• Sat. Dec 5th, 2020

Hurricane Marco forms in Gulf of Mexico; Tropical Storm Laura a formidable threat to US – Yale Climate Connections

Satellite image Marco and Laura
Visible GOES-16 satellite image of Hurricane Marco (left) and Tropical Storm Laura (right) at 12:50 p.m. EDT Sunday, August 23. (Image credit: NOAA/RAMMB)

The Hurricane Hunters found winds of 75 mph in Marco on Sunday morning, and at 11:30 a.m. EDT the National Hurricane Center upgraded Marco to hurricane status. Marco is the third hurricane of the 2020 season, arriving over two weeks before the average day of arrival of the season’s third hurricane, September 9.

At 2 p.m. EDT Sunday, August 23, the center of Marco was located about 280 miles south-southeast of the mouth of the Mississippi River in Louisiana, headed north-northwest at 14 mph with a central pressure of 992 mb. Marco had adequate conditions for intensification, with sea surface temperatures (SSTs) near 30.5 degrees Celsius (87°F), moderate wind shear of 15 – 20 knots, and a drying atmosphere with a mid-level relative humidity of 60%. Satellite images showed Marco to be a small storm, and small storms can intensify and weaken quickly.

Figure 1
Figure 1. Predicted path of Marco from the 12Z Sunday, August 23 runs of the top hurricane tracking models used by NHC. The official NHC forecast is the heavy black line. Potential landfall locations were clustered over Louisiana. (Image credit: weathermodels.com)

Marco expected to hit Louisiana as tropical storm or category 1 hurricane

Marco will continue to experience adequate conditions for intensification through Monday afternoon. Thereafter, wind shear is expected to rise, potentially causing Marco to weaken to a strong tropical storm by the time of landfall on Monday evening in Louisiana. A stronger storm is more likely to track farther to the east, near the Mississippi/Louisiana border, while a weaker storm is more likely to turn more to the northwest with the low-level flow, and make landfall in western Louisiana.

A small storm like Marco can experience large changes in intensity quickly, so a sudden ramp-up in wind shear could weaken it abruptly on Monday afternoon and cause the storm to shift towards western Louisiana.

How might Marco affect Laura?

Marco is expected to bring a storm surge of 4 – 6 feet along portions of the Louisiana coast, and it is likely that coastal water levels will still be elevated there by the time Laura passes by (see more on Laura below). Typically, it takes several days for storm surge levels to subside – longer if the storm brings heavy rains over a large area causing a great deal of runoff. As a relatively small storm not expected to drop prodigious rains over a large area, it is a good bet that Marco will leave coastal water levels elevated by 1 – 2 feet when Laura arrives, creating a modest increase in the storm surge threat in Louisiana from that storm.

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. It appears that Marco and Laura will be too far apart, and Marco will be too small, to have such an interaction.

It is exceptionally unusual to have two tropical storms or hurricanes approaching the northern Gulf Coast within two days of each other, as it appears Marco and Laura will. In fact, there are only two other times on record when the Gulf of Mexico has had two named storms at the same time. In 1933, two hurricanes made U.S. landfall within 24 hours, but they were much further apart: one struck near Brownsville, Texas, while the other hit the east coast of Florida. No other U.S. hurricanes have made landfall within 60 hours of each other, according to Phil Klotzbach (Colorado State University).

Figure 2
Figure 2. Radar image of Tropical Storm Laura taken at 1:01 p.m. EDT Sunday, August 23. Curved bands (yellow and orange colors) from a developing inner core were present between Cuba and Haiti. (Image credit: U.S. Navy)

Tropical Storm Laura a formidable threat to U.S.

Tropical Storm Laura brought torrential rains on Sunday to the nations of the Dominican Republic and Haiti, which share the island of Hispaniola, after the storm made landfall in the eastern Dominican Republic Saturday evening.

