Tropical Storm Laura grew more organized as it plowed through the Leeward and Virgin Islands Friday night through Saturday morning, but remained a minimal tropical storm with top winds of 40 mph at 11 a.m. EDT Saturday.
As of 9 a.m. EDT on Saturday, Laura brought gusts as high as 37 mph to Dominica, 46 mph to St. Croix, 40 mph to St. Thomas, 38 mph to Sint Maarten, 37 mph to Antigua, and 36 mph to Guadeloupe. Rainfall amounts of 1 – 2 inches were common across the Weather Underground personal weather station network in the Leeward and Virgin Islands.
At 2:09 p.m. EDT Saturday, Camp Santiago, located about five miles inland from the south coast of Puerto Rico, recorded sustained winds of 52 mph, gusting to 75 mph.
At 2 p.m. EDT Saturday, August 22, Laura was located about 20 miles southwest of Ponce, Puerto Rico, headed west at 18 mph with top sustained winds of 50 mph and a central pressure of 1004 mb. The storm was experiencing light upper-level wind shear of 5 – 10 knots, but wind shear occurring at middle levels of the atmosphere was still causing Laura to suffer some misalignment, with the low-level center displaced from the circulation center at mid-levels. However, this misalignment was less than on Friday, which has allowed more organization.
Satellite images on Saturday showed a modest amount of heavy thunderstorm activity slowly increasing in intensity, organization, and areal coverage. High level cirrus clouds streaming north and south away from Laura showed the storm was establishing upper-level outflow, a sign of increased organization. Radar from San Juan, Puerto Rico, showed that Laura had a well-defined center at mid-levels.
Sea surface temperatures (SSTs) were favorable for development, near 29.5 degrees Celsius (85°F), but Laura was embedded in a moderately dry region of the atmosphere, with a mid-level relative humidity of 60%. In addition, the wind shear affecting Laura was injecting dry air into the circulation, slowing development.
A track over Hispaniola and Cuba predicted
The Bermuda high, steering Laura, will be strong and will extend far to the west, steering the storm generally to the west-northwest over the next five days, and the Saturday morning runs of the models continued to mostly predict a track that will take Laura over the islands of Hispaniola and Cuba. The 0Z Saturday runs of the European, GFS, UKMET, COAMPS-TC, and HMON models all predicted that Laura would encounter significant disruption by these islands. The 12Z version of all the models, including the HWRF, were also on board with this idea.
Lots of uncertainty once Laura gets into Gulf of Mexico
Some time Monday or Tuesday, Laura will fully emerge into the Gulf of Mexico, where conditions for intensification are expected to be very favorable. The traverse over Hispaniola and Cuba may destroy Laura’s circulation, as predicted by two of the top statistical intensity models, the DSHP and LGEM. That outcome seems unlikely, though, as all the global dynamical models predict Laura will survive.
The exact timing of Laura’s emergence into the Gulf is very uncertain, as just a slight wobble in its track could mean the difference between the storm’s traversing over mountainous Cuba or tracking over the warm Gulf waters just to the north of the island. In addition, the models disagree considerably on how much of a weakness might exist in the Bermuda high steering Laura at that time. The trend in the models has been to show less of a weakness in the steering high, allowing it to push Laura more toward the west towards Texas.
Among the 12Z Saturday model runs, the HMON model had the farthest west track for Laura, calling for landfall near Brownsville, Texas, as a category 3 hurricane on Thursday morning. The HWRF and European models were the farthest east, predicting landfall in southeastern Louisiana on Wednesday; the HWRF predicted a category 4 hurricane at landfall. The GFS and UKMET models showed a weaker category 2 hurricane at landfall in central Louisiana and near the Texas/Louisiana border, respectively (though these models should not be trusted for intensity forecasts).
Ocean temperatures are a very warm 30 – 31 degrees Celsius (86 – 88°F) over much of the Gulf of Mexico, the atmosphere will be moist, and it appears that wind shear will be less of a problem for Laura than earlier thought, with moderate wind shear of 10 – 20 knots a good bet. However, Laura may experience increased wind shear from the upper-level outflow from Tropical Storm Marco, which is expected to be in the northwest Gulf of Mexico on Tuesday (see below).
