Hurricane Laura was slowly intensifying on Tuesday afternoon over the very warm waters of the Gulf of Mexico, as the storm headed towards an expected Wednesday night or early Thursday morning landfall in Texas or western Louisiana as a major hurricane.

Mandatory evacuations were in effect across large parts of southeast Texas as of Tuesday morning, August 25. These included Galveston Island and Jefferson and Orange counties, which encompass the entire Beaumont-Port Arthur-Orange metro area.

Laura had crossed over western Cuba Monday night as a tropical storm with 65 mph winds and emerged into the Gulf of Mexico around 10 p.m. EDT. Immediately upon exiting the coast of Cuba, Laura began to organize. A NOAA hurricane hunter aircraft found 75 mph surface winds in Laura Tuesday morning, leading the National Hurricane Center (NHC) to upgrade Laura to hurricane status at 8:15 a.m. EDT.

Laura is the fourth hurricane of this extremely active Atlantic hurricane season, which has set multiple records for most named storms so early in the season and for most named storms to make a U.S. landfall so early. The average date for the season’s fourth hurricane is September 21, so the current pace is nearly four weeks ahead of average for the number of hurricanes. The total accumulated cyclone energy, or ACE, so far this year – a measure of the destructive power of a season’s storms – is more than 50% above average for this point in the year. And there’s a long way to go, too – the typical half-way point of the Atlantic hurricane season is September 10.

Laura brought torrential rains and flash flooding to Cuba on Monday, with Topes de Collantes recording 5.58″ (141.7 mm) of rain in 24 hours. Jamaica, the Cayman Islands, Haiti, the Dominican Republic, and Puerto Rico have all suffered damaging flooding from Laura. The storm is being blamed for at least nine deaths in Haiti and two in the Dominican Republic.

Laura then brought heavy rain squalls and winds near tropical storm-force to the Florida Keys, with Key West recording sustained winds of 36 mph, gusting to 47 mph, at 3:16 p.m. EDT. The Key West National Weather Service office recorded a gust of 69 mph.

Laura slowly intensifying

At 11 a.m. EDT Tuesday, Laura was located over the waters of the southern Gulf of Mexico, about 620 miles southeast of Galveston, Texas. The waters there were a very warm 30.5 degrees Celsius (87°F). Laura was headed west-northwest at 16 mph with top sustained winds of 75 mph and a central pressure of 990 mb. This pressure was 10 mb lower than it was 24 hours earlier.

Satellite images early Tuesday afternoon showed a modest increase in heavy thunderstorms near Laura’s center, but the hurricane was still struggling to build heavy thunderstorms on its north side. The Hurricane Hunters did not observe an eyewall Tuesday morning, showing that Laura lacked the strong inner core needed to rapidly intensify.

High-level cirrus clouds were streaming out to the south of Laura, indicating good upper-level outflow on that side, but outflow was restricted to the north and west. Laura was embedded in a moderately dry region of the atmosphere, with a mid-level relative humidity of 55%; light to moderate upper-level wind shear of 5 – 10 knots was driving dry air to the north of Laura into the core and slowing development.

A track west-northwest, then a turn to the north

Over the next day, the Bermuda high, which is steering Laura, will force the storm to the west-northwest. On Wednesday evening, a trough of low pressure over the central U.S. will create a weakness in the high, allowing Laura to turn to the northwest and then north.

The model predictions for Laura still have a wide spread, with much of the upper Texas and west Louisiana coasts at risk for a potential landfall. The model trend in recent days has been to push Laura farther to the west, towards Texas. The 0Z and 6Z Tuesday runs of many reliable models are farther to the west than they were on Monday, implying more risk that Laura will make landfall over Texas, potentially bringing its strongest winds directly over Galveston and Houston. Note that for about one-third of NHC forecast cones, the actual track will extend outside the cone at some point. The cone is assigned a standard width each year, based on typical errors over the previous five years of tropical cyclones tracks. Some systems end up being less predictable – and some more predictable – than the cone implies.

The best-performing model for Laura continues to be the UKMET model, which has out-performed all other models and the official NHC forecast. The UKMET model’s 0Z Tuesday forecast predicted a landfall near the Louisiana/Texas border close to midnight Wednesday as an intensifying hurricane – approximately the same as in its 12Z Monday run. However, the 12Z Tuesday run of the model put landfall just north of Galveston, Texas.

The top track model in 2019, the European model (which has not performed well for Laura), had its 6Z Tuesday run predict a landfall in Texas just north of Galveston, along the Bolivar Peninsula. The European model’s 0Z and 6Z Tuesday ensemble forecasts, which generate a set of 51 runs of the model at low resolution with slightly different initial conditions to depict the potential uncertainty in the forecast, had the center of its ensemble envelope over Galveston and Houston.

Steering currents will keep Laura moving steadily after it comes ashore, so it is unlikely to stall out and produce the type of catastrophic inland rains generated by Hurricane Harvey in 2017. Instead, storm surge and wind are most likely to be the major threats posed by Laura.

Intensification into major hurricane expected

Conditions for intensification will be very favorable until Wednesday evening. Ocean temperatures are a very warm 30 – 31 degrees Celsius (86 – 88°F) across much of the Gulf of Mexico, and an upper-level high-pressure system with light winds will bring light to moderate wind shear and good upper-level outflow. The atmosphere will be somewhat dry, with a mid-level relative humidity of 55 – 60%, but once Laura establishes an eyewall and strong inner core, the dry air is unlikely to significantly impede intensification.

On Tuesday morning, Laura passed over the warm, deep waters of the Loop Current, with its tremendous amount of ocean heat content (see Sunday’s post). However, by Tuesday afternoon, much of Laura’s northern portion was over a cool eddy in the Gulf, which may slow intensification. Laura was also passing over the cool-water wake that Hurricane Marco had left behind.

