Post update on 8/19:

Hurricane Genevieve was on the decline Wednesday after peaking as a category 4 hurricane with 130 mph winds on Tuesday. At 11 a.m. EDT Wednesday, August 19, 2020, Genevieve was a category 3 hurricane with 115 mph winds, headed north-northwest at 9 mph. Genevieve’s decline may have been the result of the storm’s crossing over cool waters left by the passage of Hurricane Elida over the same region a week earlier.

Genevieve is expected to pass about 50 miles to the west of the southern tip of Mexico’s Baja Peninsula Wednesday night through Thursday morning. Since hurricane-force winds extend out 35 miles from the center, a Hurricane Warning is up for the southern tip of Baja, in case Genevieve performs a slight jog to the east. The hurricane is expected to bring dangerous surf, 3 – 6 inches of rain, and tropical storm-force winds to the southern tip of Baja. By Thursday, Genevieve will encounter significantly cooler waters, which should result in rapid weakening, and it is expected to dissipate by Sunday.

An Air Force hurricane hunter aircraft is scheduled to investigate Genevieve Wednesday afternoon.

Tropical wave 97L in the central Caribbean

A tropical wave in the central Caribbean, designated 97L by NHC, was headed west at 15 – 20 mph – a speed fast enough to impart a shearing action on the disturbance, impeding development. The system was embedded in a moderately dry atmosphere with a mid-level relative humidity of 60%, which was also interfering with development. Otherwise, conditions for development were favorable, with sea surface temperatures (SSTs) near 29 degrees Celsius (84°F) and light to moderate wind shear of 5 – 15 knots. Satellite images showed that 97L had a modest amount of heavy thunderstorm activity that had shown little change in organization since Tuesday.

Forecast for 97L

As 97L progresses west to west-northwestward over the next few days, its forward speed will slow, which should allow increased chances of development. By Friday, when 97L is expected to slow its forward speed to 10 – 15 mph and be in the northwest Caribbean, very favorable conditions for development may exist. The 12Z Wednesday run of the SHIPS model predicted that the atmosphere surrounding the system would moisten to a relative humidity of 75%, wind shear would be a light 5 – 10 knots, and SSTs would be a very warm 30 degrees Celsius (86°F). The waters of the western Caribbean have the highest heat content of any place in the Atlantic, providing ample fuel for any tropical cyclone that may spin up there.

Despite these seemingly favorable conditions, the 0Z Wednesday runs of the three best models for predicting tropical cyclone genesis – the European, GFS, and UKMET models – showed little or no development of 97L through Sunday. The system is likely to enter the Gulf of Mexico this weekend, and it is expected to be a threat to the Gulf Coast of Mexico or of the U.S. early next week. In an 8 a.m. EDT Wednesday Tropical Weather Outlook, NHC gave 97L two-day and five-day odds of formation of 40% and 80%, respectively. An Air Force hurricane hunter aircraft is scheduled to investigate 97L Thursday afternoon, if needed.

Tropical wave 98L in the central tropical Atlantic

A large tropical wave in the central Atlantic, designated 98L by NHC, was located about 1,000 miles east of the Lesser Antilles Islands at 8 a.m. EDT Wednesday, and was steadily growing more organized.

The wave will be moving west-northwest at 15 – 20 mph this week. Conditions for development of 98L were favorable, with SSTs near 28 degrees Celsius (82°F) and light wind shear of 5 – 10 knots. The system was embedded in a moist region of the atmosphere with a mid-level relative humidity of 70%. Satellite images showed that 98L had an impressive amount of heavy thunderstorm activity that had consolidated around the western side of the complex area of disturbed weather earlier in the week. The system had plenty of rotation at mid-levels of the atmosphere, but no sign of a well-developed surface circulation.

A threat to the Leeward Islands, Virgin Islands, Puerto Rico, and beyond

Now that 98L has consolidated somewhat, the system should be able to organize more rapidly. In addition, its consolidation has made the future track clearer, with the west-northwest motion of 98L likely taking the system into or very close to the Leeward Islands Friday evening through Saturday.

In an 8 a.m. EDT Wednesday Tropical Weather Outlook, NHC gave 98L two-day and five-day odds of formation of 90%. The 0Z Wednesday runs of the three best models for predicting tropical cyclone genesis – the European, GFS, and UKMET models – all showed little or no development of 98L into a tropical depression or tropical storm this week, which is a bit mystifying, given the healthy appearance of the system and favorable conditions for development. If 98L does end up developing as NHC expects, the system will be capable of intensifying quickly. The 6Z Wednesday run of the HWRF model predicted that 98L would develop into a tropical storm on Thursday afternoon, move through the northern Leeward Islands early Saturday morning as a category 1 hurricane, and affect the Virgin Islands and Puerto Rico as a category 2 hurricane on Saturday afternoon and evening.

Residents of the Leeward Islands should expect heavy rains from 98L beginning Friday night, with impacts spreading to the islands farther west Saturday and Sunday. A threat to the U.S. next week is certainly a strong possibility, as long-range model runs are showing that the Bermuda High, which is steering 98L, will be strong and will extend far to the west. The system may have to contend with high wind shear from a trough of low pressure over the U.S. next week, however, and that might interfere with intensification. A NOAA hurricane hunter aircraft is scheduled to investigate 98L Thursday evening.

The next two names on the Atlantic list of storms are Laura and Marco. The earliest twelfth storm in Atlantic tropical history was Luis on August 29, 1995. There is a tie for earliest thirteenth storms, with Lee on September 2, 2011, and Maria on September 2, 2005.

