PTC 14 Eyeing Central Gulf Coast

Odds were rising on Saturday that a disturbance in the Caribbean dubbed 91L will strengthen as it moves northward across the Gulf of Mexico toward the central Gulf Coast. The disturbance is located just north of Honduras, on the northeast end of a large, complex low called a Central American gyre (CAG) that was sprawled across Guatemala and Nicaragua on Saturday afternoon. Update (6:30 PM EDT Saturday): 91L has been upgraded to Potential Tropical Cyclone 14 (PTC 14). The PTC designation, introduced in 2017, is for systems that are not yet at depression strength but that may intensify to produce tropical storm or hurricane conditions over land within 48 hours. A tropical storm warning was in effect for Pinar del Rio and Isle of Youth, Cuba, and a tropical storm watch is in effect for the east coast of Mexico's Yucatan Peninsula from Tulum to Cabo Catoche. The National Hurricane Center now predicts that PTC 14 will approach the northern U.S. Gulf Coast at close to hurricane strength on Wednesday. The next name in the Atlantic storm list is Michael. (The discussion below was posted early Saturday afternoon, before the PTC 14 designation.)

The broad CAG has already brought torrential rains, flooding, and landslides to Costa Rica, with totals of 12” – 16” reported from Thursday into Friday. Flooding was reported in the vicinity of Puntarenas, Paquera, and Lepanto, and landslides were reported in San Ramón de Alajuela. Rainfall amounts since midnight Friday night in Costa Rica were in the 2”- 3” range.

91L was still fairly disorganized on Saturday afternoon. Thunderstorms scattered across the northwest Caribbean were just beginning to consolidate around the weak circulation center taking shape north of Honduras.

Figure 1. Surface wind estimates from the ASCAT scatterometer on Saturday morning, October 6, 2018. A center of circulation was beginning to form north of the Honduras coast, but surface winds remained weak, generally less than 20 mph. Image credit: NOAA/NESDIS.

91L likely to affect the Gulf Coast as a hurricane or tropical storm

There is strong model agreement that 91L will become a named storm approaching the central Gulf Coast later next week. Ensemble model output from 00Z Saturday is close to unanimous that 91L will track across the Gulf of Mexico as a tropical cyclone. The 00Z run of the European model brought 91L toward the Mississippi/Alabama coast late Wednesday or early Thursday, whereas the 00Z GFS and UKMET runs had a more easterly track toward the Florida Big Bend. The 12Z Saturday GFS run came in close to the previous European solution. Given the large steering features in place, there is high confidence that 91L will take a general north to north-northeast track from the Yucatan Channel across the Gulf, but it’s too soon to nail down the specifics of the track and any potential landfall.

Vertical wind shear is strong (around 25 knots), especially toward the north of 91L, where a strong subtropical jet stream is predicted to shift northward. If 91L moves slowly enough, then the subtropical jet could help support and ventilate the storm rather than tearing it apart. Overall, the shear affecting 91L is predicted to gradually decrease into the moderate range of 10 – 20 knots by early next week. Moreover, 91L will be passing over very warm water untouched by recent tropical storms or hurricanes. Sea surface temperatures are running between 29°C and 30°C (84-86°F) across the northwest Caribbean and eastern Gulf, which is 1-2°C (2-4°F) above average for this time of year. The disturbance will likely be north of the very deep oceanic heat content over the Northwest Caribbean before it becomes very organized, but it could take advantage of deep oceanic heat in the Gulf (see Figure 2 below).

Figure 2. Ocean Heat Content (OHC) levels in the Gulf of Mexico for October 6, 2018. 91L is predicted to pass over a cool eddy with low heat content on Monday after it passes through the Yucatan Channel between Cuba and the Yucatan Peninsula. This may slow its development. But on Tuesday, 91L may pass over a warm Loop Current eddy with high heat content, and if the system is well organized when it passes over the warm eddy, rapid intensification is possible. Image credit: University of Miami Rosenstiel School of Marine and Atmospheric Science.

The majority of GFS and European ensemble members predicted 91L would reach hurricane strength across the Gulf, and the GFS, UKMET, and European operational model runs all suggested that 91L could approach the coast as a strong tropical storm or a Category 1 hurricane. Coastal residents should not be complacent about the possibility of a stronger storm, though. The 06Z and 12Z Saturday runs of the HWRF—one of our top intensity models—predicted that 91L could ramp up to at least Category 2 strength shortly before landfall.

Other CAGs that became named storms

Tropical cyclones that form from CAGs are mainly a heavy rain threat, and typically do not become stronger than Category 1 hurricanes. One year ago, on October 4, a similar CAG gave birth to Tropical Depression Sixteen (TD 16) in the Southwest Caribbean off the coast of Costa Rica. TD 16 would later become Hurricane Nate, which made landfall on Louisiana’s Mississippi Delta as a Category 1 storm on October 7. Nate brought torrential rains and devastating flooding to Costa Rica, causing $562 million in damage (1% of their $57 billion GDP)--their most expensive tropical cyclone in history. Nate also did $225 million in damage in the U.S. The name Nate was retired after the 2017 season because of its impact on Central America, where 46 people died.

One other CAG from 2017 evolved into a named storm—Tropical Storm Cindy, which formed in the southern Gulf of Mexico near the northern Yucatan Peninsula. Cindy made landfall near the Texas/Louisiana border on June 22 with top winds of 50 mph, causing minimal damage. A weather.com write-up on CAGs contains a list of five other named storms that were spawned by or interacted with a CAG since 2005.

Figure 3. Natural-color satellite image of Hurricane Sergio as of 1820Z (2:20 pm EDT) Saturday, October 6, 2018. Image credit: tropicaltidbits.com.

Elsewhere in the tropics: multiple landfalls next week

Apart from 91L, at least two other tropical cyclones are poised to make landfall next week.

—More than 1000 miles west of the Mexican coast, Hurricane Sergio is proving resilient as a major hurricane. As of 11 am EDT Saturday, Sergio’s top sustained winds were 125 mph, keeping it in the Category 3 range. Global models agree that Sergio will carry out a sharp U-turn from Sunday into Monday, then accelerate toward an expected landfall on the west coast of Baja California around Thursday. By that point, Sergio will likely be a weakening tropical storm, but it could still trigger very heavy rains in far northwest Mexico and the Southwest U.S. late next week, much as Rosa did early this week.

—A system dubbed Invest 99A, located in the Arabian Sea west of the Indian coast, will be heading westward as an intensifying cyclone, perhaps reaching hurricane strength. The last several runs of the GFS model have brought 99A into the coast of Oman around Thursday, whereas the European model has been leaning toward a landfall in Yemen closer to Friday. Tropical cyclones are not very common on either coast, but the last several years have brought several destructive ones, including Chapala—which became Yemen’s first hurricane-strength landfall in November 2015—and Mekenu, which struck Oman as a Category 3 storm in May 2018, causing 31 deaths.

—Tropical Storm Leslie continues to spin in the Central Atlantic miles about 600 miles northeast of Bermuda. Leslie’s top winds were holding at 60 mph as of 11 am EDT Saturday. Leslie is not the most classically structured storm, with a large shield of showers and thunderstorms (convection) positioned well north of a more compact core. The midlatitude jet stream north of Leslie will be sagging south over the next several days, shunting Leslie toward the east-southeast under increasing wind shear. Long-range track models diverge greatly on whether Leslie will continue toward the Canary Islands as a weakening cyclone (GFS and FV3) or dip southward sooner (UKMET and European) and then accelerate toward the northeast, perhaps affecting the Azores.

Dr. Jeff Masters contributed to this post.