August 24, 2020

Marco Approaches Louisiana Monday, Laura Threatens Gulf Landfall On Wednesday


It's late and it'll be even later by the time I get this post up, so I'll dispense with the drawn-out intro and get right to it. I'm sure you won't mind.

Tropical Storm Marco  —  The Gulf One


Marco had a brief stint as a hurricane on Sunday afternoon, but wind shear buffeting the storm from the southwest put a stop to that. There's not much difference between a 70 MPH tropical storm and a 75 MPH hurricane, but our silly human brains like artificial boundaries and round numbers, so that's an unnecessarily big distinction for lots of people.

The National Hurricane Center's forecast changed a bit on Sunday, bringing the storm just to the coast before turning it west. If this forecast were to play out exactly as shown, it'd be a 50/50 shot whether or not the center of the storm comes ashore at all. It doesn't really matter, though, because it could have some disruptive impacts in an area that can't bear any disruptions right now while it awaits Laura in a few days.

Strong winds will likely lead to power outages that'll be tough for crews to restore before Laura approaches the area. Even areas that see light damage might have extended power outages simply due to the timing of the two storms and the need to keep power crews safe and out of harm's way until both storms are gone.
Source: National Hurricane Center
Coastal flooding will be an issue in areas that see an extended period of onshore winds. The NHC's 10:00 PM CDT advisory shows the potential for a storm surge of 4-6 feet between Morgan City and the Mouth of the Mississippi River, with potential surge depths tapering off to the west and east of there. That's a life-threatening surge that could easily wash out roads and inundate structures right along the coastline. Don't mess with surge—but if you live right along the coast (and you're reading me, hi!), you probably don't need to be told that.

Regarding the threat for storm surge and wind damage, there's this note from the NHC:
"It is worth noting that Marco is a small tropical cyclone. The large area of Tropical Storm and Hurricane watches and warnings along the northern Gulf Coast is a reflection of the unusually high uncertainty in the forecast, and it is unlikely that all of those regions will experience tropical-storm-force winds or life-threatening storm surge associated with Marco."
Marco's tropical storm force winds only extend 70 miles from the center of circulation, and the strongest winds only encompass a small portion of that wind field. Given its small size, the exact track of the storm will influence its effects.

Flash flooding from heavy rain is a concern near the coast, especially if the storm starts paralleling just offshore, which might slow the weakening trend more than if the core of the storm was fully over land. Any flooding issues from Marco will be compounded by the rains of Laura; many areas won't have time to fully drain the excess runoff before the second storm arrives in the middle of the week.

Marco should slowly wind-down and head off toward Texas through the middle of the week, setting the stage for Laura's arrival.

Tropical Storm Laura  —  The Caribbean One


Laura, much like Isaias a few weeks before it, really went out of its way to avoid getting torn apart by the mountains of the Greater Antilles. The latest advisory actually strengthened Laura's winds even as the center of the storm was over eastern Cuba. If the system can survive the rest of its encounter with the mountainous island nation, it'll be in a sturdier position to take root and grow in strength when it hits the Gulf of Mexico.

Not too much has changed regarding the system's track in the last day. Anyone from Houston to Mobile needs to be on high alert and fully prepared to take action just in case the storm ventures off its predicted track. Forecasters will adjust Tropical Storm Laura's track and intensity over the next few days as they get better data and the storm settles into its post-mountains routine.

My greatest fear with this system is that folks who experienced Marco on Monday will go "ah, that was nothing" and they'll be tempted to ignore Laura. The Gulf is steamy right now and the atmosphere is expected to become favorable for intensification by the time this storm arrives in the region. The NHC, which errs on the side of caution when it comes to quick intensification, notes that Laura might be near major hurricane strength (category three) when it nears the coast on Wednesday.

[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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