Marco is predicted to scrape the Louisiana coastline on Monday and is the most immediate threat. Hurricane warnings are posted from Morgan City in south-central Louisiana to the mouth of the Pearl River between New Orleans and Gulfport, Miss. A tropical storm warning is up to the west from south of Lake Charles to Morgan City, La.
A storm surge warning has been raised from Morgan City east to near Biloxi, Miss., where ocean water could rise up to six feet above normally dry land. Grand Isle, in far southeastern Louisiana, is under a mandatory evacuation order.
Downtown New Orleans is under a hurricane watch and a tropical storm warning because of Marco and will probably 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 drifts inland, Laura will follow late Wednesday or early Thursday. Parts of Louisiana could be affected by tropical storms twice in three days, for which there is no recorded precedent.
Compared with predictions on Saturday, the track forecast for Laura has shifted west, increasing the threat for western Louisiana and eastern Texas, and decreasing but not eliminating 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 Wednesday, spanning from the Central Texas coastline east toward coastal Mississippi. And while Marco is expected to approach the coast 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 storm threat could tax residents and emergency responders alike, who may already be scrambling to deal with the fallout from Marco as Laura plows ashore, probably 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 make 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 evening, as it spun about 185 miles south-southeast of the mouth of the Mississippi River. It was moving north-northwest at 12 mph.
Doppler radar revealed some of the first bands of rainfall lurking just offshore of coastal Louisiana, Mississippi, Alabama and Florida, while downpours had broken out across the Florida Panhandle.
Sea surface temperatures are abnormally warm over the waters Marco is traversing, conducive to additional strengthening. However, it appeared Marco’s intensity peaked Sunday evening, because its inclination to strengthen due to warm ocean waters was being counteracted by increasing shear — or a change in wind speed and direction with height — that is disruptive to the system.
This shear caused Marco, which briefly attained hurricane strength, to be downgraded to a tropical storm.
Marco is predicted to make landfall along the Mississippi River Delta in Louisiana on Monday as a strong tropical storm. However, the National Hurricane Center called computer model forecasts for its strength “maddeningly inconsistent” and that landfall as a hurricane or substantially weaker storm was also possible.
As Marco moves inland or parallels the coast it is forecast to rapidly weaken and slow down as it tracks westward. The Hurricane Center noted there is some possibility Marco remains offshore and disintegrates into a remnant tropical wave without actually making landfall.
Tropical-storm-force wind gusts could begin along the Louisiana coastline as soon as early Monday morning. If the eyewall moves ashore, the core of the strongest winds surrounding the storm center, damaging gusts to 70 mph or higher 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, maximized near the mouth of the Mississippi River, with two to four feet elsewhere, including as far west as Sabine Pass in Texas and into Mobile Bay, Ala. Lake Pontchartrain in Louisiana is anticipated to see a two- to four-foot surge.
Heavy rainfall of three to five inches can be expected as well, with localized amounts to seven 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.
As the center of Marco could actually parallel the coast and move westward without coming inland, the area susceptible to wind and surge could expand, though the intensity of these impacts will decrease with time.
Conditions may briefly improve Tuesday in Louisiana, when it will be sandwiched between the storms. Then Laura will approach.
Tropical Storm Laura
As Laura briskly swept across Dominican Republic and Haiti on Sunday, it looked alarmingly well-structured on satellite despite being affected by nearby terrain. That meant the storm wasn’t weakening over land as is typically the case. In fact, it had actually intensified some, and its survival puts it in a disconcerting position to strengthen even more upon arriving in the Gulf.
Centered 125 miles southeast of Camaguey, Cuba, at 11 p.m. Sunday, Laura’s maximum sustained winds had increased to 65 mph. A gust to 72 mph had recently been clocked in Guantanamo Bay.
While it pounded Cuba, its heavy rain and mudslide risk were ending in Haiti and the Dominican Republic, where warnings had been dropped but flash flooding occurred earlier in the day.
Laura is being steered west-northwestward by high pressure well off to the northeast, better news for the northern Florida Keys, Turks and Caicos Islands, and most of the southeastern Bahamas, where tropical storm watches had been discontinued.
The entirety of Cuba, however, save for the western tip, is under a tropical storm warning. A tropical storm warning is also in effect for the Cayman Islands.
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 will probably be able to strengthen with fewer limiting factors.
The National Hurricane Center is forecasting Laura to reach 105-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.
The National Hurricane Center is on board with this possibility, writing: “Although not explicitly shown, Laura could threaten the northwestern Gulf coast near major hurricane strength.”
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.
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 the 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 at the same time.