Tropical Storm Isaias Forms in the Caribbean Sea

Tropical Storm Isaias has formed in the Central Caribbean Sea, now the earliest “I” named storm in the Atlantic basin history. The previous record for the earliest ‘I’ named storm in the Atlantic basin is Irene on August 7, 2005.

Isaias is also the 5th named storm to form this July, tying the 2005 record for the most Atlantic named storms to form in July since 1851.

At 11:00 PM AST, the center of Tropical Storm Isaias was located near latitude 15.8 North, longitude 67.0 West. Isaias is moving toward the west-northwest near 20 MPH (31 KM/H), and a west-northwestward to northwestward motion with some decrease in forward speed is expected over the next couple of days. On the forecast track, the center of Isaias will move over Hispaniola on Thursday and near the Southeastern Bahamas by early Friday.

Maximum sustained winds are near 50 MPH (85 KM/H) with higher gusts. Some weakening is likely when the center moves over Hispaniola with some strengthening expected by Friday.

Tropical-storm-force winds extend outward up to 345 miles (555 KM) mainly to the northeast of the center. The estimated minimum central pressure is 1004 millibars.

Tropical Storm Isaias Forecast Cone as of 11:00 PM AST Wednesday 29th July 2020. (National Hurricane Center)
Tropical Storm Isaias Forecast Cone as of 11:00 PM AST Wednesday 29th July 2020. (National Hurricane Center)

Watches & Warnings

Tropical Storm Isaias Watches and Warnings as of 11:00 PM AST Wednesday 29th July 2020. (National Hurricane Center)

A Tropical Storm Warning is in effect for:

  • Puerto Rico, Vieques, Culebra
  • U.S. Virgin Islands
  • The British Virgin Islands
  • St. Martin and St. Barthelemy
  • Saba and St. Eustatius
  • St. Maarten
  • The Dominican Republic entire southern and northern coastlines North coast of Haiti from Le Mole St Nicholas eastward to the northern border with the Dominican Republic
  • Turks and Caicos Islands
  • Southeastern Bahamas including the Acklins, Crooked Island, Long
    Cay, the Inaguas, Mayaguana, and the Ragged Islands
  • Central Bahamas, including Cat Island, the Exumas, Long Island,
    Rum Cay, and San Salvador

A Tropical Storm Watch is in effect for:

  • Northwestern Bahamas including Andros Island, New Providence,
    Eleuthera, Abacos Islands, Berry Islands, Grand Bahamas Island, and
    Bimini

Interests in Cuba and the Florida peninsula should monitor the progress of this system.

A Tropical Storm Warning means that tropical storm conditions are expected somewhere within the warning area within 36 hours.

A Tropical Storm Watch means that tropical storm conditions are possible within the watch area, generally within 48 hours.

For storm information specific to your area in the United States, including possible inland watches and warnings, please monitor products issued by your local National Weather Service forecast office. For storm information specific to your area outside of the United States, please monitor products issued by your national meteorological service.

Hazards Affecting Land

Tropical Storm Isaias as of 11:00 PM Wednesday 29th July 2020. (Weathernerds)
Tropical Storm Isaias as of 11:00 PM Wednesday 29th July 2020. (Weathernerds)

WIND: Tropical storm conditions will continue across portions of the Leeward Islands, the U.S. and British Virgin Islands and Puerto Rico through tonight. These conditions are forecast to reach portions of the Dominican Republic and Haiti within the warning area by late tonight or early Thursday, and the southeastern Bahamas and Turks and Caicos Thursday afternoon. Tropical storm conditions are expected in the Central Bahamas beginning Friday morning and are possible in the northwestern Bahamas beginning late Friday.

RAINFALL: Isaias is expected to produce the following rain accumulations:

  • Across the British and U.S. Virgin Islands and Turks and Caicos: 3 to 6 inches.
  • Across Puerto Rico, the Dominican Republic, northern Haiti, and eastern Cuba: 3 to 6 inches, with isolated maximum totals of 8 inches.
  • Across the southeastern Bahamas: 4 to 8 inches.

These rainfall amounts may lead to life-threatening flash flooding and mudslides, as well as potential riverine flooding beginning today. Urban and small stream flooding is expected for the U.S. Virgin Islands, eastern Puerto Rico, and Hispaniola.

SURF: Swells generated by Isaias will be affecting portions of the Leeward Islands, the Virgin Islands, and Puerto Rico through Thursday. These swells are forecast to reach the north coast of the Dominican Republic, the Turks and Caicos Islands and the southeastern Bahamas tonight or Thursday. These swells are likely to cause life-threatening surf and rip current conditions. Please consult products from your local weather office.

This system is of no direct threat to the Windward Islands, including Trinidad and Tobago.

Tropical Storm Isaias Forecast Discussion

Observations from recent scatterometer passes over the system show that it now has a sufficiently well-defined center to be designated as a tropical cyclone. The current intensity is estimated to be 45 kt, but these winds are currently occurring over the Atlantic waters well to the north and northeast of the center.

Since the cyclone is expected to move over Hispaniola on Thursday some weakening is likely within the next 24 hours. However, since Isaias has such a broad wind field, the weakening will probably not be as significant as in a typical tropical cyclone with a small radius of maximum winds. Also, a re-formation of the center to the north of Hispaniola may occur. Later in the forecast period, some strengthening is likely, although this may be offset by southwesterly wind shear on the order of 20 kt in 2-3 days. The official forecast is close to the intensity model consensus but well below the latest LGEM guidance.

The scatterometer data show that the center of the system is south of the previously estimated track, so there is a lot of uncertainty in the initial motion estimate of 285/17 kt. Isaias should move on a west-northwestward to northwestward track on the southern and southwestern side of a mid-tropospheric ridge. In 2-3 days, the system is expected to turn north-northwestward due to a weakness in the ridge and an approaching trough.

Later in the forecast period, the trough should cause Isaias to turn toward the northeast. The official track forecast is a little to the east of the previous one and a little west of the simple and corrected dynamical model consensus tracks. It should be noted that further adjustments to the forecast tracks are indeed possible, especially after Isaias moves north of Hispaniola.

Key Messages:

  1. Isaias will produce heavy rains and potentially life-threatening flash flooding and mudslides across the Virgin Islands, Puerto Rico, the Dominican Republic, northern Haiti, and over the southeastern Bahamas.
  2. Tropical storm conditions are likely across portions of the Virgin Islands, and Puerto Rico through tonight and will spread westward to portions of the Dominican Republic, Haiti, and the southeastern Bahamas and Turks and Caicos and the Central Bahamas on Thursday and Friday. Tropical Storm Warnings are in effect for these areas. Do not focus on the details of the track forecast, as rainfall and wind hazards will extend far from the center of the system.
  3. While this system could bring some rainfall and wind impacts to portions of Cuba, the northwestern Bahamas, and Florida later this week and this weekend, it is too soon to determine the location or magnitude of those impacts. Interests there should monitor the progress of this system and updates to the forecast over the next few days.

Forecast Discussion by Forecaster Pasch from the NHC.

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