Hurricane warnings are up for Mexico’s Yucatan Peninsula, including Cozumel and Cancun, as Tropical Storm Zeta heads northwest across the western Caribbean toward an expected Monday night landfall in Mexico as a category 1 hurricane. Update: at 3:10 p.m. EDT Monday, Zeta was upgraded to a category 1 hurricane with 80 mph winds and a central pressure of 981 mb, based on data from an Air Force Reserve Hurricane Hunter aircraft.

Zeta is predicted to move into the Gulf of Mexico on Tuesday morning and make a second landfall in southeastern Louisiana as a category 1 hurricane or strong tropical storm on Wednesday.

If it makes landfall in Mexico’s Yucatan Peninsula, Zeta will be the third Greek-named storm to do so this month. Tropical Storm Gamma made landfall near Tulum on October 3 with 70 mph winds, and Hurricane Delta hit Cancun on October 7 as a category 2 storm with 110 mph winds.

If Zeta makes landfall in hurricane-weary Louisiana, it will be the record-breaking fifth landfall in a single season by a named storm in the state: Earlier this year, category 2 Hurricane Delta, Tropical Storm Cristobal, Tropical Storm Marco, and category 4 Hurricane Laura all hit the state. The current record for most landfalls in a single season in Louisiana is four, which 2020 shares with 2002, when Tropical Storm Bertha, Tropical Storm Hanna, Tropical Storm Isidore, and Hurricane Lili all hit the state.

At 11 a.m. EDT Monday, Zeta had top sustained winds of 70 mph, just below hurricane strength, and was headed northwest at 10 mph. The storm was spreading heavy rains over Jamaica, Cuba, and the Cayman Islands, as seen on Cayman Islands radar. Over the past two days, Zeta has brought widespread rainfall amounts of one to four inches to the Cayman Islands, Jamaica, Cuba, and northern Honduras. At 1:10 p.m. EDT Monday, winds at buoy 42056, located 140 miles southeast of Cozumel, Mexico, were sustained at 49 mph, gusting to 69 mph, with significant wave height of 22 feet.

Though Zeta’s winds were just below hurricane strength, satellite images early Monday afternoon showed the storm was struggling: Zeta did not have a well-defined core with an eyewall, and its center of circulation was nearly exposed to view, with all of its heavy thunderstorm activity restricted to the southeast side of the circulation. This condition was occurring because of upper-level winds out of the north-northwest injecting dry air into Zeta’s core, and creating 10-15 knots of wind shear. Water vapor satellite imagery showed Zeta had good upper-level outflow on all sides, but dry air over the Gulf of Mexico was apparent, and that dry air has the potential to continue inhibiting development in the coming days.

Track forecast for Zeta

A ridge of high pressure to the northeast will keep Zeta on a northwesterly track at a speed of 10-15 mph through Tuesday afternoon,  carrying Zeta over the northeastern Yucatan Peninsula between 7 p.m. and midnight CDT Monday night, and then into the Gulf of Mexico on Tuesday morning. By Tuesday night, an approaching trough of low pressure over the central U.S. is expected to turn Zeta more to the north and then north-northeast, with a landfall expected along the central Gulf Coast on Wednesday afternoon or evening.

If Zeta is a moderate-strength tropical storm in the northern Gulf of Mexico, an outcome the European model and its ensembles suggested on Monday morning, a landfall in central Louisiana is more likely, since Zeta will be steered more by the low-level flow. That flow will tend to take the storm to the north. If stronger, at hurricane strength, Zeta will “feel” the upper-level winds out of the southwest from the approaching trough. In that case, it will track more to the north-northeast, resulting in a landfall farther east, in southeast Louisiana – or perhaps as far east as Alabama.

Intensity forecast for Zeta

Zeta will have favorable conditions for intensification up until landfall Monday night in Mexico, with light to moderate wind shear of 5-15 knots, ocean temperatures of 29-30 degrees Celsius (84-86°F), and a moist atmosphere with a mid-level relative humidity of 65%. Zeta will be passing over the northwest Caribbean with the highest heat content of any waters in the North Atlantic – a strong basis for rapid intensification.

However, for it to develop in these conditions, Zeta needs to overcome its current poor structure and get rid of its tilted vortex. The wind shear affecting the storm is expected to relax late Monday afternoon, which should allow Zeta to align itself vertically, close-off an eye, and undergo some modest intensification. That said, the storm will not have much time before making landfall, so it is unlikely to be stronger than a category 1 hurricane with 90 mph winds at landfall in Mexico Monday night.

Most of the top intensity models predicted with their Monday morning runs that Zeta would range between a strong tropical storm with 65 mph winds to a category 1 hurricane with 85 mph winds from now until landfall occurs Wednesday along the U.S. Gulf Coast. If Zeta makes landfall as expected on Monday night in Mexico, considerable disruption to the storm likely will reduce the top winds by 10-20 mph, and Zeta may then need a day or so to reorganize and re-intensify over the Gulf of Mexico.

The 12Z Monday run of the SHIPS model gave good odds that Zeta would undergo rapid intensification: a 36% chance that it would intensify by 30 mph by Tuesday morning (three times higher than the climatological odds), and a 20% chance that it would intensify 50 mph by Tuesday night, into a category 3 hurricane (four times higher than the climatological odds).

On Tuesday night and Wednesday, when Zeta 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 over the Gulf could wrap into its core. However, a strong band of upper-level winds to the north of Zeta will provide a more efficient upper-level outflow channel as the storm approaches the coast, helping to counteract the increased shear.

Also see: Delta is record-setting 10th named storm to make U.S. landfall in a season

Sally, Laura, and Delta all encountered similar conditions when they made landfall earlier this year; Sally and Laura did not weaken before landfall, but Delta did. Expect Zeta to behave more like Delta, since the ocean temperature structure of the Gulf is similar to what Delta encountered. A reasonable uncertainty range for Delta’s winds at landfall in the U.S. is 60-85 mph (strong tropical storm to category 1 hurricane strength).

Zeta will be moving rapidly at landfall in the U.S., with a forward speed of 20-25 mph, which will limit the amount of rainfall. The current NHC forecast calls for two to four inches of rain in the U.S., with isolated amounts of up to six inches – an unusually low amount of rainfall for a landfalling category 1 hurricane.

Editor’s note: This post was updated at 3:20 p.m. EDT October 26, with Zeta’s upgrade to hurricane status.

Website visitors can comment on “Eye on the Storm” posts (see below). Please read our Comments Policy prior to posting. (See all EOTS posts here. Sign up to receive notices of new postings here.)

Posted on October 26, 2020 (1:07pm EDT).