Weather Outside Your Window: Grace to drench Caribbean with heavy, tropical rain

By Courtney Travis, AccuWeather senior meteorologist

Published Aug. 15, 2021 1:51 PM EDT | Updated Aug. 15, 2021 9:50 PM EDT

While Fred was traversing the Caribbean, another tropical system began to develop in the open waters of the Atlantic. Tropical Depression 7 strengthened into Tropical Storm Grace on Saturday morning, becoming the seventh named tropical system in the Atlantic Basin for the 2021 hurricane season.

Earlier in the weekend, Grace packed a punch and pushed across the Leeward Islands as a tropical storm. Periods of heavy rain hit islands from St. Lucia to the Virgin Islands, and wind gusts on islands like St. Croix and Grenada surpassed 35 mph.

As of early Sunday evening, Grace was located 155 miles southeast of Santo Domingo, Dominican Republic, and moving west at 15 mph with maximum sustained winds of 35 mph.

Grace was downgraded from a tropical storm to a tropical depression.

The above satellite image shows Grace moving across the eastern Caribbean Sea as a tropical storm on the afternoon of Sunday, August 15 (Image/CIRA/RAMMB).

On Friday and Saturday, tropical storm warnings were issued for Puerto Rico and the southern side of the Dominican Republic as Grace drew near. As of midday Sunday, tropical storm watches were also in effect for the rest of the Dominican Republic and all of Haiti.

In anticipation of Grace's impacts, Puerto Rican Gov. Pedro Pierluisi announced in a press conference last week said that the island's National Guard was standing by to assist residents during the storm, in addition to establishing storm shelters. Pierluisi also postponed the start of public school classes until Wednesday.

Into Monday, Grace is forecast to continue its westward movement, tracking near but just south of the islands of Puerto Rico and Hispaniola, a path slightly more southerly than Fred's recent trek.

The exact track of Grace could directly influence the storm's exact wind strength.

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"Thus far, Grace has managed to remain to the south of any land masses as the center is passing south of Puerto Rico. However, if Grace makes a more west-northwest turn as we expect, the center is likely to cross over Hispaniola which could disrupt Grace's circulation," said AccuWeather Senior Meteorologist Dan Pydynowski.

This close interaction with mountainous terrain, even if the center of Grace remains over water, could also be enough to downgrade Grace to a tropical depression, Pydynowski explained.

No matter the wind intensity of Grace, the tropical moisture associated with the storm will interact with the mountainous terrain of Hispanolia, as well as Puerto Rico, leading to heavy downpours and the threat for flooding and mudslides.

Grace is rated as less than one on the AccuWeather RealImpact™ Scale for Hurricanes in the Caribbean due to the heavy rain and the risk of flooding that is likely to be storm's most widespread impact to the islands.

Widespread rainfall amounts of over 2 inches (50 mm) are anticipated across the islands, with a corridor of heavier rain from southern Puerto Rico to the Dominican-Haiti border, where 4-8 inches (100-200 mm) of rain are possible. This is also the most likely location for the AcuuWeather Local StormMax™ for rain of 12 inches (300 mm).

"Unfortunately, Grace is going to bring more heavy rainfall to parts of Haiti that already were impacted by Fred’s rain and the 7.2-magnitude earthquake that occurred Saturday morning just northeast of Saint-Louis-du-Sud," said Pydynowski.

Pydynowski added that Grace is likely to send another 4 inches (100 mm) of rainfall to southern Haiti, exacerbating ongoing flooding and inhibiting cleanup and rescue efforts.

Impacts are likely to wait until later on Tuesday to spread into Cuba. Outer rain bands from Grace are expected to near the southeastern portion of the island late Tuesday, before expanding from east to west across Cuba through Thursday.

While wind gusts of 40 to 60 mph (60-100 km/h) are possible across this area, the exact strength of Grace's wind gusts will be determined by how well organized the storm is by that time.

Any infrastructure already weakened by Fred's passing last week will be more susceptible to damage at lesser wind speeds.

The Florida Keys could also see some additional tropical rainfall from Grace, should the storm pass close enough. This could bring an additional 2 inches (50 mm) of rainfall to the islands, in addition to the rain they already received from Fred over the weekend.

Later this week, Grace is expected to emerge in the Gulf of Mexico, likely putting part of the U.S. Gulf Coast next in line from the storm's impacts.

AccuWeather forecasters are continuing to monitor the atmospheric conditions across the Gulf of Mexico and how they could influence where Grace will track into next weekend. These factors include the warm waters of the Gulf of Mexico, as well as winds at various levels in the atmosphere.

Should no other weather systems in the southern U.S. steer Grace off-course, the tropical system may take aim at Texas.