The center of Laura passed over the Dominican Republic’s capital, Santo Domingo, near 12:30 a.m. EDT Sunday, bringing heavy rain and wind gusts to 40 mph. A personal weather station in Santo Domingo recorded 5.74” from Laura as of 2 p.m. EDT Sunday, and another station just southwest of the city recorded 9.29”. In Haiti, a personal weather station in the capital of Port-a-Prince recorded 6.45″. The city recorded wind gusts up to 39 mph.

Laura brought flooding rains and a few high wind gusts to Puerto Rico on Saturday. At 2:09 p.m. EDT Sunday, Camp Santiago, located about five miles inland from the south coast of Puerto Rico, recorded sustained winds of 52 mph, gusting to 75 mph, when a powerful mesoscale vortex from Laura passed overhead.

At 2 p.m. EDT Sunday, August 23, Laura was located in the waters between Haiti and Cuba, about 55 miles south of the eastern tip of Cuba. Laura was headed west-northwest at 21 mph with top sustained winds of 50 mph and a central pressure of 1004 mb. Radar from Guantanamo Bay, Cuba, showed that Laura quickly began to build an inner core in the waters between Cuba and Haiti early on Sunday afternoon, but this structure will be disrupted once the center moves over the high mountains of eastern Cuba late Sunday afternoon.

Satellite images on Sunday showed Laura as a well-organized storm. The cloud pattern showed an impressive symmetry, with high-level cirrus clouds that streamed out in all directions, indicating excellent upper-level outflow. Laura’s thunderstorms were intense with very cold cloud tops, another indication of good organization. Laura was embedded in a moderately dry region of the atmosphere, with a mid-level relative humidity of 60%, but the light to moderate upper-level wind shear of 5 – 15 knots helped keep the dry air from substantially affecting the storm.

A track over Cuba, then into the Gulf

The Bermuda high, steering Laura, will be strong and will extend far to the west, steering the storm generally to the west-northwest over the next four days, and the Sunday morning runs of the models continued to indicate a track that will take Laura over Cuba on Sunday and Monday, with the storm emerging into the Gulf of Mexico on Monday night.

Figure 3
Figure 3. The 6Z Sunday, August 23, forecast of the COAMPS-TC model, which made the best 2-, 3-, 4-, and 5-day intensity forecasts of any model in 2019, predicted Laura would traverse Cuba, reorganize over the Gulf of Mexico, and peak as a category 3 hurricane shortly before landfall Wednesday night in Louisiana. The COAMPS-TC was the third-best track model in 2019, behind the European model and UKMET model. (Image credit: Naval Research Laboratory)

Intensification likely once Laura gets into Gulf of Mexico

When Laura emerges into the Gulf of Mexico, conditions for intensification are expected to be very favorable. Ocean temperatures are a very warm 30 – 31 degrees Celsius (86 – 88°F) across much of the Gulf of Mexico, and an upper-level high pressure system with light winds will be present, bringing light to moderate wind shear and excellent upper-level outflow. The atmosphere will be somewhat dry, with a mid-level relative humidity of 60%, but with that light wind shear, dry air is unlikely to present a significant impediment to intensification. Tropical Storm Marco will be too small and too far inland to bring increased wind shear from its upper-level outflow.

It will take a day or so for Laura to recover after a long traverse over land. This recovery may be slowed by the possibility that Laura will expand in size as it traverses Hispaniola and Cuba – something that happened to Hurricane Ike when it took a similar path in 2008. When a storm’s core is over land, its outer portions tend to develop since the inner core cannot, leading to an expansion of the hurricane’s size.

A large storm takes a long time to spin up, but generates a larger storm surge than its Saffir-Simpson wind scale classification might suggest. Ike ended up bringing a devastating storm surge characteristic of strong category 3 hurricane when it made landfall in Texas on the Bolivar Peninsula as a high-end category 2 storm. If Laura behaves similarly, it may be a large storm that takes considerable time to spin up, but delivers a formidable storm surge to the coast.

