A strong tropical wave came out from West Africa on July 24, 2020. The wave looked vigorous on satellite for the next 24 hours and operational models hinted that the system could develop into a tropical cyclone in the next several days. As a result, the National Hurricane Center (NHC) started forecasting a high probability of formation in 120 hours on July 25, assigning a 70%-80% when the perturbation was located several hundred miles southwest of Cabo Verde Islands (see Figure 1). On Sunday July 26, NHC increased the chance of formation to 90% when the wave passed the Longitude 35 West. The tropical wave struggled to develop on Monday as it was interacting with a dry air mass with Saharan dust particles to the north over the Tropical Atlantic waters. 

Figure 1: Tropical Weather Outlook on Saturday July 25, 2020 at 2 PM AST, showing the disturbance near 30 West.

The next day July 28th, shower and thunderstorm activity increased several hundred miles east of the Lesser Antilles. As a result, NHC started issuing advisories on Potential Tropical Cyclone Nine by mid-morning on Tuesday. The disturbance continued disorganized and elongated during the next 24 hours as it moved fairly quickly around 20 knots over the Leeward Islands. The disturbance became Tropical Storm Isaias about 155 miles south of Ponce Puerto Rico at 11 PM AST Wednesday night (see Figure 2). 

Figure 2: Cone of uncertainty of Tropical Storm Isaias on Saturday July 29, 2020 at 11 PM AST. The center of Isaias was located about 155 miles south of Puerto Rico.

Although the center of Isaias passed further south than anticipated, it brought tropical force gusts and significant rainfall across Puerto Rico during the overnight hours of Wednesday into Thursday July 30th. The trailing trough of Isaias affected the area with the west and southwest sections of Puerto Rico receiving rainfall throughout most of the day on Thursday. The total rainfall accumulations between 8 AM on July 29th and 8 AM July 31th ranged from 4 and 8 inches, with some areas of eastern Puerto Rico reporting in excess of 10 inches during the event.  The most impacted areas in terms of flash flooding was the west, southwest, eastern interior sections of Puerto Rico. Several reports of impassable primary and secondary roads were received across those areas. Also, numerous landslides were also reported mainly in steep terrain along La Cordillera Central and over the Sierra de Luquillo affecting local roadways. 

The initial Flood Watch was issued by the Forecast Office in San Juan for all Puerto Rico and the US Virgin Islands Tuesday, July 28th, 2020 at 11:15 AM AST. It included a total rainfall forecast of 3 to 6 inches with isolated amounts up to 10 inches. The Flood Watch and rainfall forecast were successfully validated with an observed total rainfall of 12.43 inches which were reported at Río Blanco, Naguabo (LBLP4) between 8:00 AM AST Wednesday, July 29th, 2020 and 8:00 AM AST Friday, July 31st, 2020. 

Flooding began to affect portions of southeast Puerto Rico early Thursday, July 30th, 2020 as the first band of heavy rainfall moved onshore. The first Flash Flood Warning was issued at 5:18 AM AST for  Humacao, Gurabo, Yabucoa, Caguas,  Juncos, Las Piedras and San Lorenzo. By 8:30 AM AST, the eastern third of Puerto Rico was already under Flash Flood Warnings. Flooding became widespread by noon, with Flash Flood Warnings issued for southwest and west Puerto Rico.  

24 out of 110 river gauges in the USGS streamflow network rose above the minor or moderate USGS-NWS flood stage.  Every river gauge along Rio Grande de Manatí, Río Grande de La Plata, Río Grande de Arecibo, and Río Grande de Loíza rose above flood stage with several points rising above moderate flood stage. Significant flooding was observed along Río Yagüez, Río Hondo and Quebrada Grande in Mayagüez where dozens of homes were affected. A total of 31 Flash Flood Warnings and 11 Flood Warnings were issued across Puerto Rico, respectively. No warnings were issued for the US Virgin Islands. 

Isaias’ rains significantly improved the observed drought conditions across the local islands with “Moderate Drought” (D1) and “Severe Drought” (D2) classification being removed. 

Strong winds affected Puerto Rico with maximum sustained winds of 30 to 40 mph and gusts to around 60 mph. Damage reports associated with these strong gusty winds included multiple billboards, trees and power poles/lines being knocked down, as well as roofs being significantly damaged. 

Numerous floods were reported with this tropical storm mainly on July 30th. Flash flooding led to closure of main and secondary roads. The most significant flooding occurred at Rio Hondo and Quebrada Grande sectors in Mayaguez where rainfall accumulation estimates reached 8-10 inches. Several landslides were also reported mainly in steep terrain along La Cordillera Central and over the Sierra de Luquillo affecting local roadways. Nearly half a million residents of Puerto Rico reported power outages.

In contrast, the impacts over the U.S. Virgin Islands were minimal although windy conditions and squall lines were reported at times.