25 August 2020

Hurricane Laura enters Gulf of Mexico, Marco makes landfall

Marco was briefly a hurricane as it passed by the western tip of Cuba on Sunday, but began to experience decapitating wind shear after that. By the time it reached the northern Gulf coast on Monday afternoon, it was barely recognizable as a tropical cyclone. But officially, it did make landfall as a tropical storm with 40 mph peak sustained winds right at the mouth of the Mississippi River.  It was much less impactful than feared, which is great because Laura is coming up right behind it, and it means business.

Laura just became the fourth hurricane of the season on Tuesday morning as soon as it left the western tip of Cuba. All signs point to significant intensification in the coming two days before landfall.  When it does make landfall, it will be the seventh named storm to make landfall on the continental U.S. this year (joining Bertha, Cristobal, Fay, Hanna, Isaias, and Marco) so far!

NHC's 11am EDT forecast on Tuesday does bring Laura up to major hurricane status (Category 3) at landfall late Wednesday night. The hurricane and tropical storm warnings as of this advisory are shown below.  The storm surge forecast includes a peak of 9-13 feet in western Louisiana, and lower (but still dangerous) values to the east and west of that.

The global model ensembles are in close agreement on the track -- heading into northern Texas or western Louisiana late Wednesday night into early Thursday morning, and almost certainly as a strong hurricane. In the case of a strong hurricane, the biggest hazard that relies on the exact track is the storm surge -- that will have it maximum just right of center near the eyewall (maximum onshore winds). The timing with the tide cycle also plays a big role. In the area of concern, the tidal range is two feet, so if the peak surge arrives with high tide, the inundation will be two feet more than if it arrives at low tide. In western Louisiana, high tide is expected at 1am on Wednesday night... not good.

Toward the beginning of Laura's life, NHC forecasts were consistently bringing it north too quickly. But over the past couple of days, the forecasts have been essentially unwavering. The map below shows the history of all of NHC's five-day forecasts made since Tropical Depression 13 formed last Thursday.
In terms of environmental influences on Laura's intensity, Marco was too small, too weak, and too quick to have much of an effect on water temperatures in the Gulf; furthermore, Laura is following a different track, so at worst, it would just briefly cross over Marco's wake. The Gulf of Mexico is extremely warm... explosively warm for tropical cyclones. It should experience low to moderate wind shear -- not enough to do serious damage to it. There are indications from rapid intensification guidance that Laura could really get strong in a hurry. One scenario shows an increase of 40 knots in the next day (which would bring it to 105 kt by Wednesday morning). This does not seem unreasonable given the conditions.

In terms of timing, tropical storm conditions are expected to arrive along the northern Gulf coast on Wednesday evening.  In the longer range, there is some model support for Laura maintaining itself as a coherent low pressure system across the southeast US and re-emerging off the mid-Atlantic states on Saturday night.  It may no longer be a tropical cyclone, but could still have tropical storm-force winds as it zips away from the country.

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