Although the northern hemisphere tropics begin to slowly wind down in the month of October, there is still plenty of activity to discuss today. In the Atlantic, Hurricane Leslie is barely hanging onto hurricane status as it begins to turn to the north over the central Atlantic. A Central American Gyre (CAG) may also help spawn the development of a new tropical cyclone in the Northwestern Caribbean or Gulf of Mexico by early next week. In the Pacific, Hurricanes Sergio and Walaka remain active, but neither poses a significant threat to land at this time.

Leslie no threat as it holds onto hurricane status

On Wednesday morning, Tropical Storm Leslie intensified into the sixth hurricane of the 2018 Atlantic hurricane season, which is approximately the average number of hurricanes in a season for the Atlantic. However, only one of the six hurricanes so far this season – Florence – has become a Category 3 or stronger (major) hurricane. As of 11:00 a.m. EDT Thursday, Hurricane Leslie was centered near 32.5°N 57.2°W, and was moving northward at about 9 mph. Maximum sustained winds were 65 knots (75 mph), with an estimated minimum pressure of 979 mb. Although wind shear is expected to remain fairly low for the next five days, Leslie has likely peaked in intensity as it is beginning to move over sea surface temperatures cooler than 26°C (78.8°F), but it should remain a tropical cyclone for the next five days at least. Dry mid-level air – with relative humidity values near 50 percent – should also limit Leslie’s ability to maintain strong deep convection near the center. The convective cloud tops around Leslie’s ragged eye have warmed today, and Leslie likely is not far off from being downgraded back to a tropical storm. Leslie should continue moving slowly northward for the next 24-36 hours or so, then Leslie is expected to turn back to the southeast this weekend. It is possible Leslie could pass near the Azores islands in about eight days, but this remains uncertain.

Tropical development possible in Northwestern Caribbean or central Gulf of Mexico early next week

A broad area of low pressure – associated with a Central American Gyre (CAG) – has developed over the western Caribbean Sea. Surface pressures are relatively low in the area, but strong wind shear is currently preventing the development of any organized showers and thunderstorms. The wind shear is expected to remain strong for the next day or two, which should limit development during this time. However, by Sunday or Monday, some gradual development is possible as the low pressure system is expected to emerge into the Northwestern Caribbean Sea while wind shear may lessen. The 12z Thursday UKMET model run, as well as the 00z Thursday ECMWF model run both predicted some weak development with this system over the central Gulf of Mexico. The 12z Thursday GFS model run showed no development, however, many of the GEFS ensemble members did. The next name on the Atlantic naming list is Michael. The National Hurricane Center (NHC) currently gives this system a near zero percent chance of development within 48 hours, but a 40 percent chance within five days. Forecasting genesis with these gyre-related systems is often challenging, since it remains unclear if and where a well-defined center of circulation may develop. If something does develop, I suspect it will be a messy, poorly organized tropical storm with heavy rainfall the primary threat.

Hurricanes Sergio and Walaka currently spinning over the Pacific

Hurricane Sergio, the record-breaking eighth Category 4 hurricane of the extremely active 2018 Pacific hurricane season, is likely to remain a powerful hurricane over the next several days. As of 8:00 a.m. PDT (11:00 a.m. EDT) Thursday, Hurricane Sergio was centered near 15.0°N 119.3°W, and was moving northwestward at about 9 mph. Maximum sustained winds were 120 knots (140 mph), with an estimated minimum pressure of 943 mb. Gradual weakening is likely, though, as Sergio enters higher shear this weekend. It is possible that Sergio could recurve to the northeast and eventually bring tropical cyclone impacts to the Baja California Peninsula, but Sergio remains over a week away from any potential impacts to the area.

Hurricane Walaka is gradually losing strength as it moves northward over the Central Pacific. As of 8:00 a.m. HST (2:00 p.m. EDT) Thursday, Hurricane Walaka was centered near  27.9°N 166.1°W, and was moving northward at about 20 mph. Maximum sustained winds were 90 knots (105 mph), with an estimated minimum pressure of 960 mb. Walaka, once a Category 5 hurricane, is becoming significantly less organized due to increasing wind shear. A Tropical Storm Warning is currently in effect for Papahanaumokuakea Marine National Monument from Nihoa to French Frigate Shoals to Maro Reef, though this warning will likely be canceled soon. Walaka should lose tropical characteristics by late this weekend as it accelerates to the northeast.