From Tuesday evening though landfall on Wednesday night, Laura will leave the cool eddy and be passing over Gulf waters with very high heat content. The top dynamical intensity models – the HWRF, HMON, and COAMPS – continued to predict in their 0Z and 6Z Tuesday runs that Laura would be at major hurricane strength by Wednesday evening, as it approaches landfall in Texas or western Louisiana. The 12Z Tuesday run of the SHIPS model gave a 31% chance that Laura would rapidly intensify by 30 mph by Wednesday morning, becoming a strong category 2 hurricane. The official NHC forecast at 11 a.m. EDT Tuesday called for Laura to peak as a low-end category 3 hurricane with 115 mph winds at 8 p.m. EDT Wednesday. Wind shear is expected to increase to a moderately high 15 – 20 knots in the six hours before Laura makes landfall, so the hurricane may not be intensifying right up until landfall.

Laura a significant storm surge threat

Laura is expected to drive a large and destructive storm surge to the coast. The size of this storm surge will depend not only on how strong the winds are, the speed the storm is travelling, 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 Tuesday NHC forecast called for Laura’s tropical storm-force winds to span a diameter of 190 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 go to weather.com’s Jon Erdman for this information).

How to prepare for a hurricane

Inundation levels also will depend on whether Laura were to strike at high vs. low tide. Tidal ranges are low in this part of the Gulf, however, varying by roughly a foot or less between high and low tide in most locations. The broad daily high tide is centered during the overnight hours, which is when Laura is expected to arrive. Tides were already running about a foot above average at midday Tuesday across southeast Texas and southwest Louisiana.

Marco: record-earliest sixth U.S. landfall by a named storm

Tropical Storm Marco made landfall in southeast Louisiana at 7 p.m. EDT Monday, August 24, 2020, as a minimal tropical storm with 40 mph winds. During the 12 hours before landfall, Marco was nearly sheared apart by strong upper-level winds, leading NHC to drop all tropical storm and storm surge warnings a few hours before the center of Marco moved inland.

Those strong upper-level winds carried most of Marco’s heavy thunderstorms into the Florida Panhandle, where 2 – 4 inches of rain were common, with a few areas of six inches. Isolated street flooding occurred, particularly in Panama City Beach. At 5 a.m. EDT Tuesday, August 25, Marco was declared post-tropical.

Marco is the sixth named storm to make landfall in the U.S. so far this year, tying a record held by 1886 for the earliest in the season that a sixth storm has made a U.S. landfall. Here are the other Atlantic named storms in 2020 to hit the U.S., along with their preliminary damage estimates from insurance broker Aon and other sources:

• Hurricane Isaias near Wilmington, North Carolina on August 3 (85 mph winds, over $4 billion in damage to the U.S.);
• Hurricane Hanna in South Texas on July 25 (90 mph winds, $500 million in damage to the U.S. and Mexico);
• Tropical Storm Fay in New Jersey on July 10 (50 mph winds, six deaths, $350 million in damage);
• Tropical Storm Cristobal in Louisiana on June 7 (50 mph winds, one death, $325 million in damage);
• Tropical Storm Bertha in South Carolina on May 27 (50 mph winds, $200 million in damage).

The record for most U.S. landfalls in one year is nine, set in 1916; second place is jointly held by 2005, 2004, and 1985, with eight. During the period 1851 – 2019, the U.S. averaged 3.2 named storm landfalls per year, 1.6 hurricane landfalls, and 0.5 major hurricane landfalls.

North Korea at risk from Typhoon Bavi

The northwest Pacific, which is usually the most active ocean basin globally for tropical cyclones, has been unusually quiet so far this year. As of August 25, the basin had experienced nine named storms, four typhoons, and one major typhoon. According to Dr. Phil Klotzbach’s Real-Time TC Activity page, the normal tallies by this point in the year are 12 named storms, seven typhoons, and three intense typhoons.

The Accumulated Cyclone Energy (ACE) year-to-date in the basin was just 19% of average. The relative lack of activity is to be expected in a year that is trending towards La Niña conditions, when the monsoon trough that breeds typhoons shifts westwards, closer to land. As a result, the amount of time storms spend over water is shortened, limiting development.

On Saturday, August 22, the ninth named storm of the Northwest Pacific season – Tropical Storm Bavi – formed in the waters north of the Philippines and east of Taiwan, and became a typhoon Sunday. At 11 a.m. EDT Tuesday, the Joint Typhoon Warning Center (JTWC) classified Bavi as a top-end category 2 storm with 110 mph winds, and predicted that Bavi would move northwards and intensify into the season’s second major typhoon before making landfall in North Korea near 2 p.m. EDT Wednesday as a category 2 or 3 storm. According to NOAA’s historical hurricanes database, North Korea has only been hit by one typhoon in recorded history – Category 1 Typhoon Lingling in 2019.

Ocean temperatures along Bavi’s track are unusually warm – about 30 degrees Celsius (86°F), which is more than two degrees Celsius (3.6°F) above average. The warm waters are largely the result of an intense heat wave that brought all-time record heat to northeast Asia in recent weeks. On August 17, Hamamatsu, Japan tied the record set in 2018 for hottest temperature ever measured in Japan – 41.1 degrees Celsius (106°F). The unusually warm waters in front of Bavi from this heat wave will help keep the typhoon stronger than usual for a storm that affects Korea.

Long-range models suggest another strong typhoon may develop and head toward the East China Sea next week.

Bob Henson contributed to this post.

Posted August 25, 2020, at 2:11 p.m. EDT. Slight modifications made at 2:25 p.m. to add the 12Z model info.

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