Hurricane Isaias (pronounced ees-ah-EE-ahs) plowed through the Bahama Island chain Friday and Saturday, making landfall over Andros Island in the northwest Bahamas late Saturday morning. Isaias is predicted to bring heavy rains and strong winds to the entire U.S. East Coast Saturday through Tuesday.

At 11 a.m. Saturday, August 1, Isaias was pounding the northwest Bahamas as a category 1 hurricane with 80 mph winds and a central pressure of 987 mb. An observing site at Blue Lagoon on New Providence Island recorded sustained winds of 61 mph, gusting to 63 mph, at 8:30 a.m. EDT Saturday. The capital of Nassau, on New Providence Island, reported sustained winds of 36 mph, gusting to 49 mph, at 11 a.m. EDT Saturday. A personal weather station on Crooked Island received 4.24” of rain on Friday.

Bahamas radar showed that Isaias has struggled to build an eyewall from Friday afternoon through Saturday afternoon, and has typically been able to maintain only about 50% of a complete eyewall, located along the north side of its center (see the excellent radar loop from Brian McNoldy). Unfortunately, Isaias is likely to pass very close on Saturday afternoon to one of the Bahama Islands hardest-hit by last year’s Hurricane Dorian, Grand Bahama Island.

Considerable damage in the Dominican Republic and Puerto Rico

In the Dominican Republic, floods from Isaias’s heavy rains killed two people, damaged or destroyed 167 homes, and caused the evacuation of 5,595 people, according to Dominican Today. Flooding knocked out 73 aqueducts, affecting the water supply for 1.4 million people. The peak 24-hour rainfall amount in the Dominican Republic was 11.00″ (279.4 mm) at Sabana De La Mar, on the north coast.

Isaias brought torrential rains and flooding to Puerto Rico, with preliminary flood damage to the municipality of Mayagüez, on the west coast of the island, estimated at $14 million, according to El Nuevo Dia. The storm knocked out power to more than 448,000 customers at its height on Thursday.

Entire U.S. East Coast to get impacts from Isaias

Tropical storm and hurricane warnings have been hoisted for most of the east coast of Florida, and it is likely that nearly the entire U.S. East Coast will be under a watch or warning from Isaias at some time between Saturday and Tuesday. The region at highest risk is eastern North Carolina, which on Monday may lie on the strong (right-hand) side of a potential landfall by Isaias.

Isaias’s first spiral band to affect Florida moved over Miami on Saturday morning, and rainfall amounts of 2 – 4 inches are expected along the state’s east coast through Monday. In the Lake Okeechobee watershed, rains of about two inches are expected. This should not be a concern for the lake’s water level, which stood at 13.21′ on July 31 – about 0.5 feet below average for the date. The Army Corps of Engineers tries to maintain the water levels in the lake below a maximum of 15.5′ in order to relieve pressure on the aging Herbert Hoover Dike that surrounds the lake. Two inches of rain can raise the water levels in the lake a little less than a foot, so Isaias’s rains should keep the lake well below the 15.5′ level. The increased water levels in the lake resulting from Isaias’s rains will not lead to water releases, which have created toxic algae blooms along both the east and west coasts of Florida in the past. I discussed the issues bedeviling the dike in a 2017 post after Hurricane Irma’s rains of 8 – 12″ raised the lake level by 3.5′.

Forecast for Isaias

Satellite images on Saturday afternoon showed that Isaias had a large area of intense thunderstorms along the east side of the center of circulation. Isaias was in a region dominated by southwesterly upper-level winds associated with a large-scale trough of low pressure. These winds were creating unfavorable conditions for intensification, with high wind shear of 20 – 25 knots. In addition, this shear was driving dry air from the west side of the hurricane into its center, keeping heavy thunderstorms limited on its west side. Sea surface temperatures (SSTs) were a warm 29 – 29.5 degrees Celsius (84 – 85°F), and Isaias was embedded in a moderately dry atmosphere with a mid-level relative humidity of 50 – 55%. Overall, these conditions favor only slow changes to Isaias’s strength through Monday. Most of the intensity models favor slow weakening, as does the official NHC forecast.

The models are very unified in bringing Isaias to a point very close to the coast in central Florida on Sunday afternoon, when Isaias is expected to turn more to the north and north-northeast as a result of the steering influence of the trough of low pressure to its west.

On Monday and Tuesday, Isaias may experience a reduction in shear, to 10 – 20 knots, as shown in the 12Z Saturday run of the SHIPS model. With SSTs of 29 – 29.5 degrees Celsius (84 – 85°F) – unusually warm for late July – extending northwards all along the coasts of South and North Carolina, Isaias could be strengthening because of the reduced shear and warm water as it approaches a possible landfall in the Carolinas on Monday afternoon or evening.

