West Pacific: TD 20W tracking west-northwest in the West Pacific Ocean Sept. 23; close approach to Guam late Sept. 23 /update 1

Event

Tropical Depression 20W is tracking west-northwest in the West Pacific Ocean Sept. 23. As of 10:00 ChST, the storm's center of circulation was approximately 196 km (122 miles) southeast of Anderson Air Force Base, Guam. Forecast models indicate the system will turn to track northwestward and strengthen into a tropical storm before making a close approach south of Guam the evening of Sept. 23. 20W is forecast to intensify into a typhoon early Sept. 24 and gradually shift to a northerly course through Sept. 26. The storm's track and intensity forecast remain somewhat uncertain, and the system may change accordingly over the coming days.

Government Advisories
The US National Weather Service has issued a flood watch for Guam, Rota, Saipan, and Tinian through the afternoon of Sept. 24. Rainfall of 7.5-13 cm (3-5 inches) is possible for Tinian and Saipan, 5-10 cm (2-4 inches) of rain is forecast across Rota, and 2.5-7.5 cm (1-3 inches) of rain is likely over Guam. Potentially hazardous sea and coastal conditions are also forecast in the region. Dangerous rip currents are likely across the east-facing reefs of the Marianas through the afternoon of Sept. 24.

Authorities will likely issue new warnings or update existing advisories throughout the system's progression in the coming days. Weather warnings could remain active even after the system's immediate threat has diminished, as some areas may still be susceptible to rain-induced hazards. The possibility of localized evacuations cannot be discounted if weather conditions prove particularly hazardous.

Hazardous Conditions
The storm may bring heavy rainfall, strong winds, and rough seas to Guam and the Northern Mariana Islands. Sustained heavy rainfall could trigger flooding in low-lying communities near streams, creeks, rivers, and urban areas with inadequate stormwater drainage systems.

Sites located downstream of large reservoirs could experience flash flooding after relatively short periods of intense rainfall. Rain-induced landslides are possible in steeply sloped terrains. Flooding could isolate some communities for several days. Prolonged swells and storm surge generated by the system will likely result in coastal flooding as the system approaches land. Persistent onshore flow could make it difficult for the surge to recede and for water levels to decrease in coastal river catchments.

Transport
Inclement weather associated with the storm could trigger localized business, transport, and utility disruptions. Floodwaters and debris flows could render some bridges or roadways impassable, impacting overland travel in and around affected areas. Areal flooding in urban locations could also result in severe traffic congestion, while strong winds will pose a hazard to high-profile vehicles. Heavy rain and low visibility may trigger flight disruptions at regional airports.

Disruptions triggered by inclement weather and resultant hazards, such as flooding, could persist well after conditions have improved. If there is severe damage to infrastructure, repair or reconstruction efforts may exacerbate residual disruptions.

Health
Stagnant pools of water during and after flooding increase the incidence of insect- and waterborne diseases, such as dengue fever, cholera, and malaria. The threat of these disease outbreaks is usually elevated in low-income or underdeveloped areas of major urban centers due to the presence of incomplete or open sewer lines. The latent threat of waterborne contaminants from inundated industrial sites cannot be discounted; exposure to raw sewage and other hazardous materials mixed with floodwaters poses a serious health threat.

Advice

Activate contingency plans in areas where officials forecast tropical storm or typhoon conditions. Heed any evacuation orders that may be issued. Use extreme caution in low-lying coastal areas and near streams, creeks, and other waterways due to the potential for severe flooding and storm surge. Stockpile water, batteries, and other essentials in advance. Charge battery-powered devices when electricity is available; restrict the use of cellular phones to emergencies only. Power down mobile devices when not in use. Keep important documents and necessary medications in waterproof containers. Observe strict food and water precautions, as municipalities could issue boil-water advisories following flooding events. Take precautions against insect- and waterborne diseases in the coming weeks.

Plan accordingly for protracted commercial, transport, and logistics disruptions in areas in the path of the storm, especially if vital infrastructure is damaged. Seek updated information on road conditions before driving or routing shipments through areas where flooding has occurred. Confirm flights before checking out of hotels or driving to the airport; clearing passenger backlogs may take several days in some locations.

Resources

Joint Typhoon Warning Center
US National Weather Service