The Observatory said it will issue the Red Rainstorm Warning Signal at 11.10am on Thursday, replacing the black alert that has been in force for more than three hours.

"However, there will still be squally showers and thunderstorms locally. Showers will be heavy at first," the forecaster said.

An amber rain signal was issued at 5.22am and was upgraded to a red warning at 6.35am, before it was escalated to black at 7.50am.

The Standby Signal No. 1 remains in force as Tropical Storm Podul continues to move inland.