Once Isaias moves beyond the warm Gulf Stream waters to the north of North Carolina, steady weakening is likely, with most of the intensity models and the official NHC forecast predicting it by then will be a strong tropical storm with 55 – 70 mph winds during its closest approach to New England on Tuesday night through Wednesday morning. Rainfall will be increasing on the west side of Isaias over time, and totals of 4″ – 6″ will be possible along the urban corridor from Washington to New York.

Short-lived Tropical Depression Ten forms off the coast of Africa

Tropical wave 93L developed into Tropical Depression Ten between the Cabo Verde Islands and the coast of Africa on Friday afternoon. The depression is unlikely to survive into Sunday, though, since on Saturday morning it moved over cool waters with SSTs of 25 degrees Celsius (77°F).

TD 10 had only a limited amount of heavy thunderstorm activity Saturday afternoon, as seen on satellite images. At 11 a.m. EDT Saturday, with top winds of 30 mph, TD 10 was headed northwestwards at 14 mph. This track will take it into a stable air mass and over even cooler waters, and TD 10 is expected to dissipate by Sunday morning.

Next system to watch for development

A westward-moving tropical wave located about 600 miles east of the Leeward Islands on Saturday morning was producing a bit of shower activity. Some slow development of this system is possible as it turns northwestward and passes well north of the Leeward Islands by Monday.

In an 8 a.m. EDT Saturday Tropical Weather Outlook, NHC gave the system two-day and five-day odds of formation of 10% and 60%, respectively. Although the system has some model support for development, it does not appear to be a threat to any land areas.

The next name on the Atlantic list of storms is Josephine. The current record for earliest 10th Atlantic named storm formation is August 22, 2005, when Jose formed.

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7/31 Original Post

Hurricane Isaias (pronounced ees-ah-EE-ahs) made landfall as a tropical storm with 60 mph winds on Thursday afternoon on the southeast coast of the Dominican Republic, bringing damaging flooding, as well as drought relief, to Puerto Rico and the Dominican Republic. Isaias is predicted to bring heavy rains and strong winds to the entire Bahama Island chain Friday and Saturday, and to the entire U.S. East Coast Saturday through Tuesday.

At 11 a.m. Friday, July 31, Isaias was pounding the southeastern Bahamas as a category 1 hurricane with 75 mph winds. A private weather observing site at Long Bay Beach in Providenciales reported a sustained wind of 55 mph (86 km/h) and a gust to 59 mph (95 km/h) early Friday morning.

Damaging flooding, drought relief for Puerto Rico and Dominican Republic

Isaias brought torrential rains and flooding to Puerto Rico, with more than half of Puerto Rico receiving between five and 12 inches of rain (Figure 1). At the peak of the storm on Thursday, the National Weather Service in San Juan, Puerto Rico, listed seven rivers on the island that were above flood stage, and 15 others near flood stage.

Preliminary flood damage to the municipality of Mayagüez, on the west coast of the island, was estimated at $14 million, said the manager of the Municipal Emergency Management Office, Israel Martínez Cuevas, in an interview with El Nuevo Dia. The storm knocked out power to more than 448,000 customers in Puerto Rico on Thursday, Fernando Padilla, an official with the Puerto Rico Electric Power Authority, told Primera Hora. Main-trunk transmission lines that carry electricity from the south to the north of the island were down, as were local transmission lines.

In the Dominican Republic, floods from Isaias’s heavy rains knocked out power, flooded roads, and caused at least one death, according to Dominican Today. A Weather Underground personal weather station on the north shore of the Dominican Republic at Playa Bonita Beach reported 8 inches of rain on Thursday, July 30. At Hato Mayor del Rey in the east-central Dominican Republic, six feet of flooding occurred in some buildings, according to this tweet.

On the positive side, Isaias’s rains will help alleviate moderate to severe drought conditions that were affecting both Puerto Rico and the Dominican Republic, as explained in our YCC post on Wednesday.

Entire U.S. East Coast at risk from Isaias

Tropical storm and hurricane warnings have been hoisted for the entire Bahama Islands chain and portions of the Florida coast, and it appears likely that nearly the entire U.S. East Coast will be under a watch or warning from Isaias at some time between Friday and Tuesday. The region at highest risk is eastern North Carolina, which on Monday may lie on the strong (right-hand) side of a potential landfall by Isaias.

