Tropical storm warnings are up for much of the Leeward Islands, Virgin Islands, and Puerto Rico for the expected arrival on Sunday of Tropical Storm Grace, which formed at 5 a.m. EDT Saturday in the waters a few hundred miles east of the Leeward Islands.
At 11 a.m. EDT Saturday, Grace was about 265 miles east-southeast of the Leeward Islands, moving west at 23 mph, with top winds of 45 mph and a central pressure of 1005 mb. The Hurricane Hunters will fly their first mission into Grace early Saturday afternoon to better assess Grace’s strength.
Radar imagery early Saturday afternoon showed that heavy rains from Grace’s outer spiral bands were already affecting the Leeward Islands, and satellite imagery showed Grace growing more organized, with an intensifying area of heavy thunderstorms surrounding the center, low-level spiral bands growing more pronounced, and an upper-level outflow channel developing to the north. Wind shear was light, 5-10 knots, and this lack of shear has allowed Grace to wall off the dry air to its north and form an inner core. Grace’s fast forward speed is likely inhibiting development to some degree, but sea surface temperatures have warmed to 28 degrees Celsius (82°F), and will grow even warmer as Grace progresses westward. Grace will likely begin building an eyewall Saturday afternoon and evening.
Forecast for Grace
Grace has been following a path very similar to Fred’s, steered by a broad ridge of high pressure over the western Atlantic. This ridge will keep Grace moving steadily west to west-northwest over the next five days, bringing the storm near or over Puerto Rico on Sunday and Hispaniola on Monday. Grace’s forward speed will slow to about 15 mph by Sunday morning as the ridge weakens, which should allow more intensification to occur. On Sunday and Monday, wind shear is expected to be light to moderate, 5-15 knots, the atmosphere will grow moister, and sea surface temperatures will warm to 29 degrees Celsius (84°F). These conditions are favorable for development, and Grace has good model support to intensify until it encounters the high mountains of Puerto Rico and the Dominican Republic. The SHIPS model at 12Z Saturday was giving a 42% chance that Grace would rapidly intensify into a category 1 hurricane by Sunday morning. Small storms like Grace are capable of rapid swings in intensity, both upward and downward.
The biggest question concerning Grace’s longer-range intensity is whether or not it will hit Hispaniola and get shredded by the island’s high mountains, as happened to Tropical Storm Fred. Since Grace is a small storm, with tropical storm-force winds that extend out just 35 miles from the center, it will be more prone to weakening through interaction with land. If Grace avoids significant interaction with Hispaniola, intensification into a hurricane by Monday is quite possible. However, current forecasts carry Grace close to Hispaniola, and the National Hurricane Center is forecasting a peak intensity of 60 mph as a result of land interaction.
Unfortunately, Grace’s expected track near or over Hispaniola will make heavy rains likely in Haiti, which was hit by a devastating magnitude 7.2 earthquake on Saturday morning. These rains will begin on Monday night and extend through all of Tuesday. However, Grace’s small size means that southwestern Haiti, where the earthquake struck, will likely miss the storm’s heaviest rains.
Once Grace clears the high mountains of Hispaniola on Tuesday, the storm will have the opportunity to intensify if its path carries it through the Bahamas. However, if Grace’s circulation is completely disrupted by land interaction, it will take the storm at least a day to reorganize. Waters will be warm and the atmosphere moist in the Bahamas, favoring intensification, but wind shear from strong upper-level winds out of the north from a trough of low pressure over the western Atlantic may be strong enough to keep any intensification slow. Grace’s potential strength when it reaches the Southeast U.S. on Thursday is difficult to assess given these uncertainties, and the storm could well be a category 1 hurricane then – or a tropical wave, destroyed by the high mountains of Hispaniola.
2021 well behind 2020’s record pace for named storms
After setting a record for the earliest appearance of a season’s fifth named storm (Elsa on July 1), 2021 has thankfully fallen well behind the record pace of 2020, which ended with an astounding 30 named storms. Last year, the season’s seventh named storm (Gonzalo) formed on July 22, setting the record for the earliest appearance of the Atlantic’s seventh storm of the season. Grace’s formation date of August 14 lags the 2020 pace by over three weeks, but comes about three weeks before the average September 2 arrival of the Atlantic’s seventh named storm (using a 1991-2020 climatology), according to Phil Klotzbach of Colorado State University.
Fred degenerates to a tropical wave
Tropical Depression Fred degenerated into a tropical wave on Saturday morning after an extended encounter with wind shear and the high mountains of Cuba tore the storm apart. At 11 a.m. EDT Saturday, the remains of Fred were located about 125 miles southwest of Key West, Florida, moving west-northwest at 12 mph. Key West radar showed total rainfall from Fred was mostly less than an inch over South Florida, including the Keys, with a few hot spots receiving 2-4 inches.
Forecast for ex-Fred
Ex-Fred will start rounding the southwest corner of a steering ridge of high pressure, allowing it to angle more toward the northwest and then north over the next three days, resulting in a Monday landfall on the Gulf Coast in the Florida Panhandle, Alabama, or Mississippi. Models agree that Fred will regenerate into a tropical depression on Sunday, aided by a moist atmosphere and very warm sea surface temperatures of 30 degrees Celsius (86°F). Hindering development will be upper-level winds out of the west to southwest from an upper-level trough of low pressure to the northwest of Fred, which is expected to bring 15-20 knots of wind shear. This shear will keep the bulk of Fred’s heavy rains to the east of the center, and areas to the west of Fred’s center will see few impacts from the storm. Given the shear and the time it will take Fred to reorganize, it is unlikely that Fred will be stronger than a 60-mph tropical storm at landfall on the Gulf Coast.
Linda cranks up to Category 4 in Eastern Pacific
Major Hurricane Linda was the best kind of category 4 hurricane on Saturday: the kind that spins harmlessly out to sea. Linda, centered about 500 miles southwest of Cabo San Lucas, Mexico, intensified rapidly from minimal hurricane strength on Thursday night, August 12, to category 3 strength on Friday night, August 13. At 8 a.m. PDT Saturday, August 14, Linda’s peak winds reached 130 mph, putting it just inside the category 4 bracket. On satellite, Linda presented as a classically intense, symmetric hurricane.
Linda has overperformed, taking advantage of light wind shear (around 5 knots) and warm sea surface temperatures (around 28 degrees Celsius, or 82°F): as soon as early Friday, it was predicted to reach only category 2 strength. In typical fashion for Eastern Pacific hurricanes, Linda will weaken gradually later in the weekend as it moves westward atop the cooler waters that prevail farther offshore. NHC predicts Linda to be a category 2 storm by Sunday morning and category 1 by Monday.
Linda is the second category 4 storm in the Eastern Pacific this year, coming after 145-mph Felicia. It’s also the 12th named storm in the basin, a mark that is typically reached on September 19 based on NHC’s 1971-2009 climatology.
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