typhoon Chanthu was undergoing extremely rapid intensification on Tuesday afternoon, now predicted to reach category 4 strength by Wednesday. On Friday and Saturday, Chanthu is predicted to pass between the northern Philippines and southern Taiwan as a dangerous category 4 storm.
Thus far, 2021 is behaving much like 2020. While the North Atlantic is experiencing a year almost as active as 2020 (and even more destructive, after the rampage of Hurricane Ida alone), the Northwest Pacific has been remarkably tranquil. Through September 7, according to the real-time tropical cyclone activity page at Colorado State University, the North Atlantic was running at 153% of its average ACE for the season up to this point, based on a 1981-2020 climatology. Meanwhile, the Northwest Pacific had only racked up a measly 50% of its average ACE to date. The Northeast Pacific was running close to average (95%). Because the Northwest Pacific typically accounts for roughly half of the hemispheric ACE, the entire Northern Hemisphere is actually on the slow side for the year to date: about 83% of average. (Of course, try telling a resident of New Orleans or New York that it’s been a slow season for tropical cyclones globally.)
Although a warming climate is already increasing the number of hurricanes, typhoons, and cyclones vaulting to intense levels, as reiterated by the IPCC last month, there is no sign that the total global number of tropical cyclones is changing. In the Northern Hemisphere, the El Niño/Southern Oscillation (ENSO) cycle and other factors tend to modulate areas of peak activity such that the Northwest Pacific is often quiet when the North Atlantic is active, and vice versa. The year 2020 was a classic example: the total amount of accumulated cyclone energy (ACE) was 145% of average in the hyperactive North Atlantic but only 67% of average in the Northwest Pacific, according to climatlas.com.
La Niña conditions were likely partially responsible for the quiet 2020 typhoon season; during La Niña, the breeding grounds for typhoon formation shift closer to the coast of eastern Asia, reducing the time a storm will have over water, and thus limiting intensification potential and the number of typhoons. This shift in the tropical cyclone formation areas has also been observed so far in 2021, despite the fact neutral ENSO conditions have been in place in 2021. However, conditions are tending towards La Niña (NOAA issued a La Niña Watch this summer), and this shift towards La Niña may be partially responsible for tropical cyclones forming closer to the coast of eastern Asia in 2021.
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