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PTC-9 Projected to Become Tropical Storm Isaias Tonight or On Wednesday

| July 28, 2020 @ 4:08 pm

SUMMARY OF 500 PM AST…2100 UTC…INFORMATION

LOCATION…14.4N 55.9W
ABOUT 435 MI…700 KM ESE OF THE LEEWARD ISLANDS
MAXIMUM SUSTAINED WINDS…40 MPH…65 KM/H
PRESENT MOVEMENT…WNW OR 285 DEGREES AT 23 MPH…37 KM/H
MINIMUM CENTRAL PRESSURE…1007 MB…29.74 INCHES

WATCHES AND WARNINGS

A Tropical Storm Warning is in effect for…
* Puerto Rico, Vieques, Culebra
* U.S. Virgin Islands
* British Virgin Islands
* Antigua, Barbuda, Montserrat, St. Kitts, Nevis, and Anguilla
* Guadeloupe, Martinique, St. Martin, and St. Barthelemy
* Saba and St. Eustatius
* St. Maarten
* Dominica
* The Dominican Republic from Cabo Caucedo northward along the northern coast to the Dominican Republic/Haiti border

DISCUSSION AND OUTLOOK

The overall structure of the disturbance has not changed much since this morning. New clusters of convection have developed over the northern portion of the elongated circulation and the Air Force Reserve reconnaissance aircraft has been unable to find a well-defined center. The aircraft measured some SFMR winds of 30-35 kt well to the northwest of the trough axis, and these data along with the earlier ASCAT form the basis of the 35 kt initial intensity. The latest dynamical model guidance still suggests that the system will consolidate over the next 12-18 hours and the disturbance is expected to become a tropical storm before it reaches the Leeward Islands on Wednesday.

The particulars of the track forecast remain uncertain since the system lacks a well-defined center. The initial motion estimate is a highly uncertain 285/20 kt. The overall track forecast reasoning remains the same, with the disturbance expected to move west-northwestward over the next several days to the south of a subtropical ridge that extends westward over the western Atlantic. The 12Z track guidance is in agreement on the overall scenario, but some differences are noted due primarily to the system’s strength and vertical depth. Models such as the UKMET and HWRF, which depict a stronger cyclone, are along the northern side of the guidance envelope, while the weaker solutions remain more equatorward. The updated NHC track forecast is similar to the previous advisory and lies a little south of the various consensus aids. Regardless of the exact track, the system is expected to bring locally heavy rainfall to much of the Lesser Antilles, and tropical-storm-force winds to portions of the Leeward Islands, the Virgin Islands, and Puerto Rico within the next 24-36 hours.

The global model guidance suggests that the system will consolidate overnight and should acquire a better-defined center. As this occurs, the disturbance is likely to gradually strengthen within the low vertical wind shear environment that it is situated in. Between 36 and 72 hours, the strength of the system will largely be dependent on the amount of land it encounters. If the system moves over the Greater Antilles it is likely to be weaker than indicated in the official forecast, but a path north or south of Hispaniola could result in a stronger system. Later in the period, some southwesterly shear could also temper further strengthening. The GFS and ECMWF models generally weaken the system in the longer-range due to these negative factors. The NHC intensity forecast is unchanged from the previous advisory, but a usually high degree of uncertainty regarding the future track and intensity of the system remains. Interests in Hispaniola, the Bahamas, Cuba, and Florida should continue to monitor forecasts as changes to both track and intensity are likely.

KEY MESSAGES

1. Potential Tropical Cyclone Nine will produce heavy rains and potentially life-threatening flash flooding and mudslides across the northern Leeward Islands, the Virgin Islands, Puerto Rico, and the Dominican Republic.

2. Tropical storm conditions are likely across portions of the Leeward Islands, the Virgin Islands, Puerto Rico, and portions of the Dominican Republic beginning Wednesday and spreading westward through Thursday. 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. The details of the long-range track and intensity forecasts are more uncertain than usual since the system does not have a well-defined center and it could move over portions of the Greater Antilles later this week. However, this system could bring some rainfall and wind impacts to portions of Hispaniola, Cuba, the Bahamas, and Florida by the end of the week. Interests there should monitor its progress and updates to the forecast over the next few days.

Category: ALL POSTS, Severe Weather, Tropical

About the Author ()

Scott Martin is an operational meteorologist, professional graphic artist, musician, husband, and father. Not only is Scott a member of the National Weather Association, but he is also the Central Alabama Chapter of the NWA president. Scott is also the co-founder of Racecast Weather, which provides forecasts for many racing series across the USA. He also supplies forecasts for the BassMaster Elite Series events including the BassMaster Classic.

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