Hurricane Laura powered its way to major hurricane status overnight, putting on an impressive display of rapid intensification over the very warm waters of the Gulf of Mexico.

Laura is headed towards a landfall expected Wednesday night or early Thursday morning in northeastern Texas or western Louisiana as a major category 4 hurricane, and is expected to cause “catastrophic” wind and storm surge damage, according to the National Hurricane Center (NHC). Rain squalls from Laura’s outer spiral bands were already affecting the coasts of Texas and Louisiana on Wednesday morning, and they will increase in intensity throughout the day.

Laura rapidly intensified by an impressive 50 mph in the 24 hours ending at 11 a.m. EDT Wednesday, with the winds rising from 75 mph to 125 mph and the pressure falling from 990 mb to 956 mb. This far exceeds the definition of rapid intensification, which is a 24 mb drop in 24 hours. Buoy 42395, located just east of Laura’s eye on Wednesday morning, reported sustained winds of up to 76 mph, wind gusts as high as 107 mph, and a wave height of 37 feet (11 meters).

At 11 a.m. EDT Wednesday, Laura was already generating a storm surge of 1 – 3 feet along much of the Texas and Louisiana coasts; the largest surges, between 2.5 – 3 feet, were at Shell Beach, Louisiana, located to the southeast of New Orleans, and Freshwater Canal Locks, on the south-central coast of Louisiana. Laura’s storm surge can be tracked using the Trabus Technologies Storm Surge Live Tracker or the NOAA Tides and Currents page for Laura.

Laura continues to rapidly intensify

At 11 a.m. EDT Wednesday, Laura was located over the waters of the central Gulf of Mexico, about 235 miles southeast of Galveston, Texas. These waters were a very warm 30 degrees Celsius (86°F). Laura was headed northwest at 15 mph with top sustained winds of 125 mph and a central pressure of 956 mb, putting it just 5 mph below category 4 strength.

Satellite images and data from the Hurricane Hunters showed that Laura has closed off a large eye about 30 miles in diameter. Intense thunderstorms with very cold cloud tops surrounded the eye and extended high into the atmosphere. The eye had not yet fully cleared out, which likely will occur by Wednesday afternoon as the hurricane continues to intensify.

High-level cirrus clouds were streaming out to the east and south of Laura, indicating good upper-level outflow on that side. Upper-level outflow was steadily improving to the north and west, showing that Laura was establishing a second outflow channel that connected up with the trough of low pressure over the central U.S. This improved outflow structure will help Laura intensify further on Wednesday afternoon. Laura was embedded in a moderately dry region of the atmosphere, with a mid-level relative humidity of 60%, but this dry air was not hindering the hurricane anymore, as the moderate wind shear of 10 – 15 knots affecting the storm was not high enough to drive the dry air into the well-developed inner core.

Landfall near the Texas/Louisiana border imminent

There’s not much mystery on where Laura is going. The hurricane has made its expected turn to the northwest, and is headed toward a landfall near the Texas/Louisiana border around midnight Wednesday night. After landfall, Laura will turn to the north, following steering currents from a trough of low pressure over the central U.S. The rapidly weakening storm will then turn to the east on Friday, passing through the Tennessee Valley on its way to the mid-Atlantic coast, where it will move out to sea by Sunday.

Intensification expected until just before landfall

Ocean temperatures are a very warm 30 degrees Celsius (86°F), and Laura will be passing over Gulf waters with very high heat content on Wednesday. Conditions for intensification will be very favorable until four to six hours before landfall, when strong upper-level winds from the trough of low-pressure steering Laura will bring a high 20 – 25 knots of wind shear and likely halt the intensification process. Interaction with land may also slow intensification at that time. Data from the Hurricane Hunters late Wednesday morning showed that Laura might be starting to develop concentric eyewalls, a process common in intense hurricanes, which leads to a temporary halt in intensification when the double eyewalls become fully developed. This process could slow down Laura’s intensification by Wednesday night. The top dynamical intensity models – the HWRF, HMON, and COAMPS -predicted in their 0Z and 6Z Wednesday runs that Laura would be a category 4 hurricane at landfall, with the 6Z COAMPS forecast calling for a category 5.

Laura a catastrophic storm surge threat

Laura will drive a massive catastrophic storm surge to the coast, with the 11 a.m. EDT Wednesday NHC advisory calling for a peak surge of 15 – 20 feet along the western Louisiana coast. The magnitude of this storm surge will depend not only on how strong the winds are, the speed the storm is traveling, and the angle at which it approaches the coast, but also on the size of the storm. A large storm with winds blowing over a wide area of ocean will typically generate a higher storm surge that covers a larger area than a smaller hurricane.

The 11 a.m. EDT Wednesday NHC forecast called for Laura’s tropical storm-force winds to span a diameter of 220 nautical miles (nm) at landfall. For comparison, Hurricane Rita in 2005 – a low-end category 3 hurricane with 115 mph winds which drove a 10 – 15-foot storm surge to the southwest Louisiana coast – had tropical storm-force winds that spanned a diameter of 300 nm. Hurricane Ike in 2008 – a high-end category 2 hurricane with 110 mph winds that drove a storm surge of 15 – 20 feet to Texas’ Bolivar Peninsula – had a tropical storm-force wind field 390 nm across. (Thanks to weather.com’s Jon Erdman for this information) Given that Laura likely will be a much stronger storm than Rita at landfall, but smaller in areal size, a peak storm surge of 15 – 20 feet – larger than the 10 – 15-foot storm surge Rita brought to southwest Louisiana (see below) – is a reasonable forecast.

Inundation levels also will depend on whether Laura strikes at high or low tide. Tidal ranges are low in this part of the Gulf, however. The tidal range at Lake Charles, Louisiana is 1.5 feet, and high tide is at 6:09 a.m. CDT Thursday.

An impact greater than Hurricane Rita’s in 2005 likely

After peaking as a category 5 storm with 180 mph winds and a central pressure of 895 mb – the fourth lowest pressure ever measured for an Atlantic hurricane – Hurricane Rita weakened to a category 3 storm with 115 mph winds before making landfall in western Cameron Parish, Louisiana, near the Texas border, on September 24, 2005. Rita caused an estimated $25.2 billion in damage, making it the eleventh-costliest hurricane in U.S. history. Seven direct deaths were blamed on the storm, and the chaotic evacuation of Houston was blamed for 107 deaths.

When a hurricane threatens, don’t underestimate storm surge risk

The small town of Cameron (population 2,000 and elevation three feet) was the largest town along the stretch of southwest Louisiana coast that received Rita’s peak storm surge, estimated to be 10 – 15 feet. The storm surge destroyed 90% of the homes in Cameron, and destroyed all of the structures in Holly Beach (population 300), with the only human-made features remaining after the storm being power poles, concrete slabs, and roads. Rita’s surge penetrated more than 30 miles inland, reaching Interstate 10. The surge, combined with freshwater flooding, inundated downtown Lake Charles, located 30 miles from the ocean, up to six feet deep. Rita also produced a storm surge of 4 – 7 feet in coastal areas of southeastern Louisiana, flooding some spots that had already been impacted by the surge from Hurricane Katrina about one month earlier.

Posted August 26, 2020, at 12:35 p.m. EDT.

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