Tropical Storm Laura formed in the waters just east of the Leeward Islands at 9:05 a.m. EDT Friday, August 21, 2020, making it the earliest twelfth storm on record for an Atlantic hurricane season. The previous record was held by Luis on August 29, 1995.
Laura poses a threat as a tropical storm this weekend to the Leeward Islands, Virgin Islands, and Puerto Rico, where Tropical Storm Warnings were posted. A more southerly predicted track lessens the threat early next week to the Bahamas and the Florida east coast, but increases the threat to Hispaniola and Cuba.
At 11 a.m. EDT Friday, August 21, Laura was located about 210 miles east-southeast of the northern Leeward Islands. Laura was headed west at 18 mph, with top sustained winds of 45 mph and a central pressure of 1007 mb. The storm was experiencing more upper-level wind shear than on Thursday: a moderate 10 – 20 knots. More detrimental to Laura was wind shear occurring at middle levels of the atmosphere, where strong east-northeasterly winds moved in opposition to the low-level flow out of the southeast. This mid-level shear was causing Laura to suffer misalignment, with the low-level center considerably to the northwest of the circulation center at mid-levels.
On Friday morning, the Hurricane Hunters found that Laura’s surface center had relocated farther to the south, closer to the mid-level center. This better alignment of the storm was reflected in an improved satellite presentation, as a modest amount of heavy thunderstorm activity slowly grew in intensity, organization, and areal coverage. High level cirrus clouds streaming away from Laura to both north and the south showed the storm was establishing upper-level outflow, a sign of increased organization.
Sea surface temperatures (SSTs) were favorable for development, near 28 degrees Celsius (82°F), but Laura was embedded in a moderately dry region of the atmosphere, with a mid-level relative humidity of 60%. The wind shear dogging Laura was injecting dry air into the circulation, interfering with development.
Laura primarily a heavy rain threat to the islands
Laura is primarily a heavy rain threat to the islands in its path, with the Virgin Islands, Puerto Rico, and Hispaniola (shared by the Dominican Republic and Haiti) likely to receive 3 – 6 inches of rain with isolated higher amounts. For the islands on the weaker (left) side of Laura’s circulation, wind damage should be limited. For a fast-moving system like Laura, winds on the right (north) side likely will be at least 15 mph higher than those on the left (south) side.
A more southerly track predicted
Laura did not develop into a tropical storm as early as models had originally predicted, and its center has reformed farther to the south. As a result, the Friday morning runs of the models predicted a more southerly track for Laura than they did previously. A more southerly track increases the odds that the system will encounter disruption from the islands of Puerto Rico, Hispaniola, and Cuba. The 0Z Friday runs of the European, GFS, UKMET, and HMON models all predicted that Laura would encounter significant disruption by these islands. The HWRF and COAMPS-TC models kept Laura just north of the islands, and both of these models predicted that Laura would become a major hurricane in the Gulf of Mexico on Tuesday after passing along the north coast of Cuba and through the Florida Keys on Monday. The 6Z Friday runs of these two models were similar. The HWRF model had the advantage of initializing its forecast with Doppler radar data from Thursday night’s NOAA P-3 hurricane hunter mission, something no other model ingests.
The Bermuda high, which is 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. A weakness in the high will allow Laura to turn more to the northwest on Monday, when it should be near the Florida Keys. At that time, Laura may have to contend with higher wind shear from a trough of low pressure over the U.S., which might interfere with intensification. Laura may also have to contend with increased wind shear resulting from the upper-level outflow from Tropical Depression 14, which is expected to be in the northwest Gulf of Mexico on Tuesday (see below). Until that time, upper-level wind shear for Laura is predicted to be a light to moderate 5 – 15 knots. However, strong mid-level wind shear will likely still be a problem for the storm through Saturday.
Ocean temperatures will steadily warm over the next five days, reaching a very warm 30 – 31 degrees Celsius (86 – 88°F) by Monday. Warm waters extend to great depth across the Bahamas, giving the ocean a high heat content ideal for fueling rapid intensification. The main detriment to Laura’s intensification beginning on Saturday will be interaction with land.
NOAA hurricane hunter aircraft will continue to investigate Laura though the weekend, feeding data from their Doppler radars in real time to the HWRF model. The NOAA jet is to make its first mission into Laura on Friday evening. Data from the dropsondes launched by the NOAA jet have been shown to improve track forecasts by as much as 15%.
Tropical Depression 14 struggling in western Caribbean
Tropical Depression Fourteen (TD 14) was struggling to organize in the western Caribbean on Friday because of dry air and land interaction. At 11 a.m. EDT Friday, August 21, the center of TD 14 was located off the north coast of Honduras, about 325 miles southeast of Cozumel, Mexico. TD 14’s close proximity to land was inhibiting the flow of moist air into the system, which was headed northwest at 14 mph, away from the coast, favoring development.
Although the pockets of dry air in the western and central Caribbean interfere with its development, TD 14 was in a relatively 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 favored development. Satellite images showed that TD 14 was still disorganized, and had only modest heavy thunderstorm activity.
Forecast for TD 14
As TD 14 progresses northwestward through Saturday, its forward speed will slow as a result of the steering influence of a large trough of low pressure over the central U.S. The upper-level southwesterly winds ahead of this trough will take TD 14 to the northwest over Mexico’s Yucatan Peninsula on Saturday night, and into the Gulf of Mexico on Sunday.
Approaching the Yucatan Peninsula on Saturday, TD 14 will find very favorable conditions for development. The 12Z Friday run of the SHIPS model predicted that the atmosphere surrounding the system would be a moist 70 – 75%; wind shear would be a light 5 – 10 knots; and SSTs would be a very warm 30 – 30.5 degrees Celsius (86 – 87°F). These conditions are expected to allow TD 14 to be a strong tropical storm at landfall Saturday evening in the Yucatan Peninsula.
Crossing the Yucatan will weaken TD 14, and it may take a day for the storm to reorganize over the warm waters of the Gulf of Mexico, where SSTs are a warm 30 degrees Celsius (86°F). An upper-level trough of low pressure over the Gulf at that time will bring dry air and high wind shear to TD 14, limiting how much re-intensification can occur. The long-range intensity forecast is uncertain, but TD 14 could be a strong tropical storm or a category 1 hurricane when it makes its expected landfall on Tuesday along the U.S. Gulf Coast.
The next name on the Atlantic list of storms is Marco, the thirteenth named storm of 2020. There is a tie for 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 that reached tropical storm strength on September 1, just before Lee was discovered in post-season analysis.)
Will Laura and TD 14 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.
Some model runs have shown that Laura and TD 14 may be of similar strength and less than 900 miles apart in the Gulf of Mexico on Tuesday, resulting in a Fujiwara interaction. The 0Z Friday run of the German ICON model showed this, with TD 14 slowing its approach to the Texas coast as a result of its rotating counter-clockwise around a common center with Laura.
A new tropical wave off coast of Africa has potential to develop
A strong tropical wave located near the coast of Africa on Friday morning has some modest potential to develop into a tropical depression this weekend or early next week. The disturbance will move west-northwest at 15 – 20 mph over the tropical Atlantic and bring heavy rain showers and the threat of flash flooding to the Cabo Verde Islands over the weekend. The 0Z Friday operational runs of the GFS, European, and UKMET models did not show development of this wave during the coming five days. In an 8 a.m. EDT Friday Tropical Weather Outlook, NHC gave the new African tropical wave two-day and five-day odds of formation of 20% and 40%, respectively.
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