Hurricane Florence is becoming a major concern for the southeastern United States coast. Florence regained hurricane strength Sunday morning, and is expected to rapidly intensify into a powerful Category 4 hurricane by Tuesday. Florence is now moving into more favorable conditions, and intensification appears to be starting once again. Florence is likely to make landfall in the Carolinas as a major hurricane late Thursday night or early Friday morning, though it is still not out of the question that Florence misses landfall. Also in the Atlantic, Tropical Storms Helene and Isaac are gradually strengthening, and both are likely to become hurricanes by Monday.

Florence regains hurricane strength as it poses a major threat to the Southeastern United States late this week

As of 11:00 a.m. EDT Sunday, Hurricane Florence was centered near 24.4°N 56.3°W, and was moving westward at about 6 mph. Maximum sustained winds were 65 knots (75 mph), with an estimated minimum pressure of 984 mb. There have been hints of a ragged eye in satellite imagery this morning, and Florence is likely starting an intensification trend. Due to wind shear of less than 10 knots and sea surface temperatures near 29°C (84.2°F), steady or perhaps rapid intensification is expected. The only potential limiting factor for intensification in the short term is a fairly dry mid-level atmosphere, with relative humidity values near 50%. Nearly all of the intensity guidance predicts significant intensification with Florence over the next two days, and Florence is likely to regain major hurricane strength on Monday. By Thursday, an increase in wind shear to above 20 knots (25 mph) is likely as Florence approaches the Southeastern United States, and this could cause Florence to weaken slightly as it approaches the coast.

There is still uncertainty with Florence’s eventual track, but models have come into better agreement with Florence approaching the Carolinas by Thursday as a dangerous major hurricane. Florence is currently moving westward, but is expected to gradually turn to the west-northwest or northwest over the next few days. The GFS model, which has been the best performing model with Florence so far, predicted in its 06z run that Florence would make landfall along the Outer Banks of North Carolina. The ECMWF and HWRF models in their most recent runs showed a landfall near the North Carolina-South Carolina border. The UKMET model in its 00z run predicted that The NHC (National Hurricane Center) forecast for Florence predicted a landfall near Wilmington, which is a blend of the model scenarios. It should be stressed that the potential landfall is still over four days out, so there are likely to be more changes in the track forecast over the next few days. Regardless, it is looking increasingly likely that the Southeastern United States, especially the Carolinas, will have serious impacts from Florence. Even if Florence does not make landfall, rains from its outer bands could cause flooding problems near the coast. If the current models are correct, Florence could produce hurricane-force winds and life-threatening storm surge along the coast of the Carolinas, and flooding well inland. Steering currents are expected to become very weak by Friday, which could lead to a slowing of Florence’s movement or perhaps even a stall, which would lead to a prolonged heavy rainfall threat. It is also a possibility that if Florence turns to the north as suggested by the GFS model, it could lead to major inland flooding in the mid-Atlantic United States as well. No watches and warnings are in effect yet, but we will likely see Hurricane Watches posted for parts of the southeastern U.S. coast by Tuesday if current trends continue. Residents from Georgia to the North Carolina Outer Banks should have a Hurricane Plan ready, and coastal residents should be ready to evacuate if needed.

Large TS Helene gradually becoming better organized south of the Cabo Verde Islands

Tropical Storm Helene is gradually strengthening over the eastern tropical Atlantic, south of the Cabo Verde Islands. As of 11:00 a.m. EDT Sunday, Tropical Storm Helene was centered near 13.2°N 25.0°W, and was moving westward at about 13 mph. Maximum sustained winds were 55 knots (65 mph), with an estimated minimum pressure of 997 mb. Helene is a sprawling tropical storm, but lacks a well-defined inner core at this time. Fairly low wind shear (less than 15 knots) and sea surface temperatures above 27°C (80.6°F) over the next two days should allow gradual strengthening to continue, and Helene should become a hurricane later today or on Monday. The cyclone’s large size may allow dry air to get entrained into the storm, which would slow intensification. Helene should move west-northwestward for the next few days, before a gradual turn to the north around Wednesday. After moving past the Cabo Verde Islands, Helene should remain away from land, at least for the next week or so. Wind shear is expected to increase to above 30 knots (35 mph) in about four days, and this should cause a gradual weakening trend for the large cyclone. It is interesting to note that the global models keep Helene a powerful hurricane through the shear. A Hurricane Watch and Tropical Storm Warning remains in effect for the Cabo Verde islands of Santiago, Fogo, and Brava, though these watches and warnings will likely be canceled soon.

Tropical Storm Isaac expected to strengthen into a hurricane soon, likely to pose a threat to the Central Lesser Antilles on Thursday

The ninth named storm of the 2018 Atlantic hurricane season, Isaac, was named Saturday afternoon. As of 11:00 a.m. EDT Sunday, Tropical Storm Isaac was centered near 14.3°N 39.1°W, and was moving westward at about 9 mph. Maximum sustained winds were 55 knots (65 mph), with an estimated minimum pressure of 999 mb. Isaac is gradually strengthening over the tropical Atlantic, and recent microwave imagery indicate that Isaac has developed a well-defined low-level ring. Wind shear of less than 10 knots, the cyclone’s relatively small size and sea surface temperatures above 27°C (80.6°F) favor intensification over the next 48 hours or so, and rapid intensification cannot be ruled out. It is interesting to note that the ECMWF and GFS models show no significant intensification despite the apparently favorable environment. The HWRF model, on the other hand, predicts that Isaac will rapidly intensify into a Category 4 hurricane by Wednesday. HWRF, while one of the most reliable intensity models for tropical cyclones, appears to be an outlier with Isaac’s strength. In about 72 hours or so, wind shear (a result of Hurricane Florence’s outflow) is expected to quickly increase to above 25 knots (30 mph), and this should cause a gradual or perhaps rapid weakening trend. Isaac could be a hurricane as it moves through the central Lesser Antilles on Thursday, but it is also possible that shear increases sooner than anticipated and Isaac quickly weakens before reaching the islands. Both the GFS and ECMWF models dissipate Isaac over the central Caribbean Sea by next Saturday. Air Force Hurricane Hunter aircraft are scheduled to begin investigating Isaac on Wednesday. Interests in the Lesser Antilles should closely monitor the progress of Isaac over the next few days.

Non-tropical low southwest of Azores could acquire tropical or subtropical characteristics this week

The Atlantic has the potential to see four simultaneously active named storms at once. A non-tropical area of low pressure is expected to form southwest of the Azores on Wednesday. This low pressure system could gradually acquire subtropical or tropical characteristics while the low meanders over the central Atlantic. The NHC gives this system a near zero percent chance of development within 48 hours, and a 30 percent chance within five days. The next name on the Atlantic naming list is Joyce.