Hurricane Delta rapidly intensified into a category 4 hurricane over the warm waters of the western Caribbean on Tuesday morning, with the potential to land a devastating blow to Cancun, Mexico, on Wednesday morning. Delta is expected to turn to the north by Thursday and make landfall on the central Gulf of Mexico coast of the U.S. on Friday or Saturday.
Delta, which was just a tropical depression at 5 a.m. EDT Monday, has put on a rare feat of rapid intensification, increasing its winds by 70 mph (from 40 mph to 110 mph) in its first 24 hours since becoming a named storm at 8 a.m. EDT Monday. According to Dr. Phil Klotzbach, this is the most intensification in a 24-hour period for an October Atlantic named storm since Hurricane Wilma in 2005. According to Tomer Berg, Delta did the fastest intensification from a 35-mph tropical depression to a 130-mph category 4 hurricane in modern Atlantic records, accomplishing the feat in just 30 hours. The previous record was Keith in 2000 (42 hours).
At 11:20 a.m. EDT Tuesday, Delta was located about 125 miles southwest of Grand Cayman Island, moving west-northwest at 16 mph with top winds of 130 mph and a central pressure of 954 mb.
Delta was a relatively small hurricane, with hurricane force winds that extended out 25 miles from the center and tropical storm-force winds that extended out 90 miles. Because of this small size, Delta has not been a heavy rain-maker for Jamaica and the Cayman Islands. Rainfall amounts in these islands have generally been less than two inches. As of 11 a.m. EDT Tuesday, the highest two-day rainfall amount on Grand Cayman Island among the eight personal weather stations in the Weather Underground network was 2.08 inches in Georgetown. Delta is the first named storm to move within 120 miles of the Cayman Islands since Hurricane Paloma passed 25 miles to the southeast of Grand Cayman Island as a category 3 storm with 115 mph winds on November 8, 2008.
Delta’s impact in Mexico’s Yucatan Peninsula
The ridge of high pressure steering Delta will keep the storm on a west-northwest to northwest track at a forward speed near 15 mph through Wednesday afternoon; that track would put the storm over the northeastern tip of Mexico’s Yucatan Peninsula near Cancun between 4 a.m. and 8 a.m. EDT Wednesday.
Delta will have nearly ideal conditions for intensification until landfall in Mexico, with wind shear a light to moderate 5 – 15 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 northwestern Caribbean waters having the highest heat content of any waters in the North Atlantic – an ideal setup for rapid intensification. The 12Z Tuesday run of the SHIPS model gave an unusually high probability that Delta would rapidly intensify: a 77% chance that it would intensify by 30 mph by Wednesday morning (7 times higher than the climatological odds), and a 36% chance of increasing by 45 mph (likely putting it at category 5 strength at landfall).
The NOAA Hurricane Hunters were reporting late Tuesday morning that Delta had a tiny “pinhole” eye with a diameter of four miles. Hurricanes with eyes this small are able to rapidly intensify more readily, since there is less mass to accelerate. The Hurricane Hunters found no signs of an outer spiral band wrapping into the core to potentially form a new eyewall and trigger an eyewall replacement cycle (ERC). During an ERC, a common occurrence in intense hurricanes, the inner eyewall collapses and is replaced by a larger-diameter outer eyewall from a former spiral band. This effect often leads to a weakening of the maximum sustained winds by 10 – 20 mph. Given these factors, Delta is likely to continue to intensify until landfall Wednesday morning in Mexico, with category 5 strength at least 30% probable.
Wind, storm surge, and flooding rains will all be significant threats from Delta, and the hurricane may make a direct hit on Cancun (population 900,000), the center of Mexico’s tourism industry. If it does, Delta is likely to be a multi-billion-dollar disaster for Mexico, and potentially the nation’s most expensive hurricane since category 4 Hurricane Wilma of 2005 did $6.5 billion in damage (2020 dollars).
The National Hurricane Center is predicting a storm surge of six to nine feet near and to the right of where the eye makes landfall. If Delta makes landfall slightly to the north of Cancun, the city will mostly be spared the worst of Delta’s storm surge. Likewise, wind damage from the hurricane will be significantly less in Cancun if the city experiences the weaker left eyewall of Delta, instead of the stronger right eyewall. Tidal range in Cancun is only about one foot (0.3 meters) between low tide and high tide, so the timing of Delta’s landfall is not that important with regard to the storm surge impact.
Flash flooding from Delta’s torrential rains is a major concern in the northern Yucatan, which is already waterlogged from the passage of Tropical Storm Gamma, which dumped over 300 mm (11.81 inches) of rain over portions of the peninsula during the past week. NHC is predicting Delta will dump four to six inches of rain over the region, with isolated amounts of up to 10 inches.
Track forecast for Delta as it approaches the U.S.
