The warning follows an announcement issued by the Thai Meteorological Department forecasting heavy to very heavy rain and rough seas from today through Thursday (May 19-22).
Officials warned that waves in the upper Andaman Sea, including waters off Phuket, are expected to reach two to three metres, rising above three metres in storm areas. Small boats have been advised to remain ashore.
The Meteorological Department also warned of flash floods, runoff and possible landslides in vulnerable areas due to persistent heavy rain and strong winds generated by an intensifying low-pressure system near Myanmar and a strengthening southwest monsoon.
Beach safety warnings have also been intensified across Phuket, with red flags raised at several beaches including Patong. Lifeguards warned swimmers to stay out of the water because of dangerous rip currents generated by monsoon surf conditions.
Video footage shared online by local officials showed rough surf conditions at Patong Beach, with warnings telling tourists not to enter the water.
The Department of Disaster Prevention and Mitigation urged visitors and residents to monitor weather conditions closely, obey beach warning flags and avoid risky water activities during the storm period.
The agency reiterated that double red flags indicate extremely dangerous swimming conditions and that entering the sea is strictly prohibited.
Heavy rain and strong winds have already caused damage in several parts of Phuket since Sunday night.
In Patong, municipal workers were deployed to remove trees that fell onto houses along Phetchakood Rd after strong winds swept through the area. Officials inspected damage at affected homes and said assistance would be considered under disaster relief regulations.
Elsewhere on the island, emergency crews responded to numerous reports of fallen trees blocking roads and damaging power lines in Phuket Town, Sri Soonthorn, Chalong, Mai Khao and Pa Khlok.
In Chalong, municipal disaster officials cut away trees that had collapsed onto electricity lines in Soi Kluai Mai Thai to prevent electrocution hazards and restore safe traffic access.
Phuket City Municipality reported crews worked throughout the night clearing large trees that had fallen across roads and residential areas in Talad Nuea and Talad Yai during heavy rain and strong wind gusts.
Rawai Municipality also confirmed that flood-prevention projects in several flood-prone areas are being accelerated ahead of peak monsoon conditions. Officials said four major drainage improvement projects are under way to reduce recurring flooding in low-lying communities.
The Meteorological Department said the next weather update would be issued later today.