More heavy rain is forecast for the lower North, Northeast and Central regions through to Tuesday as Tropical Storm Conson approaches and makes landfall on mainland Southeast Asia.
The Meteorological Department said on Friday that Conson was expected to hit central Vietnam on Monday, and then degrade.
Small boats were warned to stay ashore in both the Andaman Sea and Gulf of Thailand until Tuesday, the department said.
The latest weather warning extended the period of the storm's impact from Sunday to at least Tuesday.
Rain has covered most parts of the country since Tuesday, with flooding in 20 provinces but no fatalities reported.
Seven provinces in the northern region were hit hardest, with 630 out of 916 households in 19 districts suffered from flooding, the Disaster Prevention and Mitigation Department reported on Friday.
Lom Sak district in Phetchabun and Wang Thong district in Phitsanulok were among the worst hit, with both municipal districts flooded on Thursday night. Soldiers were called in to help rescuers and local officials to evacuate people and their belongings to safety zones.
The Meteorological Department said rain was expected over about 60% of Greater Bangkok during the next week.