It will take a day or so for Laura to recover after a long traverse over land. This recovery may be slowed by the possibility that Laura will expand in size as it traverses Hispaniola and Cuba – something that happened to Hurricane Ike in 2008 when it took a similar path. Conditions were favorable for development of Ike during its traverse over the islands, but since its core was over land, the outer portions of the storm developed instead. That led to a big expansion of the hurricane’s size, resulting in Ike’s taking a very long time to spin up once its core emerged over water. A large storm generates a larger storm surge, and Ike ended up bringing a devastating storm surge, characteristic of a much stronger hurricane, when it made landfall in Texas on the Bolivar Peninsula as a high-end category 2.
How to make an evacuation plan
Once Laura establishes an inner core over the Gulf of Mexico, rapid intensification is a good possibility. Residents along the Gulf Coast should anticipate the possibility that Laura will rapidly intensify right up until landfall, potentially reaching major hurricane status.
Marco intensifies in western Caribbean
Tropical Storm Marco formed in the northwest Caribbean at 11 p.m. EDT Saturday, August 21, making it the earliest thirteenth storm on record for an Atlantic hurricane season. There is a tie for previous record-earliest thirteenth storm of the season, with Lee on September 2, 2011, and Maria on September 2, 2005. (Lee was originally the twelfth storm of the 2011 season, but an unnamed system reached tropical storm strength on September 1, just before Lee, was discovered in post-season analysis.)
At 2 p.m. EDT Saturday, August 22, the center of Marco was located about 90 miles east-northeast of Cancun, Mexico, headed north-northwest at 12 mph with a central pressure of 992 mb. Marco was taking advantage of favorable conditions to quickly intensify. In addition, Marco was in a moist large-scale environment, with a mid-level relative humidity of 70%. SSTs near 30 degrees Celsius (86°F) and light wind shear of 5 – 10 knots also favoring development. Marco is a small storm, and small storms can intensify and weaken quickly. Radar images from Cuba showed that Marco had built nearly a complete eyewall on Saturday morning.
Marco expected to ‘shoot gap’ between Cuba and Mexico
Although earlier model runs had Marco angling left and passing over the Yucatan Peninsula, Marco’s center has consolidated further east than expected. Thus, Marco is now predicted to shoot the gap between Cuba and Mexico, which should allow the storm to intensify into a category 1 hurricane over the warm waters of the southern Gulf of Mexico, where SSTs are 30 degrees Celsius (86°F).
An upper-level trough of low pressure over the Gulf will bring dry air and high wind shear to Marco beginning on Sunday, though, limiting how much intensification can occur. With wind shear expected to be a high 20 – 30 knots Sunday night through Tuesday, a small storm like Marco should weaken considerably, and be at most a strong tropical storm when it makes its expected landfall on Monday or Tuesday along the U.S. Gulf Coast.
Will Laura and Marco perform a Fujiwara dance in the Gulf of Mexico?
When two tropical cyclones approach within about 900 miles of each other, they tend to rotate counter-clockwise around a common center, then go their separate ways, in a process called the Fujiwara effect. In rare cases they may merge into one storm, but the resulting storm will not be stronger than either of the original two storms, since wind shear from each weakens the other.
More commonly, when two storms interact, one will weaken or destroy the other with its wind shear, just as Hurricane Wilma did to Tropical Storm Alpha in 2005. The Saturday morning model runs showed limited support for a Fujiwara interaction between Marco and Laura.
A tropical wave off coast of Africa has zero potential to develop
A tropical wave located near the coast of Africa Saturday morning has diminished in intensity. The disturbance will move west-northwest at 15 – 20 mph over the tropical Atlantic and bring heavy rain showers and a threat of flash flooding to the Cabo Verde Islands over the weekend. The 0Z Saturday operational runs of the GFS, European, and UKMET models did not show development of this wave during the coming five days. In a 2 p.m. EDT Saturday Tropical Weather Outlook, NHC gave the new African tropical wave two-day and five-day odds of formation of 0%.
South Korea at risk from Tropical Storm 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 22, the basin had experienced nine named storms, three 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 14% of average. The relative lack of activity is 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, the ninth named storm of the Northwest Pacific season – Tropical Storm Bavi – formed in the waters north of the Philippines and east of Taiwan. The Joint Typhoon Warning Center (JTWC) predicted in its 11 a.m. EDT Saturday, August 22, forecast that Bavi would develop into the northwest Pacific’s second major typhoon of the season, and move northwards and make landfall in South Korea on Wednesday as a category 2 storm. Ocean temperatures along Bavi’s track are unusually warm – about 30 degrees Celsius (86°F), which is approximately two degrees Celsius (3.6°F) above average.
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