Hurricane activity in the Atlantic typically weakens sharply toward the end of October. On average, only 14% of the Atlantic season's activity occurs after October 15 (as measured by accumulated cyclone energy). However, things may not calm down so quickly this year. One riot is approaching the Leeward Islands later this week, and another could develop in the western Caribbean days later.
The unusually active 2020 Atlantic season is a reminder of what can happen in late October and November in a warming climate. After October 15, three major hurricanes swept through the Caribbean and Gulf of Mexico, and another hurricane (Epsilon) swept through the northwest Atlantic. Zeta, Eta, and Iota all made landfall in the Caribbean, with Zeta and Eta subsequently hitting the U.S. Gulf Coast. Iota is the last named Atlantic storm of the year. It was also the last storm in the Atlantic to be named after a Greek letter, as the “overflow” list of Greek letters (created to be used when a season has completed all 21 designated names but has not retired names in 2017 mind) was Replaced by new supplementary list in 2021.
Intensifying La Niña events in 2020, as well as unusually warm sea surface temperatures, helped boost late-season activity. This October, La Niña conditions appear to be forming again, and despite the recent ravages of Helen and Milton, average sea surface temperatures in the Gulf of Mexico remain record-high to date. The Atlantic shipping lanes have certainly been active since the climatic midpoint of the season (around September 10th), putting the year well behind normal into the busier than average category.
So far this year, there have been 13 named storms, 9 hurricanes, 4 major hurricanes, and an accumulated cyclone energy (ACE) index of 141 in the Atlantic Ocean. Hurricanes, ACE Index is 106. The long-term average for the entire season from 1991 to 2020 is 14.4 named storms, 7.2 hurricanes, 3.2 major hurricanes, and an ACE Index of 123.
Cape Verde's late-season wave deserves attention
Late October is well past the normal season for Cape Verde disturbances, which traverse the main development zone of the tropical Atlantic and can affect the Caribbean and Central and North America as long-range hurricanes. This year, temperatures in major development areas will be roughly the same as in 2023 (about 0.5 degrees Celsius, or 1°F, warmer than normal).
A Cape Verdean disturbance in the mid-Atlantic called “Invest 94L” will move west midweek. On Monday, this disturbance had a well-defined surface circulation but was embedded in a very dry air mass, keeping severe thunderstorm activity limited, as shown in the satellite loop. But by Wednesday and Thursday, sea surface temperatures at 94L will reach a record high of around 28 degrees Celsius (82°F). SHIPS mode predicts smaller wind shear, wind speeds of 5-10 knots, and the mid-level atmosphere should gradually become moist (relative humidity increases from around 40% to over 50%). Although tropical cyclones in the region typically stay well away from land so late in the season, an unusually strong subtropical ridge will keep 94L moving generally westward, which may cause the disturbance to bring heavy rain to the northern Leeward Islands.
By early next week, 94L will encounter two major obstacles to its development: a hostile wind-shear wall (Figure 2) if it attempts to move west-northwest toward Florida (Figure 2); In Florida, the majestic mountains of Hispaniola appear. 94L is unlikely to affect the East Coast of the United States. As Michael Lowry discussed in his Monday Substack post, hurricanes that hit the continental United States late in the season have historically come from the western Caribbean, not the Atlantic. After mid-October, only four U.S. hurricanes originated from systems in the eastern or northern Caribbean: Unnamed (1887), Hazel (1954), Kate (1985), and Nicole (2022).
In its Tropical Weather Outlook issued at 8 a.m. ET Monday, the National Hurricane Center projected a 10 percent chance of 94L developing within two days and a 50 percent chance of developing within seven days. The next name on Atlantic’s list is Nadine.
Western Caribbean could cause trouble by Thursday
An area of weather disturbance in the southwestern Caribbean Sea is associated with a large area of low pressure known as the Mesoamerican Gyre. An ensemble of GFS and European models shows the possibility of a tropical disturbance developing into a tropical depression, which could separate from a circulation off the coast of Nicaragua on Thursday. The disturbance may move west or west-northwest, bringing heavy rain to Central America and Mexico's Yucatan Peninsula early next week. However, the National Hurricane Center did not emphasize this developing threat in its tropical weather outlook at 8 a.m. EDT Monday.
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