Next up: a new tropical wave over Africa

A strong tropical wave located over extreme western Africa on Wednesday morning has the potential to develop into a tropical depression early next week. The new wave is predicted to move off the coast of Africa on Thursday, then move west to west-northwest at 15 – 20 mph over the tropical Atlantic, passing to the north of the Lesser Antilles Islands around Wednesday, August 26. In an 8 a.m. EDT Wednesday Tropical Weather Outlook, NHC gave the new African tropical wave two-day and five-day odds of formation of 0% and 20%, respectively.

Original Post on 8/18

Hurricane Genevieve put on an impressive show of rapid intensification Monday afternoon through Tuesday morning, exploding into a category 4 hurricane with 130 mph winds by 11 a.m. EDT Tuesday, August 18, 2020. Genevieve is tied with the northeast Pacific’s Hurricane Douglas as the most intense tropical cyclone observed in the Western Hemisphere so far this year.

Genevieve had near-ideal conditions overnight for rapid deepening, with light wind shear less than 5 knots, a very moist atmosphere, and sea surface temperatures (SSTs) of 29 – 30 degrees Celsius (84 – 86°F). These favorable conditions will persist through Wednesday morning, when, the National Hurricane Center (NHC) has predicted, Genevieve will peak with 150 mph winds. Genevieve could grow stronger than that, becoming a category 5 storm with 160 mph winds, though an eyewall replacement cycle could interrupt its intensification process before that can occur.

Genevieve will track northwestward this week, passing on Wednesday evening close to the southern tip of Baja California, where a Tropical Storm Warning was posted. Genevieve is expected to bring dangerous surf, 1 – 4 inches of rain, and tropical storm-force winds to the southern tip of Baja, but it will pass far enough offshore to spare the region of hurricane-force winds. By Thursday, Genevieve will encounter significantly cooler waters, which should result in rapid weakening, with it expected to dissipate by Sunday.

Tropical wave 97L in the eastern Caribbean

A tropical wave in the eastern Caribbean, designated 97L by NHC, brought heavy rain showers and wind gusts near tropical storm-force to the southern two-thirds of the Lesser Antilles island chain on Monday afternoon through Tuesday morning.

On Tuesday, 97L was headed west at about 20 mph – a speed fast enough to impart a shearing action on the disturbance, impeding development. Otherwise, conditions for development were favorable, with sea surface temperatures near 29 degrees Celsius (84°F) and moderate wind shear of 10 – 15 knots. The system was embedded in a moderately dry atmosphere with a mid-level relative humidity of 60%. Satellite images showed that 97L had a modest amount of heavy thunderstorm activity that was beginning to show signs of organization, though dry air was significantly impeding the process.

Forecast for 97L

As 97L progresses west to west-northwestward over the next few days, it will slow its forward speed, which should allow increased chances of development. By Friday, when 97L is expected to slow its forward speed to 15 mph and be in the western Caribbean, very favorable conditions for development may exist. The 12Z Tuesday run of the SHIPS model predicted that the atmosphere surrounding the system would moisten to a relative humidity of 75%, wind shear would be a light 5 – 10 knots, and SSTs would be a very warm 29.5 degrees Celsius (85°F). The waters of the western Caribbean have the highest heat content of any place in the Atlantic, providing ample fuel for any tropical cyclone that may spin up there.

The 0Z Tuesday runs of the three best models for predicting tropical cyclone genesis – the European, GFS, and UKMET models – had one model, the GFS, predict development on Friday, when 97L is expected to be in the western Caribbean, approaching Mexico’s Yucatan Peninsula. In an 8 a.m. EDT Tuesday Tropical Weather Outlook, NHC gave 97L two-day and five-day odds of formation of 20% and 50%, respectively. No hurricane hunter missions into 97L were scheduled.

Tropical wave 98L in the central tropical Atlantic

A large and complex tropical wave in the central Atlantic, designated 98L by NHC, was located about 900 miles west-southwest of the Cabo Verde Islands at 8 a.m. EDT Tuesday.

The wave will be moving west to west-northwest at 15 – 20 mph this week. Conditions for development of 98L were favorable, with sea surface temperatures near 28 degrees Celsius (82°F) and moderate wind shear of 10 – 15 knots. The system was embedded in the Intertropical Convergence Zone (ITCZ), a moist region of the atmosphere with a mid-level relative humidity of 75%. Satellite images showed that 98L had an impressive amount of heavy thunderstorm activity, but the system was complex, with multiple regions of spin and heavy thunderstorms along an axis nearly 1,000 miles long. The multiple clumps of thunderstorms were competing with each other, slowing development.

Forecast for 98L

The future track of 98L will depend heavily on when and where a well-defined center of circulation develops. That prospect is very uncertain, given the complex nature of the wave on Tuesday. A general west to west-northwest motion can be expected over the next several days, with a stronger system that consolidates earlier more likely to track farther to the north. The 0Z Tuesday runs of the three best models for predicting tropical cyclone genesis – the European, GFS, and UKMET models – all supported intensification of 98L into a tropical depression or tropical storm by Friday, when it nears the Leeward Islands. In an 8 a.m. EDT Tuesday Tropical Weather Outlook, NHC gave 98L two-day and five-day odds of formation of 70% and 90%, respectively. Residents of the Leeward Islands should expect heavy rains from 98L on Friday and Saturday, with impacts potentially spreading to the islands farther west Saturday and Sunday.

The next two names on the Atlantic list of storms are Laura and Marco. The earliest twelfth storm in Atlantic tropical history was Luis on August 29, 1995. There is a tie for earliest thirteenth storms, with Lee on September 2, 2011, and Maria on September 2, 2005.

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