Tropical Storm Delta formed in the western Caribbean on Monday morning with the potential to rapidly intensify into a dangerous hurricane that will affect the Cayman Islands, northeast Mexico, and western Cuba on Tuesday and Wednesday and the central Gulf of Mexico coast of the U.S. on Thursday and Friday.

At 11 a.m. EDT Monday, Delta was headed west-northwest at seven mph, spreading heavy rain showers over Jamaica, Cuba, and the Cayman Islands, as seen on Cayman Islands radar. Satellite images showed Delta had nearly closed off a cloud-free center, and the storm was likely beginning to build an eyewall. Delta’s heavy thunderstorm activity was steadily growing more intense and expanding in areal coverage.

The first hurricane hunter mission into Delta is to occur Monday afternoon, and the first dropsonde mission by NOAA’s jet is scheduled for Tuesday afternoon.

The 2020 parade of record-early named storms continues

Delta’s October 5 arrival marks the earliest date that any Atlantic season has produced its 25th tropical storm, topping the record held by Gamma from November 15, 2005. In total, 22 of 2020’s 25 named storms set records for being the earliest-arriving for their respective letter; only Arthur, Bertha, and Dolly fell short.

With the Atlantic hurricane season three-quarters done, we’ve already had 25 named storms, eight hurricanes, two intense hurricanes, and an accumulated cyclone energy (ACE) index of 108 (26% above average for the date). Only one Atlantic hurricane season since 1851 has had more named storms during an entire season: 2005, with 28 named storms. According to Colorado State University hurricane scientist Phil Klotzbach, the averages for this point in the season are nine named storms, five hurricanes, two intense hurricanes, and an ACE index of 86.

Track forecast for Delta

The ridge of high pressure steering Delta will keep the storm on a west-northwest to northwest track at a forward speed between 7 – 16 mph through Wednesday, which will put the storm very near Grand Cayman Island between 6 and 10 a.m. EDT Tuesday, and near the western tip of Cuba or near the northeast tip of Mexico’s Yucatan Peninsula after 8 p.m. EDT Tuesday night.

Beginning on Tuesday night, Delta will draw close enough to Tropical Storm Gamma for the two storms to interact. When two tropical cyclones approach within about 900 miles of each other, they tend to rotate counter-clockwise around a common center, then go their separate ways, in a process called the Fujiwara effect.

In rare cases they may merge into one storm, but the resulting storm will not be stronger than either of the original two storms, as wind shear from each weakens the other. In the case of Delta and Gamma, Delta is likely to sling Gamma inland to the southeast over the Yucatan Peninsula, and Gamma is likely to push Delta on a more westerly track, closer to Texas. However, with Gamma now expected to make landfall in the Yucatan Peninsula on Tuesday, the storm may be small and weak enough that it will have only a negligible steering effect on Delta.

Once Delta enters the Gulf of Mexico on Wednesday, steering of the storm will be increasingly affected by a trough of low pressure over the south-central U.S.; the southwesterly winds will turn Delta sharply to the north or north-northeast on Wednesday night or Thursday morning. The exact timing and location of this turn are crucial for determining where on the U.S. coast Delta will make landfall. This turn cannot be predicted with accuracy this far in advance, as it will depend upon how much interaction Delta and Gamma have, and models make such forecasts poorly.

Intensity forecast for Delta

Delta will have nearly ideal conditions for intensification through Wednesday morning, with wind shear less than five knots, ocean temperatures of 30 degrees Celsius (86°F), and a moist atmosphere with a mid-level relative humidity of 70 – 75%. Delta will be passing over waters in the northwest Caribbean that have the highest heat content of any waters in the North Atlantic – an ideal setup for rapid intensification.

The top five intensity models predicted with at least one of their Monday morning runs that Delta would intensify into a hurricane by Wednesday, with a category 1 hurricane the dominant prediction. However, the 0Z Monday run of the top intensity model from 2019, the COAMPS-TC, predicted that Delta would peak as a category 4 hurricane on Wednesday in the southern Gulf of Mexico, and then weaken on approach to Louisiana.