Once Delta enters the Gulf of Mexico on Wednesday, steering of the storm will be increasingly controlled by a trough of low pressure over the south-central U.S.; the southwesterly winds of the trough will turn Delta sharply to the north or north-northeast on Thursday. The exact timing and location of this turn are crucial for determining where on the U.S. coast Delta will make landfall. The models have been trending farther westwards with their forecasts, keeping eastern Texas in play for a potential landfall, but the most likely landfall location is still Louisiana, with Mississippi and Alabama possible, but less likely.
The NHC cone of uncertainty for Delta includes the cities of New Orleans, Lake Charles, and Gulfport – the sixth time in 2020 this has occurred (with Cristobal, Laura, Marco, Sally, and Beta being the other storms). Thanks go to Heather Marie Zons for this stat.
It now appears that the remnants of Tropical Storm Gamma, located inland over the Yucatan Peninsula, will not have a significant steering influence on Delta. It is possible that Delta could absorb some of Gamma’s spin, which could help Delta expand from its current small size into a medium-sized hurricane.
Intensity forecast for Delta on approach to the U.S.
Passage of Delta over the tip of the Yucatan Peninsula will potentially disrupt the inner core of the small hurricane, resulting in a weakening by a full Saffir-Simpson category. However, the terrain Delta will be passing over is not mountainous, and the hurricane may spend only a few hours over land, resulting in little appreciable weakening.
Once in the southern Gulf of Mexico, Delta will be over shallow continental shelf waters with relatively low heat content; these waters will have also cooled as a result of the passage of Tropical Storm Gamma over the weekend. However, by Wednesday evening, Delta will be back over warm, deep waters with high heat content, and will have this high-octane fuel through Thursday. Wind shear is predicted to be a low 5 – 10 knots through Friday, but ocean temperatures along Delta’s path by then will be cooler, 28 degrees Celsius (82°F), and the atmosphere more dry, to a mid-level relative humidity of 60%.
On Friday, when Delta will be nearing 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 to a high 15 – 30 knots, and dry air to Delta’s west will have an 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.
Sally and Laura both encountered similar conditions when they made landfall earlier this year, and neither storm weakened before landfall. The top intensity models generally predicted on their Tuesday morning runs that Delta would be a weakening category 3 or 4 hurricane on Friday as it approached the U.S. northern Gulf Coast.
Delta could be anywhere between a category 1 and category 3 hurricane at landfall in the U.S., and will likely be a highly destructive multi-billion dollar storm with wind damage, heavy rains, and storm surge all being significant threats.
Gamma weakens, becomes post-tropical before making a second landfall in Mexico’s Yucatan Peninsula
Tropical Storm Gamma weakened to a tropical depression on Monday afternoon, then became post-tropical Monday night before moving inland along the north shore of Mexico’s Yucatan Peninsula on Tuesday morning, bringing very heavy rains over much of the peninsula. Gamma brought five-day rainfall amounts in excess of 300 mm (11.81 inches) over portions of the Yucatan Peninsula, with Cancun receiving over 200 mm (7.87 inches).
Yucatan Peninsula hurricane history
The northeastern Yucatan Peninsula is no stranger to intense hurricanes. In 1988, category 5 Hurricane Gilbert made a double landfall on Cozumel Island and the mainland Yucatan Peninsula with winds of 160 – 165 mph. Gilbert was one of only three category 5 hurricanes on record to hit the Yucatan, the others being Dean in 2007 and Janet in 1955.
The most recent year that a hurricane made landfall in the northeastern Yucatan Peninsula was 2005, when two major hurricanes hit. The first was category 4 Hurricane Emily, which made landfall on July 18, 2005, about 60 miles south of Cancun.
After peaking as the most intense Atlantic hurricane of all-time – a category 5 storm with 185 mph winds and a central pressure of 882 mb – Hurricane Wilma underwent an eyewall replacement cycle and hit Cozumel on October 21, 2005, as a category 4 storm with 140 mph winds and a 926 mb central pressure. Moving very slowly, Wilma made landfall the next day on the Yucatan Peninsula about 15 miles south of the Cancun airport with 135 mph winds and a 933 mb central pressure. Over a 24-hour period, Wilma dumped 1,634 mm (64.33 inches) of rainfall at Isla Mujeres, the highest 24-hour accumulation ever recorded in the Western Hemisphere. Parts of the Yucatán Peninsula experienced tropical storm-force winds for nearly 50 hours, and Wilma brought sustained winds of 100 mph, gusting to 130 mph, to Cancun. A storm surge of up to 15 feet battered the coast, accompanied by 16- to 26-foot waves that reached the third stories of some buildings.
Wilma killed four people and did $6.5 billion (2020 dollars) in damage in Mexico, making it the most expensive hurricane in the country’s history (0.6% of Mexico’s GDP). Wilma left 300,000 people homeless in the city of Cancun, and damaged or destroyed 110 of the hotels in the city.
Editor’s note: this post was updated at 1:15 p.m. October 6, to update Delta’s rapid intensification stats.
Posted on October 6, 2020 (12:53pm EDT).