A non-tropical area of low pressure was developing on Thursday afternoon near the Southeast U.S. coast along an old frontal boundary, and this low has the potential to evolve into a tropical storm by Saturday as it heads north toward North Carolina. This system was given the designation Potential Tropical Cyclone Sixteen (PTC 16) by the National Hurricane Center (NHC) at 11 a.m. EDT Thursday.
Regardless of whether PTC 16 gets named, it will bring impacts typical of a tropical storm to the coast, and NHC was predicting PTC 16 would make landfall on Saturday morning in eastern North Carolina with top sustained winds of 60 mph. The highest probabilities from NHC of tropical storm force winds (sustained winds of at least 39 mph) were in North Carolina, including 59% at New River, 57% at Cape Hatteras, and 54% at Morehead City. The latest NOAA quantitative precipitation forecast calls for two to five inches of rain along the coast from North Carolina to eastern Massachusetts for the five-day period ending at 8 a.m. EDT Tuesday, September 26. Totals of more than two inches could drench the Washington, D.C., area as well.
An extended period of strong onshore winds from PTC 16 will bring a storm surge of one to four feet to a swath of the coast from the North Carolina-South Carolina border to southern New Jersey. The highest storm surge, predicted to be about four feet by the NOAA Extratropical Storm Surge 2.2 model, is predicted to cause major coastal flooding Saturday at three Virginia tide gauges in southern Chesapeake Bay. Moderate flooding is predicted at 18 tide gauges in Virginia, Delaware, and New Jersey, and minor flooding is predicted from Cape Hatteras, North Carolina, to northern New Jersey, including central portions of Chesapeake Bay (Figure 2). The highest water levels will occur near the time of high tide; at Norfolk, Virginia (at the Sewells Point tide gauge), the greatest flooding is predicted during the 4 p.m. EDT Saturday high-tide cycle (Figure 3).
Conditions for development of PTC 16 are mixed. Though the atmosphere is reasonably moist and sea surface temperatures are very warm — near 29 degrees Celsius (84°F) — wind shear is a high 30-35 knots, and wind shear is predicted to be a very high 30-45 knots through Saturday.
In its 11 a.m. EDT Thursday advisory, the National Hurricane Center gave PTC 16 two-day and seven-day development odds of 60%. If development does occur, it would most likely lead to a tropical storm late Friday or Saturday. If it were designated as a subtropical storm, the system would still be named and impacts would be similar. The next name on the Atlantic list of storms is Ophelia.
New African tropical wave a concern for the Caribbean
A tropical wave that emerged from the coast of Africa on Wednesday was located just west of the Cabo Verde Islands early Thursday afternoon. This system was headed west at about 10-15 mph and is likely to develop into a tropical depression early next week.
This weekend, the tropical wave is predicted to interact with an area of disturbed weather several hundred miles to its west that is embedded in the Intertropical Convergence Zone. The result of this interaction is very difficult to predict at this time and will be crucial for determining the ultimate track and intensity of the tropical depression that is expected to form. If the interaction is relatively swift, and the tropical wave ends up moving westward at a relatively fast pace, the ridge of the pressure to its north is likely to force the wave to the west, resulting in a threat to the Lesser Antilles Islands by Wednesday. This was the solution offered by multiple runs of the GFS model in recent days, although the recent 6Z Thursday forecast has backed off on this idea. A more prolonged interaction between the wave and the ITCZ disturbance, potentially resulting in a slower forward speed, is likely to result in a more northwesterly track and a weaker system that would pass several hundred miles to the northeast of the Leeward Islands by the middle of next week. This is the solution that has been advanced by the European model and a number of the ensemble members of the GFS model.
If the wave stays to the south of the track that Hurricane Lee and Hurricane Nigel took, there are record-warm waters with a high heat content, capable of supporting rapid intensification if dry air and high wind shear do not interfere with development. Water temperatures are about 30 degrees Celsius (86°F) near the Leeward Islands, about 2 degrees Celsius above average.
In its 8 a.m. EDT Thursday Tropical Weather Outlook, the National Hurricane Center gave 99L two-day and seven-day development odds of 20% and 70%, respectively. The next name on the Atlantic list of storms after Ophelia is Philippe.
Nigel on the downswing in the remote North Atlantic
Hurricane Nigel has embarked on what’s expected to be a gradual weakening as it spins harmlessly in the North Atlantic about 500 miles southeast of Cape Race, Newfoundland. At 11 a.m. EDT Thursday, Nigel was a Category 1 hurricane with top sustained winds of 85 mph, moving northeast at 30 mph. Nigel peaked as a Category 2 storm with top sustained winds of 100 mph late Tuesday. As Nigel accelerates northeastward on Thursday, it will quickly encounter much stronger wind shear as well as sea surface temperatures below the standard threshold for supporting a tropical storm (26 degrees Celsius or 79 degrees Fahrenheit). NHC predicts that Nigel will become post-tropical by Friday as it interacts with a mid-latitude storm system.
NHC has been tracking one or more named Atlantic storms around the clock for more than a month now — since August 20 — except for a span of just 30 hours on September 4-5, between the dissipation of former Tropical Storm Gert and the formation of Tropical Storm Lee, which ended up going to Category 5 strength. It’s likely that any break between Nigel and a future Ophelia would be a brief one, keeping the NHC forecasters busy for at least another week.