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Atlantic disturbances 95L and 96L expected to soon develop » Yale Climate Connections

Nicholas is no more, after being downgraded to a post-tropical low over coastal Louisiana on Thursday morning (for a full accounting of Nicholas’ meteorological impacts, see the latest NOAA storm summary). However, two tropical disturbances in the Atlantic – 96L off the coast of North Carolina and 95L in the tropical eastern Atlantic – are expected to become tropical cyclones within the next two days.

Hurricane hunters to investigate disturbance 96L, off North Carolina

Heavy thunderstorm activity increased Thursday morning in association with an area of disturbed weather, designated 96L, located about 250 miles south-southeast of the North Carolina Outer Banks. Satellite imagery showed that 96L had multiple surface swirls, but no well-organized surface circulation. A hurricane hunter aircraft mission into 96L on Wednesday did not find the system qualified as a tropical depression, but a second mission scheduled for Thursday afternoon has a better chance of finding that a tropical depression has formed.

96L has good model support for development through Saturday as it moves north to north-northeast at 10-15 mph, remaining a few hundred miles offshore of the mid-Atlantic coast. Conditions are marginally favorable for development, with sea surface temperatures around 28 degrees Celsius (82°F), a moist mid-level atmosphere with a relative humidity around 70%, and moderate to high wind shear of 15-25 knots. Thursday night through Saturday, 96L likely will bring one to two inches of rain to North Carolina’s Outer Banks, plus waves of 5 to 10 feet to the waters along the mid-Atlantic coast.

By Sunday, 96L is expected to turn to the northeast and cross beyond the northern boundary of the Gulf Stream, where waters are cooler than 25 degrees Celsius (77°F). Wind shear will also increase on Sunday, and 96L may then take on characteristics of a subtropical storm. Steering currents favor a track a few hundred miles offshore from the Canadian Maritime Provinces, and 96L could bring rains of one to two inches to portions of Nova Scotia and Newfoundland on Sunday. Steering currents are expected to collapse on Monday, resulting in 96L’s moving slowly and erratically a few hundred miles south of Newfoundland for multiple days. In its 8 a.m. EDT Thursday Tropical Weather Outlook, the National Hurricane Center  gave 96L 2-day and 5-day odds of development of 70%. The next name on the Atlantic list of storms is Odette.

Figure 1. Visible satellite image of disturbance 95L in the eastern Atlantic at 11:35 a.m. EDT September 16, 2021. The disturbance was poorly organized and had a limited amount of heavy thunderstorms. (Image credit: Tropical Tidbits)

Little change to 95L in the eastern tropical Atlantic

A tropical wave in the eastern Atlantic, about 800 miles west-southwest of the Cabo Verde Islands at 8 a.m. EDT Thursday morning, was headed west at 15 mph. This wave, which NHC designated as 95L, has not become any more organized over the past day, despite seemingly favorable conditions for development. Satellite imagery showed that 95L has a degree of spin, but a relatively meager area of heavy thunderstorms.

95L, with favorable conditions for development this week, has modest model support for development. From Thursday through Sunday, the disturbance will have sea surface temperatures near 28 degrees Celsius (82°F), mostly moderate wind shear of 10-20 knots, and a moderately moist atmosphere with a mid-level relative humidity of 55-65%. These conditions are expected to allow the system to develop into a tropical depression by Sunday. However, 95L is expected to face a major obstacle to further intensification by Monday, when it approaches the Leeward Islands, as a result of a large upper-level trough of low pressure. This trough is predicted to have high wind shear and dry air, and many model forecasts have 95L being destroyed when it encounters the trough. In its 8 a.m. EDT Thursday Tropical Weather Outlook, the NHC gave 95L 2-day and 5-day odds of development of 70% and 80%, respectively. The next name on the Atlantic list of storms after Odette is Peter.

Bob Henson contributed to this post.

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