The 12Z Monday run of the SHIPS model gave an unusually high probability that Delta will undergo rapid intensification: a 62% chance that it would intensify 75 mph by Thursday morning (12 times higher than the climatological odds). The official National Hurricane Center forecast called for Delta to achieve hurricane status on Tuesday afternoon, and become a category 2 hurricane by Wednesday morning. A reasonable uncertainty range for Delta’s intensity on Tuesday night and early Wednesday morning, during its closest approach to western Cuba and northeastern Mexico, is as a category 1 to category 3 hurricane.

By Thursday, when Delta will be approaching the U.S. Gulf Coast, the storm will encounter more hostile conditions for intensification. Waters beneath the storm will be significantly cooler, wind shear will rise, and dry air to the west of Delta will have the opportunity to wrap into its core. However, a strong band of upper-level winds to the north of Delta will provide a more efficient upper-level outflow channel as the storm approaches the coast, helping counteract the increased shear.

Both Sally and Laura encountered similar conditions when they made landfall earlier this year, and neither storm weakened before landfall. Delta could be anywhere between a strong tropical storm and a category 3 hurricane at landfall in the U.S., with the official NHC forecast of a category 2 hurricane with 100 mph winds a reasonable prediction.

The role of ocean heat content in Delta’s intensification

In the Gulf of Mexico, the deepest warm water is found in the Loop Current – an ocean current that transports warm Caribbean water through the Yucatan Channel between Cuba and Mexico. The current flows northward into the Gulf of Mexico, then loops southeastward just south of the Florida Keys (where it is called the Florida Current), and then goes just west of the westernmost Bahamas. From there, the waters of the Loop Current flow northward along the U.S. coast and become the Gulf Stream. During summer and fall, the Loop Current provides a deep (80 – 150 meter) layer of very warm water that can provide a huge energy source for a hurricane to rapidly intensify into a major hurricane.

The Loop Current commonly bulges out in the northern Gulf of Mexico and sometimes will shed a clockwise rotating ring of warm water that separates from the main current. This ring of warm water slowly drifts west-southwestward towards Texas or Mexico at between two and three miles per day. This feature is called a “Loop Current ring”, “Loop Current eddy”, or “warm core ring”, and it can provide a key source of energy to fuel rapid intensification of hurricanes that cross the Gulf (in addition to the Loop Current itself).

Also see: Extreme events ‘presage worse to come’ in a warming climate

In 2020, fortunately, no warm Loop Current eddy is present in the central Gulf (though there is a weak warm eddy in the western Gulf). Instead, a prominent counter-clockwise rotating cool eddy lies in the central Gulf. The current track forecast calls for Delta to pass along the southern edge of that cool eddy, which may slow the intensification process. Once Delta approaches the U.S. Gulf Coast, the storm will encounter another region of cool waters caused by cool air flowing over the Gulf over the past week in the wake of the passage of the season’s first major cold front. Near the Florida-Alabama border, waters have also been cooled by the passage of Hurricane Sally in mid-September.

Gamma expected to make a second landfall in Mexico’s Yucatan Peninsula

Tropical Storm Gamma made landfall in Mexico’s Yucatan Peninsula early Saturday afternoon with top winds of 70 mph, just below hurricane strength, bringing heavy rains to portions of the Yucatan Peninsula, western Cuba, and Central America.

According to Conagua, 24-hour rainfall amounts of 11.41 inches of rain were recorded at Tizimin, and 9.63 inches at Cozumel. At 11 a.m. EDT Monday, Gamma was over the southern Gulf of Mexico, just off the coast of the Yucatan Peninsula, with top winds of 45 mph, moving south-southwest at 2 mph.

Forecast for Gamma

Conditions for development of Gamma will be unfavorable through Wednesday, with high wind shear of 15 – 30 knots and a dry atmosphere (a mid-level relative humidity of 50 – 60%). The top intensity models and the official NHC forecast all predict slow weakening of Gamma through Wednesday. This process will be hastened if Gamma makes landfall over the Yucatan Peninsula on Tuesday, as predicted by NHC.

Gamma, caught in an area of weak steering currents, is expected to move at less than 5 mph for the remainder of its existence, resulting in heavy rains across much of Mexico’s Yucatan Peninsula and portions of Central America.

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Posted on October 5, 2020 (1:21 pm EDT).