On Monday night and Tuesday morning, Laura will be passing over the deep, warm waters of the Loop Current (see below), but will not be sufficiently organized to take full advantage. By Tuesday afternoon, Laura will likely be crossing over a cool eddy in the Gulf with limited heat content, which will make rapid intensification unlikely. However, the storm will have a long stretch of warm waters with high heat content Tuesday night until landfall Wednesday night or Thursday morning, and rapid intensification during that time is a good possibility. The 12Z Sunday runs of the HWRF and HMON intensity models predicted that Laura would peak as a category 4 hurricane; the 6Z Sunday run of the COMAPS-TC model predicted a category 3 peak. Residents along the Gulf Coast should anticipate the possibility that Laura will rapidly intensify right up until landfall, potentially reaching major hurricane status.

Figure 4
Figure 4. Predicted path of Laura from the 6Z August 23 run of the European model’s 51 ensemble members (colored lines, which show minimum central pressure). Model members showed a wide range of potential landfall locations along the U.S. Gulf Coast, mostly in Texas and Louisiana. (Image credit: weathermodels.com)

Where will Laura go?

The NHC cone of uncertainty should probably be wider than depicted for Laura, since model solutions for the storm have a wide spread covering much of Texas and Louisiana for potential landfall locations. The trend in the models has been to build the Bermuda high steering Laura farther to west, pushing the storm more westward, towards Texas.

The best-performing model for Laura so far has been the UKMET model, which at all times has out-performed all other models and the official NHC forecast. The UKMET model’s 12Z Sunday forecast predicted a landfall near the Texas/Louisiana border Wednesday night as an intensifying hurricane. It appears that steering currents will keep Laura moving after it comes ashore, so it is unlikely to stall out and produce the type of catastrophic inland rains generated by Harvey in 2017. Instead, storm surge would more likely be the greater threat with Laura.

Figure 5
Figure 5. Ocean Heat Content (OHC) levels in the Gulf of Mexico on August 23, 2020. The warm waters of the Loop Current lay just off the northwest coast of Cuba, and Laura is expected to pass over this high heat content water before encountering a cool eddy with limited heat content in the central Gulf. OHC values higher than 75 kilojoules per square centimeter (light green colors) are highly favorable for rapid intensification of hurricanes. Near-shore waters along the continental shelf are too shallow to have a relevant number for OHC, and are left white in this image. (Image credit: University of Miami Rosenstiel School of Marine and Atmospheric Science)

The role of ocean heat content in Laura’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.

With current speeds flowing at about 1.8 mph (0.8 m/s), the Loop Current is one of the fastest currents in the Atlantic Ocean. The current is about 200 – 300 km (125 – 190 miles) wide, and 800 meters (2,600 feet) deep, and is present in the Gulf of Mexico about 95% of the time. 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 any hurricanes aspiring to become rapidly intensifying major hurricanes.

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 about 2 – 3 miles per day. This feature is called a “Loop Current ring,” “Loop Current eddy,” or “warm core ring,” and can provide a key source of energy to fuel rapid intensification of hurricanes that cross the Gulf (in addition to the Loop Current itself).

Satellite imageOcean heat as ‘fuel’ for hurricanes

The Loop Current pulsates in a quasi-regular fashion and sheds rings every 6 to 11 months. When a Loop Current eddy breaks off in the Gulf of Mexico at the height of hurricane season, it can lead to a dangerous situation where a vast reservoir of energy is available to any hurricane that might cross over. That is what occurred in 2005, when a Loop Current eddy separated in July, just before Hurricane Katrina passed over and “bombed” into a Category 5 hurricane. The eddy remained in the Gulf and slowly drifted westward during September. Hurricane Rita passed over the same Loop Current eddy three weeks after Katrina, and also explosively deepened to a Category 5 storm.

In 2020, fortunately, no warm Loop Current eddy is present in the central Gulf (though there is a warm eddy in the western Gulf, southeast of the Texas/Mexico border). Instead, a prominent counter-clockwise rotating cool eddy lies in the central Gulf. Passage of Laura over this cool eddy may slow the intensification process. However, once Laura leaves the cool eddy, the storm will have a long stretch of waters with high ocean heat content until landfall, and rapid intensification is a good possibility.

Bob Henson assisted with writing content for this posting. 

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Topics: Weather Extremes