Satellite images on Friday afternoon showed that Isaias had a large area of intense thunderstorms, which were building over the center of circulation after it had been partially exposed on Friday morning by an intrusion of dry air. Isaias had begun bumping into a region dominated by southwesterly upper-level winds associated with a large-scale trough of low pressure. These winds were creating less favorable conditions for intensification, with moderate to high wind shear of 15 – 25 knots. Sea surface temperatures (SSTs) remained a warm 29 – 29.5 degrees Celsius (84 – 85°F), and Isaias was embedded in a moderately dry atmosphere with a mid-level relative humidity of 50 – 55%. Bahamas radar showed that Isaias had built about 50% of a complete eyewall.

Overall, these conditions favor only slow changes to Isaias’s strength through Saturday as the storm tracks to the northwest through the Bahamas and close to the east coast of Florida. Most of the intensity models favor slow weakening, but the official NHC forecast favors slow strengthening. A weaker Isaias will tend to track more to the west, potentially allowing a landfall in Florida, while a stronger storm will tend to feel the influence of the upper-level southwesterly winds on its west side, resulting in a more easterly track, keeping Isaias more to the east and preventing a Florida landfall. Unfortunately, Isaias is likely to pass close to the Bahama Islands hardest-hit by last year’s Hurricane Dorian, Grand Bahama Island and Great Abaco Island (Figure 4).

On Sunday and Monday, when Isaias is expected to turn more to the north and north-northeast as a result of the steering influence of the trough of low pressure to its west, the storm may experience a reduction in shear, to 10 – 20 knots (the technical reasons for this were well-explained in a Thursday night video discussion by Levi Cowan). SSTs of 29 – 29.5 degrees Celsius (84 – 85°F) – unusually warm for late July – extend northwards all along the coasts of South and North Carolina, so Isaias could be strengthening because of reduced shear and warm water as it approaches a possible landfall in the Carolinas on Monday.

Once Isaias moves beyond the warm Gulf Stream waters to the north of North Carolina, steady weakening is likely, with most of the intensity models and the official NHC forecast predicting it by then will be a strong tropical storm with 60 – 70 mph winds during its closest approach to New England on Tuesday.

Bahamas still recovering from 2019’s Hurricane Dorian

Isaias will be an unwelcome menace in the Bahama Islands, which are still recovering from the catastrophic impact of category 5 Hurricane Dorian of last year. After hitting St. Thomas in the U.S. Virgin Islands as a category 1 hurricane last August 28 and causing $150 million in damage to the Caribbean islands, Hurricane Dorian rapidly intensified into a category 5 mega-hurricane that powered ashore on Great Abaco Island in the Bahamas on September 1, 2019, with sustained winds of 185 mph. Dorian tied for third-strongest landfalling tropical cyclone in world history.

At landfall, Dorian was moving at just 5 mph, and portions of Dorian’s eyewall lashed Great Abaco and Grand Bahama islands with category 5 winds for a total of 22 hours before the great hurricane finally weakened to category 4 strength. Dorian’s extreme winds, storm surge of 20 – 25 feet, and rains of up to 3 feet (0.9 m) of rain combined to bring the Bahamas their most devastating natural disaster in history.

A November 15 report from the Inter-American Development Bank put damage in the Bahamas from Dorian at $3.4 billion – over 25% of their $12 billion GDP, and their most expensive disaster in history. Insurance broker Aon put Dorian’s damages at over $8 billion. Dorian killed 74 and left 282 people missing in the Bahamas, according to reliefweb.com. The authors of that report said that as of May 20, water, electricity, sanitation, and shelter continued to be challenges on Abaco Island. Medical care is also a problem, particularly with the COVID-19 pandemic occurring. The Bahamas began seeing a sharp increase in COVID-19 cases beginning in July, and had 508 total cases and 14 deaths as of July 30, according to worldometers.info.

Invest 93L off the coast of Africa unlikely to develop

A tropical wave with a well-defined surface circulation between the Cabo Verde Islands and the coast of Africa, designated Invest 93L, was suffering from high wind shear on Friday, and had only a limited amount of heavy thunderstorm activity, as shown on satellite images. The system was headed northwards into a stable air mass and over cooler waters, and it is likely to dissipate early next week. In an 8 a.m. EDT Friday Tropical Weather Outlook, NHC gave 93L two-day and five-day odds of formation of 20%.

A westward-moving tropical wave located about 1,000 miles east of the Lesser Antilles on Friday morning was producing a limited amount of shower activity. Some slow development of this system is possible as it turns northwestward over the western Atlantic by early next week. In an 8 a.m. EDT Friday Tropical Weather Outlook, NHC gave the system two-day and five-day odds of formation of 0% and 20%, respectively. Although the system has some model support for development, there does not appear to be a threat to any land areas.

The next name on the Atlantic list of storms is Josephine. The current record for earliest 10th Atlantic named storm formation is August 22, when Jose formed in 2005.

Thanks go to Bob Henson for assisting with this post.

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