On September 10, a fairly well-organized tropical wave entered the Atlantic from the east. It had an ample amount of moisture associated with it and moved steadily westward south of the Cape Verde islands. But as with several previous systems in the 2020 season, there was another tropical wave right on its heels, which had an influence on the first's ultimate development and track. That second wave ultimately became Tropical Storm Vicky. By the next day, the system had a low-pressure center associated with it. Convection wrapped halfway around the center in a curved band over the western semicircle; on the 12th, further organization led to the designation of Tropical Depression Twenty.
There was a little bit of northerly shear affecting the depression, which kept the convection confined to the southern semicircle throughout the next day. Further, the large circulation was still partially embedded in a monsoon trough located in the Intertropical Convergence Zone, which slowed intensification in the short term. Eventually, thunderstorm activity became more concentrated and began to wrap around the circulation. The depression was upgraded to Tropical Storm Teddy early on September 14. This was the earliest nineteenth, or "T" storm, surpassing the record set by the unnamed Azores subtropical storm of 2005, which formed on October 4 of that year.
Teddy had a lot going for it: light shear, warming waters, and plenty of humidity. It began to intensify quickly later that day as it continued just north of west, bringing it to a high-end tropical storm. The increase in winds paused on September 15 as the system took some time to build an eyewall and fight off incursions of dry air. Teddy turned northwest and strengthened again overnight to category 2 hurricane strength. It was a large system, with excellent outflow in all directions but the west. The storm passed well to the northeast of the Lesser Antilles on the 16th, sparing them of any impacts save large waves. Starting that evening, more deep convection developed and wrapped around the eye, which itself warmed and cleared out by early on September 17. Teddy became a very powerful hurricane, rocketing up to category 4 strength by the afternoon and reaching its peak intensity of 140 mph winds and a pressure of 945 mb.
The cyclone underwent some structural changes overnight and became a bit less defined, leading to a bit of weakening. Aircraft and satellite measurements indicated the next day that an eyewall replacement cycle (EWRC) began, in which a new outer eyewall formed, contracted, and eventually replaced the old. These EWRC's are typical for strong hurricanes. They usually lower its top winds, but expand its windfield, and indeed Teddy's wind radii had increased by that afternoon, even though it was down to a category 3. The new outer eyewall became more complete over the next day as the cyclone fluctuated in intensity, but Teddy's EWRC was quite slow. By the afternoon of September 19, the inner eyewall had finally collapsed for the most part, leaving the outer one surrounding a much larger, ragged eye.
Teddy had taken a turn back toward the west-northwest that day, but it soon resumed its northwest course. Environmental factors were becoming less conducive for a hurricane: the storm was tracking over the cold wake left by Hurricane Paulette about a week prior, the air was becoming drier, and wind shear was increasing. As a result, Teddy's eye filled in and it weakened to a category 2. The storm rounded the edge of the retreating subtropical ridge that evening, turning north and then east of north. On September 21, this fortunately brought the center well east of Bermuda, which had just had a direct hit from Paulette. Even so, Teddy was large enough to bring tropical storm conditions to the island from 150 miles away.
The storm began a rather complex interaction with an frontal low approaching from the northwest that day. The lows rotated a bit around one another, accelerating Teddy northward and bending its track back to the left. At the same time, warm Gulf stream waters and a baroclinic energy boost deepened the hurricane even as it began to dsiplay extratropical characteristics. The minimum central pressure dropped back to 950 mb at its lowest during the morning of September 22. By that time, Teddy was a few hundred miles south of Nova Scotia, but was slowing down. The cyclone was certainly a hybrid of tropical and extratropical, though it had enough of a warm core to still be considered a hurricane. Maximum winds decreased to category 1 strength, but Teddy was now extremely massive: rain extended as far west as eastern Maine and tropical storm force winds reached over 300 miles in every direction from the center, including over 500 (!) miles in the northeast quadrant. The entire island of Nova Scotia was within the wind field.
Luckily, Teddy soon lost its baroclinic energy source and also moved over much colder waters, bringing about a steady decay of the cyclone, as well as extratropical transition late that night. The low that had been Teddy made landfall in central Nova Scotia with maximum winds of around 65 mph on September 23 and continued into the Gulf of St. Lawrence later that day. The low was absorbed by a larger system on the 24th.
The image above shows Teddy as a major hurricane over the open Atlantic.
This image is Teddy on September 22 shortly before completing extratropical transition. It was a large, hybrid cyclone, ranking among the largest recorded by gale force wind diameter. The U.S. northeast is visible at top left, along with some smoke in the upper atmosphere from the 2020 California wildfires.
Teddy hit Nova Scotia as an extratropical cyclone, but by then it was weakening significantly, so imapacts were limited.
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