La Niña conditions favor active Atlantic hurricane seasons with higher-than-average U.S. landfalling hurricane activity, particularly along the U.S. East Coast north of Florida. With two named storms and four other threat areas in the Atlantic, today’s very active situation is typical of what one expects during the climatological peak week of a La Niña hurricane season.
La Niña expected to last through spring 2021
Over the past week, sea surface temperatures in the benchmark Niño 3.4 region of the eastern tropical Pacific (5°N-5°S, 170°W-120°W) were 0.9 degrees Celsius below average, well below the 0.5 degree below-average threshold for a weak La Niña, and near the 1.0 degree below-average threshold for moderate La Niña conditions. Forecasters at NOAA and at Columbia University’s International Research Institute for Climate and Society favor La Niña conditions continuing through the winter (75% chance), and peaking as a borderline weak/moderate La Niña.
Earth’s most recent La Niña event, from September 2017 through March 2018, was a weak one. That said, the La Niña Atlantic hurricane season of 2017 was an exceptionally brutal one, with three destructive hurricanes that ranked in the top five for most expensive weather-related disasters in world history: Harvey ($128 billion), Maria ($92 billion) and Irma ($51 billion).
The tweet below by Steve Bowen of Aon shows that U.S. landfalling hurricanes have historically been considerably more common during La Niña years:
Paulette a threat to Bermuda
In the central Atlantic, Tropical Storm Paulette, with 50 mph winds at 11 a.m. EDT Thursday, was headed west-northwest at 10 mph toward Bermuda, well within the National Hurricane Center’s five-day cone of uncertainty. Paulette was struggling with very high wind shear of 30 – 40 knots from an upper-level trough of low pressure. That very high wind shear is expected to continue through Saturday morning, which should cause Paulette to weaken. The shear is predicted to relax to a moderate 10 – 20 knots Saturday afternoon through Sunday, and then drop to less than 10 knots by Monday, when Paulette will be nearing Bermuda. The lower shear likely will allow Paulette to re-strengthen, and the majority of the top intensity models predict that Paulette will be a hurricane on Monday, with several predicting it could be a category 2 hurricane.
Steering currents appear well-positioned to turn Paulette more to the northwest and then north early next week, and the storm is not expected to be a landfall threat in the U.S. Fewer than 5% of the 72 ensemble members of the 0Z or 6Z Thursday runs of the European and GFS model ensemble forecasts showed Paulette making landfall along the U.S. East Coast next week; Bermuda currently appears to be the only land area facing a possible Paulette landfall.
Rene not a threat to land
In the eastern Atlantic, Tropical Storm Rene, a strengthening tropical storm with 50 mph winds at 11 a.m. EDT Thursday, was headed west-northwest at 12 mph into the central Atlantic, far from any land areas. With adequately warm waters near 26.5 degrees Celsius (80°F), light to moderate wind shear, and a moist atmosphere, conditions appear favorable for Rene to intensify into a hurricane by Saturday. Rene is expected to turn more to the northwest by Sunday and begin weakening; the storm is unlikely to affect any land areas.
According to floodlist.com, the tropical wave that became Rene produced torrential rains and deadly flooding in West Africa. Six flood deaths occurred in Senegal, with up to eight inches of rain falling in 24 hours on September 5. Three flood deaths occurred in Burkina Faso.
Two areas of interest near the U.S. worth watching
The weak area of low pressure approaching the North Carolina coast this week, 94L, was moving ashore on Thursday afternoon and is no longer a threat to develop.
NHC was monitoring two other areas of interest near the U.S., both with low chances of development. A large area of disorganized showers and thunderstorms centered a few hundred miles northeast of the central Bahamas was forecast to move westward, crossing the Bahamas and Florida on Friday, and moving into the eastern Gulf of Mexico over the weekend. Upper-level winds are expected to become conducive for some development of this system while it moves slowly west-northwestward over the eastern Gulf of Mexico early next week. In an 8 a.m. EDT Thursday Tropical Weather Outlook, NHC gave this system two-day and five-day odds of development of 0% and 30%, respectively.
Another disturbance that developed over the northeastern Gulf of Mexico, off the Florida coast, was producing a few disorganized showers and thunderstorms. Some slow development is possible while this system moves westward and then southwestward over the northern and western Gulf of Mexico through early next week. In an 8 a.m. EDT Thursday Tropical Weather Outlook, NHC gave this system two-day and five-day odds of development of 10% and 20%, respectively.
The future Sally likely to be a tropical wave that emerged from Africa on Thursday
Top models for forecasting tropical cyclone genesis strongly support development of a new tropical wave that emerged from the coast of Africa on Thursday.
This wave is predicted to move mostly westward at low latitude at 15 to 20 mph next week. Given its lower-latitude position compared to Paulette and Rene, this new tropical wave may be a long-range concern for the Caribbean and North America. Over 30% of the 51 ensemble members from the 0Z Thursday, September 10, run of the European ensemble forecast showed that this new system would be a named storm in or just north of the Caribbean late next week.
However, the future track of the system could be affected by the position and strength of Paulette and Rene, by the structure of the wave once it organizes into a tropical depression, and also by the path and intensity of another tropical wave likely to move off the coast of Africa on Saturday – variables very difficult to accurately predict. In an 8 a.m. EDT Thursday Tropical Weather Outlook, NHC gave the new wave two-day and five-day odds of development of 60% and 90%, respectively. The wave behind it, expected to emerge from the coast on Saturday, was given two-day and five-day odds of development of 0% and 40%, respectively.
The next two names on the Atlantic list of storms are Sally and Teddy.
Posted on September 10, 2020 (2:06pm EDT).