• Thu. Nov 26th, 2020

Hurricane warnings issued as Gulf Coast prepares for back-to-back strikes from Marco and Laura – The Washington Post

A storm surge warning has been hoisted from Morgan City east to near Biloxi, Miss., where the ocean water could rise up to 6 feet above normally dry land. Grand Isle, in extreme southeast Louisiana, is under a mandatory evacuation order.

Downtown New Orleans is under a hurricane watch and a tropical storm warning because of Marco and is likely to contend with tropical-storm-force winds and heavy rain, which could cause flash flooding. The city’s levee system, however, should protect it from the predicted surge.

After Marco sweeps inland, Laura will follow late Wednesday or early Thursday. Parts of Louisiana could be affected by hurricanes twice in three days for which there is no recorded precedent.

Compared with predictions on Saturday night, the track forecast for Laura has shifted west, increasing the threat for western Louisiana and eastern Texas, and decreasing the threat for New Orleans. Houston should pay particular attention to Laura.

The track forecast for Laura remains uncertain, and landfall is plausible over a wide zone, spanning from the Central Texas coastline east toward coastal Mississippi. And while Marco is expected to come ashore as a Category 1 hurricane, there is an increasing risk that Laura could rapidly intensify into a more dangerous storm, rated Category 2 or higher.

Laura is not only likely to be a more intense storm than Marco at landfall, but also substantially larger, bringing impacts over a much broader area.

The dual hurricane threats could tax residents and emergency responders alike, who may already be scrambling to deal with the fallout of Marco as Laura plows ashore, likely at even greater intensity, all in the midst of the coronavirus pandemic. The National Hurricane Center warned of a “prolonged period of hazardous weather” for areas affected by both storms.

The successive blows could be most problematic with regard to storm surge inundation, with a second spike in water levels coming just as the first rise may be subsiding.

The shape of the seafloor and slope of the continental shelf along the Gulf Coast makes that area very prone to hazardous storm surge flooding even from low-end tropical systems.

Additionally, areas that experience heavy rainfall from both storms will be particularly prone to flooding.

Tropical Storm Marco

Tropical Storm Marco had sustained winds of 70 mph late Sunday morning, nearing hurricane strength as the small but potent storm spun about 325 miles south-southeast of the mouth of the Mississippi River. It was moving north-northwest at 14 mph.

On satellite, Marco was blistering with strong convection — or shower and thunderstorm activity — with updrafts so strong that they penetrated into the second layer of the atmosphere, known as the stratosphere. That’s a signal of a powerful storm system, and one whose strength will have the potential to be highly problematic once it hits land.

Marco blossomed on Saturday, intensifying over some of the hottest ocean waters in the world. Sea surface temperatures are near 90 degrees in the western Gulf of Mexico, conducive to additional strengthening. However, this intensification may be halted or even reversed by an abrupt increase in wind shear, or a change of wind speed and/or direction with height.

Because Marco is a small storm, it is subject to rapid changes in strength from these competing forces, but it is predicted to make landfall along the Mississippi River Delta in Louisiana on Monday as a Category 1 hurricane. As it moves inland, on a northwestward track across southern Louisiana, it is forecast to rapidly weaken and slow down.

Tropical-storm-force wind gusts could begin along the Louisiana coastline as soon as early Monday morning. As the eyewall moves ashore, the core of the strongest winds surrounding the storm center, damaging gusts to 80 mph could affect some coastal locations.

Those strong winds will help push ashore a “life-threatening” storm surge, according to the National Hurricane Center. The surge could reach up to four to six feet in southeastern Louisiana, with two to four feet elsewhere, including as far west as Sabine Pass, Tex., and into Mobile Bay, Ala. Lake Pontchartrain is anticipated to see a two- to four-foot surge.

Heavy rainfall of three to six inches can be expected, as well, with localized amounts to eight inches. The intense tropical downpours could cause localized inland flooding. There is also the risk of an isolated tornado or waterspout, as is often the case with landfalling tropical systems.

Although the most likely track forecast for Marco brings it into southeast Louisiana on Monday, there is still substantial uncertainty in its exact landfall location, and some model simulations suggest it could come ashore somewhat farther west.

Conditions may briefly improve Tuesday in Louisiana, when it will be sandwiched between the storms. Then Laura will approach.

Tropical Storm Laura

On Sunday morning, Laura, currently located over the Dominican Republic and Haiti, looked alarmingly well-structured on satellite despite being affected by nearby terrain. That means the storm isn’t weakening over land as much as once thought, giving it a head start on restrengthening once it emerges over the gulf.

Centered 70 miles northwest of Port Au Prince, Haiti, at 11 a.m. Sunday, Laura had maximum sustained winds of 50 mph. It was bringing heavy rain and a mudslide risk to Haiti and the Dominican Republic. Four to eight inches of rain with localized one-foot totals is leading to flooding, and the mudslide threat is greatest in Haiti, where rampant deforestation has left much of the hill landscape barren and unstable.

Laura is being steered west-northwestward by high pressure well off to the northeast, and the entirety of Cuba, save for the western tip, is under a tropical storm warning. A tropical storm watch is up also for the Florida Keys, which could be brushed by some wind and rain on Monday.

While Laura remains over land, its intensification prospects will be held in check, but once it emerges over the warm water of the Gulf of Mexico early Tuesday, it may be able to strengthen unfettered.

The National Hurricane Center is forecasting Laura to reach 100 mph Category 2 intensity on Wednesday over the Gulf of Mexico.

However, there is a chance that Laura may attain major hurricane status as a Category 3 or stronger hurricane, the tropical system set to be located in an environment favorable for rapid intensification due to the unusually warm ocean waters combined with weak upper-level winds.

There is a chance that the upwelling of colder waters left in the wake of Marco could make a tiny dent in Laura’s propensity to strengthen when their paths intersect, but this effect is likely to be minimal because Marco is such a small storm.

Laura is something to watch even for residents of Houston in case the track continues shifting westward.

Below-average forecast confidence exists associated with Laura, particularly regarding the system’s strength, toward midweek. Residents along the Gulf of Mexico, especially in central/northeast Texas and Louisiana, should stay tuned to the changing forecast in the days ahead.

Historical perspective

Both Laura and Marco are exceptional storms, the earliest L and M systems in the Atlantic on record. They are the latest dominoes to fall in a season that has already featured the earliest C, E, F, G, H, I, J, and K tropical storms and hurricanes. With already more tropical systems than in an average year, the season has been twice as active as average.

Five named tropical systems have made landfall along U.S. shores in 2020. If Laura and Marco follow suit, as forecast, 2020 will break the record for most continental U.S. landfalls in a single year.

If Laura and Marco churn through the Gulf of Mexico simultaneously, it will mark just the third time on record that two storms coexisted there. The other two times were in September 1933 and June 1959, according to Colorado State University hurricane researcher Phil Klotzbach. If both storms manage to become hurricanes in the Gulf of Mexico at the same time, it would be a first. However, the storms may be spread far enough apart that they end up not being in the gulf together at the same time.