The second hurricane of the 2021 Atlantic season is Hurricane Grace, which intensified into a low-end category 1 hurricane with 75 mph winds at 11 a.m. EDT Wednesday as it passed just south of Grand Cayman Island. Two hurricane hunter aircraft in Grace on Wednesday morning found hurricane-force winds in the storm, and documented that Grace had built a completely closed eyewall.

Grace is the second hurricane of the season, joining Elsa from early July. According to the National Hurricane Center, the average arrival date of the season’s second hurricane is August 28 (using a 1966-2009 climatology), so 2021 is over a week ahead of schedule in that metric.

Grace made landfall along the north shore of Jamaica on Tuesday afternoon, passing five miles south of Montego Bay at 5 p.m. EDT with sustained winds of 50 mph. Kingston, Jamaica recorded peak sustained winds of 47 mph Tuesday afternoon, with gusts as high as 53 mph; the peak gust at Montego Bay was 47 mph. The Kingston airport measured 9.76” (248 mm) of rain in the 24 hours ending at 8 a.m. EDT Wednesday. The Jamaica Gleaner reported that eastern Jamaica suffered widespread flash flooding, tree damage, and downed power lines, which led to the closing of numerous roads (see the high water rescue performed yesterday in the Tweet below). No deaths or injuries were reported.

At 8 a.m. Wednesday morning, Grace passed just 20 miles southwest of Grand Cayman Island, putting the island in the powerful right-hand side of the storm. Wind gusts exceeding hurricane force were reported on Grand Cayman, and the main airport reporting station was knocked offline. There is considerable damage being reported on the island. A personal weather station on Grand Cayman recorded 4.74” of rain in the 24 hours ending at noon EDT Wednesday.

Satellite imagery early Wednesday afternoon showed Grace’s heavy thunderstorms much more organized than on Tuesday, with increased intensity and more low-level spiral bands; a faint eye was also apparent.

Grace brought heavy rains to Haiti, the Dominican Republic, and Trinidad

Official weather observations are sparse in Haiti, but there are three personal weather stations in southwest Haiti in the Weather Underground network. The highest 24-hour rainfall amount recorded at these three stations from Grace was 3.13”. It is likely that heavier rains fell over portions of this region, adding significant misery for the survivors of a devastating magnitude 7.2 earthquake on Saturday morning that killed an estimated 1,941 people, with more fatalities expected. There are as yet no reports of deadly flooding in Haiti from Grace, though a social media video (below) showed floodwaters from the storm rampaging through Cayes-Jacmel, 75 miles east-southeast of the earthquake’s epicenter.

According to an excellent storm summary at floodlist.com, a landslide attributed to Grace killed one person on Trinidad. In the Dominican Republic, Grace dumped up to 250 mm (9.84”) of rain in the Barahona Peninsula, where Grace made landfall as a tropical depression on Monday. Floods and landslides from Grace’s rains damaged 558 homes in the country.

Forecast for Grace

A broad ridge of high pressure over the western Atlantic is steering Grace. The ridge is expected to keep the storm on a west to west-northwesterly path, resulting in a landfall in the northeastern Yucatan Peninsula on Thursday morning. Grace will emerge into the Gulf of Mexico by Thursday night; steering currents over the Gulf may force Grace on more of a westerly or west-southwesterly track on Friday, resulting in a second Mexican landfall in the Veracruz state on Friday night or Saturday morning.

With warm waters near 30 degrees Celsius (86°F), an atmosphere with a mid-level relative humidity of 55-60%, and moderate wind shear of 10-15 knots, Grace is expected to undergo at least modest strengthening before making landfall in Mexico’s Yucatan Peninsula. The waters of the western Caribbean that Grace will be traversing have the highest heat content of anywhere in the Atlantic, giving Grace plenty of fuel for rapid intensification. The 12Z Wednesday run of the SHIPS model gave a 25% chance that Grace would rapidly intensify by 35 mph into a 105-mph category 2 hurricane in the 24 hours ending at 8 a.m. EDT Thursday. However, the atmosphere is somewhat on the dry side for rapid intensification, and with upper-level winds out of the northwest bringing moderate wind shear, Grace is more likely to hit the Yucatan Peninsula as a category 1 hurricane than as Cat 2. NHC predicted a landfall intensity in the Yucatan Peninsula of 85 mph in its 11 a.m. EDT Wednesday forecast.

