Moderate to heavy rain is expected in the next few days, and this could cause flash floods in several low-lying areas, the National Environment Agency says.
Places include Kampong Lorong Buangkok, Moutbatten Road and Everitt North Road.
Other areas like Chinatown, Boat Quay, Geylang and Jalan Besar could be affected too.
This is even though there's the Marina Barrage acting as a tidal barrier to alleviate flash floods, PUB says.
Residents and shop-owners are advised to take the necessary precaution.
However PUB says it has stepped up the monitoring, inspection and cleansing of 90 hotspots island-wide to ensure the drainage system is not blocked.
For the latest weather reports, visit the NEA website at www.nea.gov.sg or call 6542 7788.
To report obstruction in drains or to check the flood situation, call the PUB hotline at 1800-284-600.