Even as Post-Tropical Cyclone Debby continued to dump flooding rains from the Southeast U.S. to Southeast Canada on Friday, the next tropical threat from the Atlantic was already looming. Computer models predict that a large tropical wave in the central tropical Atlantic is encountering too much wind shear and dry air to develop through Saturday, but the wave will encounter a more favorable environment for development by Sunday and Monday as it approaches the Lesser Antilles Islands.
Saturday morning update: The disturbance was designated Invest 98L by NHC on Friday night. Conditions were marginal for development on Saturday, with high wind shear of 20-25 knots and some dry air to the north. Satellite images showed a modest amount of heavy thunderstorm activity that was clumpy and beginning to show some spin. Conditions for development will improve on Sunday and Monday, with wind shear falling to a moderate 10-20 knots with ocean temperatures a very warm 28.5-29 degrees Celsius (83-84°F). There is good model support for development of 98L into Tropical Storm Ernesto by Tuesday, when it is likely to be moving through the Lesser Antilles Islands.
A sharp turn to the northwest and then north is predicted by most of the models after 98L reaches the vicinity of Puerto Rico and Hispaniola, but the steering pattern is complicated and we cannot not be confident of 98L’s long range track yet (see Tweet by Eric Webb below). In their 8 a.m. EDT Saturday Tropical Weather Outlook of the National Hurricane Center gave the wave two-day and seven-day odds of development of 30% and 80%, respectively.
Original post (Fri. 8/9/24):
The tropical wave, which was not yet organized enough on Friday morning to be given an “Invest” designation by the National Hurricane Center, was centered a few hundred miles west-southwest of the Cabo Verde Islands, headed west at about 15 mph. Satellite imagery showed that the wave lay to the south of a large region of dry air from the Saharan Air Layer, and the disturbance had a limited amount of heavy thunderstorms with only a slight amount of rotation. High wind shear of 15-25 knots was inhibiting development, but the wave was over warm waters of 28 degrees Celsius (82°F). Sea surface temperatures remain 1-2 degrees Celsius (1.8-3.6°F) warmer than average for early August across most of the western tropical Atlantic.
Track forecast for the disturbance
A strong ridge of high pressure associated with the Azores-Bermuda High will steer the disturbance to the west through Sunday, then more to the west-northwest beginning on Monday. On this track, the wave will begin spreading heavy rains in the Lesser Antilles on Monday, with the center of the disturbance likely passing through the Lesser Antilles Islands on Tuesday. Puerto Rico, the Dominican Republic, Haiti, the Turks and Caicos Islands, and the southeastern Bahamas are all likely to see heavy rains from the system from Wednesday through Thursday. At that time, a trough of low pressure passing to the north is likely to steer the disturbance more to the northwest. It is too early to assess the potential threat to the coast of the U.S. and Canada, but the system is a threat to make landfall. A recurving path that stays offshore is also possible.
Intensity forecast for the disturbance
The atmosphere is expected to steadily moisten, and with mostly moderate wind shear and near-record sea surface temperatures, the disturbance has decent model support for development before reaching the Lesser Antilles Islands, though it appeared unlikely it would affect the islands as a hurricane. Disruption of the circulation by passage over the mountains of Puerto Rico and Hispaniola would slow development if the system tracks more to the south, while a more northerly track would allow for a stronger storm.
In their 8 a.m. EDT Friday Tropical Weather Outlook of the National Hurricane Center gave the wave two-day and seven-day odds of development of 0% and 60%, respectively. The first Hurricane Hunter mission into the wave is scheduled for Monday. The next name on the Atlantic list of storms is Ernesto. Based on 1991-2020 data, the average date of the “E” storm (i.e., the fifth named system of an Atlantic season) is August 22. Update: As of 2 a.m. EDT Saturday, Invest 98L had two- and seven-day development odds of 20% and 80%.
Debby’s price tag will be in the billions
Former Hurricane Debby, now a post-tropical cyclone, will wrap up its North American journey on Friday and Saturday as it hurtles across the U.S. Northeast and into Southeast Canada. Post-Tropical Cyclone Debby was centered in north central Pennsylvania at 11 a.m. EDT Friday, racing northeast at 35 mph. A corridor of two to five inches of rainfall, with local amounts topping six inches, is expected near Debby’s path through central and far northern New York and along the St. Lawrence Valley of southeastern Ontario and southern Quebec, Canada, including Montreal. Update: Montreal had its single wettest day by far in more than 140 years of recordkeeping on Friday, as Debby delivered a whopping 152 millimeters (5.98 inches) of rain, smashing the all-time calendar-day record of 93.5 mm (3.68 inches) set on Nov. 8, 1996.
Trailing rains – still enough to trigger flash flooding in some cases – were expected as far south as the coastal Carolinas on Friday. Intense thunderstorms associated with Debbie affected the Washington, D.C., area on Friday morning, with a tornado warning issued.
On Friday afternoon, major flooding from Debby’s rains was occurring at two rivers in North Carolina, one in South Carolina, and one in Georgia. Major flooding was predicted to occur by Monday at three additional rivers in North Carolina, two in South Carolina, and one in Florida. Debby’s rains were predicted to cause moderate river flooding at dozens of rivers across Virginia, Maryland, Pennsylvania, New York, and New Jersey. A storm surge combined with high runoff from heavy rains caused moderate coastal flooding in Annapolis, Maryland, on Friday.
Debby is expected to join the ranks of billion-dollar U.S. hurricanes – in fact, this year’s two Atlantic hurricanes thus far, Beryl and Debby, are both in that category. However, Debby’s damage has been somewhat less than feared, as the projected areas of 20-inch-plus rainfall totals failed to materialize, and no state set its all-time record tropical-cyclone rainfall. The most recent summary of preliminary storm totals includes these state-by-state peaks through 10 a.m. Friday:
Florida: 18.86 inches, Parrish
South Carolina: 18.25 inches, 5.3 miles southeast of Summerville
Georgia: 14.82 inches, 1.4 miles east-southeast of Oliver
Maryland: 6.95 inches, 1 mile south of Damascus
New Jersey: 8.67 inches, 1.7 miles southeast of Pennington
North Carolina: 15.25 inches, Kings Grant
Delaware: 6.57 inches, 5.3 miles north of Newark
Virginia: 7.6 inches, 1.3 miles east-northeast of Afton
West Virginia: 5.98 inches, 12.8 miles south-southwest of Harpers Ferry
Pennsylvania: 5.39 inches, 5 miles north-northwest of Rainsburg
According to Gallagher Re, insured damages for Debby are likely to be $1-$2 billion, with wind-related damages contributing just a few hundreds of millions to that total. Most of Debby’s damage was from flooding, and only about 10-50% of the properties in the most heavily affected areas had flood insurance, so Debby’s total damages (both insured and uninsured) will be much higher than $1-$2 billion.
For the next few days, we will be posting updates on the disturbance threatening the Lesser Antilles Islands within this post.
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