Tropical activity in the Atlantic typically increases as July turns into August. As usual, a large tropical wave called Invest 97L will build up near Haiti at noon Thursday and move toward the southeastern Gulf of Mexico this weekend. The system appears likely to become the fourth named Storm Debbie this year. Its future is highly uncertain, but models are starting to favor the idea of a long-lived, slow-moving tropical storm or hurricane that could bring significant rainfall to parts of the eastern Gulf Coast and nearby areas. The possibility of a coastal scrape route along or near the southeastern coast of the United States cannot be ruled out at this time.
On Thursday, 97-force winds brought heavy rain to Hispaniola, the southeastern Bahamas and adjacent areas. However, with mid-level relative humidity of only about 55%, the system encounters and draws in too much dry air, preventing these scattered showers and thunderstorms (convection) from coalescing into a more organized system. Otherwise, conditions were favorable for development, with wind shear speeds of 5-15 knots and sea surface temperatures near a record high of 29 degrees Celsius (84°F).
In the Tropical Weather Outlook issued at 2 p.m. ET Thursday, the National Hurricane Center projected that 97L has at least a 30 percent chance of becoming a tropical depression by Saturday and a 70 percent chance by next Thursday. As the weekend approaches, these odds, especially on both days, are expected to rise even further. The first hurricane search flight to 97L is tentatively scheduled for Friday afternoon.
Prediction 97L
97L's route over the next week or more will be determined by three systems, all to its north:
- A powerful ridge of high pressure across the western and central United States
- Strong ridge in the Atlantic Ocean associated with the Azores-Bermuda High
- Weaknesses in between (low pressure trough)
Initially, 97L will be guided primarily by the Atlantic Ridge, which is positioned to travel west-northwest along the Cuba Length Drive system. Along the track, the 97L will encounter increasingly humid conditions, with humidity expected to climb to 65-70% by Saturday. Wind shear should remain light to moderate through the weekend and land interactions with the islands will decrease as 97L moves into the open southeastern bay on Saturday and Sunday. All of these factors will make it easier for 97L to consolidate into a tropical depression or tropical storm.
The main track uncertainty for 97L involves an upper-level trough moving across the Great Lakes on Thursday. The trough will lengthen as it moves into vulnerable areas over the weekend in the Appalachians and East Coast. By early next week, as upper-level high pressure gradually returns, the weakening trough will move away from the U.S. East Coast, potentially slowing the movement of 97L.
Predictive models have been swinging back and forth between two camps, which pose distinct threats:
- scene one: If 97L develops quickly and reaches far enough north by late weekend, an eastern U.S. trough could pull the system into the Florida Gulf Coast and then slowly move northeast along or near the southeastern U.S. coast. As the week unfolds, such a track could bring heavy rain, high seas and other impacts to eastern Georgia and the Carolinas, depending on how far offshore (or how far inland) 97L's track is and how far inland it moves speed.
- Scenario 2: If 97L is slower in intensity and/or develops a bit further south, it is more likely to become trapped in the Gulf early next week as the western/central U.S. ridge forms eastward. This may cause 97L to linger in the northeastern Gulf of Mexico and adjacent land areas for many days, causing a severe threat of heavy rain.
Record-high sea surface temperatures in the eastern Gulf of Mexico (see Figure 1 below) will provide ample moisture for heavy rains. Warmer waters also extend deep into parts of the eastern Gulf, which could help exacerbate the situation's rapid intensification.
Over the past day or so, European and GFS forecast models have been leaning towards Scenario 2 (Eastern Gulf Stall). But these and other models operate very differently. So, at this point, it's best to think of these models as providing very general guidance on what to do. Can Will happen next week, not as any definite prediction. The operation of the model may specifically change until the 97L develops a more centralized circulation center.
By the end of the week, it should become more obvious which of the two scenarios above is more likely to happen. Even beyond this point, weak steering currents, slow motion, and nearshore positioning can make it extremely difficult to accurately predict 97L's track, intensity, and impact.
Carlotta expected to be Northeast Pacific's first hurricane of 2024
After the slowest start on record, hurricane season is finally gaining traction in the Northeast Pacific. As of 11 a.m. ET Thursday, Tropical Storm Carlotta was located about 455 miles west-southwest of Manzanillo, Mexico, with maximum sustained winds of 60 mph. Carlotta is expected to become the first hurricane of the year in the Northeast Pacific on Friday as it moves safely west-northwest away from land and could reach or approach major hurricane strength (Category 3) on Saturday before encountering Cooler waters.
As Phil Klotzbach (Colorado State University) points out, this is only the second time in satellite-era data since 1970 that the Northeast Pacific has not experienced a hurricane in August. The first recently recorded hurricane in the area was Ignacio, which reached hurricane intensity on August 24, 2003.
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