With only three days until the climatological peak of the Atlantic hurricane season, the Atlantic basin is extremely busy today, with four tropical cyclones currently active – TS Florence, TD Gordon, TD Eight and TD Nine. This is the first time the Atlantic has had four simultaneously active tropical cyclones since 2008. Of these, Florence is the most imminent land threat.

TS Florence expected to restrengthen this weekend, increasing risk to the United States East Coast

Tropical Storm Florence has changed little in organization today, but it is expected to begin restrengthening by Sunday. As of 5:00 p.m. EDT Friday, Tropical Storm Florence was centered near 24.8°N 52.5°W, and was moving westward at about 8 mph. Maximum sustained winds were 55 knots (65 mph), with an estimated minimum pressure of 996 mb. Florence is beginning to move towards a more favorable environment, and this should allow Florence to begin to re-strengthen soon. Florence is currently not vertically stacked, which should slow intensification for the next 24 hours or so. After that time, vertical shear is expected to decrease to less than 10 knots, while Florence moves over sea surface temperatures near 29°C (84.2°F). This is a conducive environment for intensification, and all of the intensity guidance shows significant strengthening by early next week. The official National Hurricane Center (NHC) forecast brings Florence up to Category 4 strength by next Wednesday. The center of Florence is expected to pass well south of Bermuda on Tuesday, and significant impacts to Bermuda are becoming less likely. Florence is currently moving westward, but should turn more to the west-northwestward early next week.

Florence is an increasing concern for the United States east coast. Due to a strong ridge of high pressure over the Northeastern United States, there will not be an easy “escape route” for Florence to recurve out to sea. Much of the model guidance is currently clustered to a potential landfall in the Carolinas or mid-Atlantic U.S., with the GFS predicting a landfall in the Outer Banks of North Carolina and the ECMWF model predicting a landfall in central South Carolina. It should be stressed that the potential impacts are still about six days away, and are subject to large changes at this lead time. It is still too soon to rule out the possibility of Florence turning out to sea without significant impacts for the U.S., though this scenario appears to becoming less likely. A NOAA aircraft is scheduled to investigate the upper-level environment around Florence tomorrow, which could help improve track forecasts. We will probably not have an idea of where Florence is going until early next week, but a landfall along the east coast as a significant hurricane is certainly a possibility. Interests in the Eastern United States should closely monitor the progress of Florence over the coming days, and have a hurricane plan ready.

TD 8 forms southeast of Cabo Verde Islands, Tropical Storm Warnings in effect

The eighth tropical depression of the surprisingly active 2018 Atlantic hurricane season developed Friday afternoon southeast of Cabo Verde. As of 5:00 p.m. EDT Friday, Tropical Depression Eight was centered near 13.2°N 18.6°W, and was moving westward at about 10 mph. Maximum sustained winds were 30 knots (35 mph), with an estimated minimum pressure of 1002 mb. TD Eight is the easternmost tropical cyclone to form in the Atlantic basin since Hurricane Fred in 2015. Eight is a very large system. During the next 72 hours, environmental conditions are expected to remain conducive for development, with sea surface temperatures above 27°C (80.6°F), wind shear less than 15 knots, and mid-level relative humidity values near 70 percent. After that time, wind shear is expected to increase, while Eight moves into cooler waters and a drier, more stable environment. That should limit intensification, or perhaps cause weakening, after that time. While most of the intensity guidance predicts that Eight will remain below hurricane strength, the GFS and ECMWF global models both strengthen Eight into a hurricane by Monday morning. Eight should become a tropical storm very soon, likely tonight or early Saturday morning. A Tropical Storm Warning is currently in effect for the Cabo Verde Islands of Santiago, Fogo, and Brava. Even though the center of Eight may remain south of the Cabo Verde Islands, tropical storm conditions are still possible because of the system’s large size. I highly doubt TD Eight will pose any threat to North America or the Caribbean in the long term, though it could affect the Azores in about 8-9 days. The next name on the Atlantic naming list is Helene.

Invest 92L develops into Tropical Depression Nine, potential threat to the Lesser Antilles next week

Invest 92L, a well-defined low pressure system over the central tropical Atlantic, has developed a well-defined surface circulation, and has now been classified as Tropical Depression Nine. As of 5:00 p.m. EDT Friday, Tropical Depression Nine was centered near 13.6°N 34.9°W, and was nearly stationary. Maximum sustained winds were 30 knots (35 mph), with an estimated minimum pressure of 1007 mb. The center of Nine is near the eastern edge of the convective mass, though deep convection has been increasing near the center in the past couple of hours. This is due to some moderate easterly shear, which should slow intensification for the next 24 hours or so. The shear is expected to decrease afterwards, and much of the intensity guidance as well as the global models show deepening after that time. Both the ECMWF and GFS models predict that TD Nine will be near the central Lesser Antilles as a tropical storm in about six days. However, it’s also a possibility Nine could turn to the north before reaching the Lesser Antilles as suggested by some GEFS ensemble members, but this scenario does not seem likely at this time.

Invest 94L has a low chance to develop

A trough of low pressure, currently located southwest of Bermuda, is producing disorganized showers and thunderstorms. As of 18:00 UTC Friday, Invest 94L was located near 30.8°N 68.2°W, and was nearly stationary. Maximum sustained winds were 20 knots (25 mph), with an estimated minimum pressure of 1015 mb. The NHC only gives 94L a 10 percent chance of development within 48 hours, and a 20 percent chance within five days. No reliable global models predict development of 94L, but if it develops, it could influence Florence’s track. Though 94L is unlikely to develop into a tropical cyclone, it could moisten the environment around Florence, preventing dry air entrainment.

I will be back with an update on the tropics tomorrow evening.