Grace will need time to reorganize after crossing the Yucatan Peninsula, but conditions for intensification over the southern Gulf of Mexico will be favorable, with warm waters near 30 degrees Celsius (86°F) and a moister atmosphere with a mid-level relative humidity of 65%. Landfall in Mexico’s Veracruz state will likely come at hurricane strength; NHC predicted a landfall intensity of 85 mph in its 11 a.m. EDT Wednesday forecast.

Henri’s forecast track inching toward New England

Tropical Storm Henri, holding its own with 65-mph winds southwest of Bermuda at 11 a.m. EDT Wednesday, may become a threat to New England by this weekend. Henri was originally expected to recurve around Bermuda well east of the U.S. coast, but model forecasts have been consistently pushing westward, and NHC forecasts have followed suit.

“Given the uncertainty in the longer-range track forecasts, users should be prepared for additional adjustments to the NHC track forecast in future forecast cycles,” according to the NHC discussion posted at 11 a.m. EDT Wednesday, August 18.

Henri’s short-term future is more predictable. The storm is moving west-southwest over the open Atlantic as it rounds the base of an upper-level ridge that separates it from the remnants of Fred, located in western Pennsylvania on Wednesday morning. Henri and ex-Fred will continue to rotate around this ridge through Thursday, as Fred’s remnants move into New England while Henri continues west-southwest near latitude 30°N.

Giving Henri a boost will be water temperatures around 28-29 degrees Celsius (82-84°F), which are around 0.5-1.0 degree Celsius above average for the time of year. The air around Henri is only moderately moist (mid-level relative humidity around 50%), and high wind shear of around 20 knots on Wednesday and 25 knots on Thursday will keep Henri from any rapid strengthening. The storm will continue to expand with age: after originating as a tiny tropical depression, Henri should be an average-sized tropical storm by Thursday.

The forecast for Henri becomes vastly more complicated from Friday onward. The remnants of Fred in New England will be replaced by a building upper-level ridge, extending southeastward to connect with the Bermuda-Azores High. It now looks increasingly possible that Henri will be marooned west of that ridge axis, hindering it from recurving like most storms in its location would. Moreover, the ridging will help decrease shear over Henri, which NHC predicted will become a hurricane by Friday. Through the weekend, Henri will remain over unusually warm waters, and the midlevel atmosphere will moisten to a relative humidity of 65-70%, so a day or two of at least gradual strengthening appears likely.

Over the weekend, a slight weakness in the ridge is predicted to develop over the mid-Atlantic, and a number of models now are moving Henri northward around this weakness. Of our top track models, the 0Z and 06Z GFS runs bring Henri into southeast New England as a strong tropical storm or hurricane; 0Z UKMET briefly stalls Henri over Cape Cod, again as a strong tropical storm or hurricane; and the 0Z Euro essentially dissipates Henri, though there are signs of its remnants offshore. The high-resolution regional HWRF and HMON models, which are good track models, concur on a New England threat and agree that Henri could be a strong tropical storm or Cat 1 hurricane on approach.

The consistent trends we’re seeing across models and over time is certainly enough to raise concerns for New England, which is far from immune from tropical storms and even the occasional hurricane. The official track forecast has not yet been nudged as far west as the model solutions above would imply, in part because of NHC’s tendency to implement large forecast shifts like this in a gradual manner (a habit that has served it well over time, as NHC track forecasts are typically better than that of any single model). NOAA will be launching flights with its Gulfstream-IV aircraft to sample the environment around Henri in more detail; the data will be incorporated in future model runs, giving us a better sense over time of Henri’s future.

Fred downgraded to a post-tropical storm

At 11 a.m. Wednesday, Fred was downgraded to a post-tropical cyclone as it moved over western Pennsylvania. Ex-Fred was moving northeast at 20 mph, and is expected to dump 2-4” of additional rain along portions its track.

Fred made landfall in the Florida Panhandle 25 miles west of Apalachicola at 3:15 p.m. EDT Monday, August 16, with sustained 65 mph winds and central pressure of 994 mb. The peak storm surge at a NOAA tide gauge occurred at Apalachicola, where the surge peaked at 4.67’, causing minor coastal flooding. Four coastal locations reported wind gusts of 70-73 mph. The highest rainfall amount reported was 11.34” at Vernon, Florida. Grace caused major flooding in western North Carolina; Asheville reported a four-day rainfall total of 8.10”, its fifth-wettest four-day period on record.

As of noon EDT Wednesday, NOAA’s Storm Prediction Center had logged 16 preliminary tornado reports from Fred. Fred may spawn additional tornadoes on Wednesday over Pennsylvania, Maryland, and Northern Virginia, where SPC has given a “Slight” risk for severe weather.

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