August 22, 2020

Hurricane Watches For Northern Gulf Coast Ahead Of Marco; Laura Not Far Behind


You know how meteorologists always say to check the forecast frequently because things can change in a hurry? Today is why. The National Hurricane Center's forecast for Tropical Storm Marco changed quite a bit from this time yesterday, bringing the storm toward the northern Gulf Coast as a hurricane on Monday, making landfall in almost the same spot they expect another hurricane to make landfall a day or two later.

Two Storms In Two Days


This is an...odd...situation, and it's not one we've dealt with in living memory. Current forecasts show two different hurricanes making landfall on the same stretch of coastline within about two days of each other. It's certainly possible as far as these scenarios go—two different storms taking two different approaches to the same area—but it's one we've been lucky enough not to have encountered before.

All that stuff about the storms merging and the Fujiwhara effect—which I discussed yesterday—seems much less likely than it did a few days ago. The storms should lag each other by at least a day, putting enough space between each storm that they'll both be able to enter the Gulf of Mexico individually without trying to destroy one another or throw the other off-track.

Laura and Marco both originated from tropical waves that rolled off the western coast of Africa a few days apart. The first wave, Marco, took a more southerly track that brought it across the length of the Caribbean Sea, while the second wave, Laura, skirted a little to the north and brought it across the region's northern islands.
Source: Tropical Tidbits

Even though both systems took different paths, their ultimate tracks will be influenced by a large ridge of high pressure sitting off the coast of the southeastern United States. These ridges act like a cross between guard rails and conveyor belts; the ridge prevents the storm from curving out to sea while also guiding it along the outer perimeter of the ridge, which is what'll likely guide both systems into the Gulf Coast early next week.

The forecasts will likely change over the next couple of days. Communities that aren't currently in the predicted path of these two storms shouldn't exhale just yet, especially since the models suddenly lurched Marco's path east this morning and Laura is still a full four days away from land. Lots has changed in the last day and lots more can change in the next four days. 

If you live in or near the paths of these storms, it's important to prepare now so it's not a mad scramble to get groceries or secure property when it's too late and there's a thousand things to do at once. Watching these storms for days at a time can be maddening—things keep changing!—but it's great fortune that we can watch these storms approach land so far in advance. Take advantage of the advanced warnings and prepare for the potential for extended power outages, tree damage, and flooding. Hurricane prep isn't as daunting as it can seem; I wrote up a small list the other day to help you along with your plans.

Tropical Storm Marco —The Gulf One


Tropical Storm Marco entered the Gulf of Mexico late this afternoon, squeezing through the Yucatan Channel without ever having made landfall near Cancun like yesterday's forecasts predicted. The storm followed the outside edge of the cone of uncertainty—which is the historical margin of error in a track forecast—and it's now heading north toward the northern Gulf Coast. 

The Gulf of Mexico is a vat of bathwater right now. Sea surface temperatures across most of the Gulf are hovering in the upper 80s, which is plenty warm enough to allow a storm to take off if environmental conditions allow. While Marco isn't expected to undergo rapid intensification right now, storms can unexpectedly intensify, especially when they're passing over such warm waters. This is why it's so important not to under-prepare for a storm thinking it's "only" going to be a certain strength when it reaches land.

The latest forecast shows the storm reaching hurricane strength before it makes landfall along the Louisiana coast during the day on Monday, slowly working its way inland through the middle of the week. The forecast track can and probably will change a bit over the next couple of days, so don't assume you're out of the woods if you're in or near the cone of uncertainty.

Here are the greatest risks with Marco as it makes landfall:

➤ Winds: Wind damage is likely during Marco's landfall. The winds of a high-end tropical storm or category one hurricane will lead to widespread power outages. The unique situation of having two storms hit the same area at once will make it difficult for crews to safely restore electricity between storms. If you live in an area expecting these storms, anticipate days-long power outages and get enough flashlights, batteries, food, and water to last for the duration.

Source: National Hurricane Center

➤ Storm Surge: Any landfalling storm can generate a life-threatening surge of seawater into the coast. The NHC's calling for a 3-5 foot storm surge to the east of where the storm makes landfall, which right now would stretch from Grand Isle to Mobile Bay.

The surge forecasts will change as the storm draws closer to landfall. The depth of a storm surge depends on a number of factors, including its size and intensity at landfall, as well as the shape and makeup of the coastline it hits and whether landfall occurs during high or low tide.


➤ Flooding: Flash flooding is likely from the storm's heavy rain, even in communities hundreds of miles inland from the point of landfall. The rainfall is going to be exacerbated by two storms hitting the same region. The map above shows this afternoon's 7-day rainfall forecast from the Weather Prediction Center.

➤ Tornadoes: Portions of eastern Louisiana, southern Mississippi, and southern Alabama could see an extended risk for tornadoes this week. As we saw with Hurricane Isaias earlier this month, tornadoes are common when a tropical cyclone makes landfall, especially in the right-front quadrant, which is the eastern side of the storm in this case.

Tropical tornadoes occur quickly and they often come with reduced lead time. Make sure you're always close to safe shelter if you're under threat from this storm, and keep the alerts activated on your smartphone in case your location receives any tornado warnings at night when you're asleep.

Tropical Storm Laura  —  The Caribbean One


Tropical Storm Laura is still chugging along despite interacting with the rough terrain of the Greater Antilles. The system has maximum sustained winds of 50 MPH right now and the latest forecast drags the tropical storm over the length of Hispaniola and Cuba through Monday morning.

These are mountainous islands and Laura isn't exactly a juggernaut right now, so this storm's forecast is much more uncertain than Marco's. It wouldn't take much for the high mountains of these islands to severely disrupt Laura's circulation, making an open question of its future track and intensity. The tropical storm could easily survive its island pass, though, if its inner core misses the islands' roughest terrain, and there's plenty of time and space for the storm to redevelop if it does get disrupted.

Right now, the National Hurricane Center's forecast expects the storm to handily survive its encounter with the mountains and emerge over open waters near the Florida Keys on Monday, where tropical storm watches are in effect tonight. 

If things play out as predicted, Laura will lag behind Marco by about two days, making landfall in the same general vicinity as Marco. The angle of approach would be different this time, which matters for the effects of strong winds and storm surge, but the risk for tornadoes and inland flooding from heavy rain would remain the same.

Laura is still about four days away from the Gulf Coast no matter what it does, so we have more time to watch and the forecast has more time to change. Anyone in Florida or along the Gulf Coast needs to watch this storm's developments closely. As we saw with Marco's change in track between yesterday and today, it doesn't take much of a shift for a situation to change dramatically in either direction.

It's worth noting that the Gulf will be ripe for intensification by the time Laura gets there, whatever state it arrives in. If Laura is healthy when it reaches the Gulf, the combination of low wind shear and bathwater sea temperatures could allow the storm to gather strength in a hurry as it closes in on the coast.

The National Hurricane Center releases fresh forecasts every six hours (at 5 and 11 AM/PM Eastern), with position and intensity updates every three hours in between.

[Satellite: NOAA]

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I graduated from the University of South Alabama in 2014 with a degree in political science and a minor in meteorology. I ran Gawker's The Vane for two years and I've contributed to Mental Floss, Forbes, Popular Science, and the Washington Post's Capital Weather Gang. I also teamed up with Outdoor Life to write a book called The Extreme Weather Survival Manual, which came out in October 2015.

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