A severe thunderstorm warning for heavy rain is now in force for the interior and southern parts of Viti Levu from Sigatoka through Coral Coast to Navua and the southern parts of Vanua Levu from southern Bua and Cakaudrove.
The Nadi Weather Office says a trough of low pressure affects the Fiji group while a northeast wind flow prevails over Fiji.
They say daytime heating has caused thunderstorms to develop over the inland areas of Viti Levu and Vanua Levu.
The weather Office says these thunderstorms are expected to become severe, drift south and affect the southern parts of the two main islands.
They say severe thunderstorms may produce heavy rainfall, which could lead to flash flooding of low-lying areas within the next few hours, while frequent and dangerous lightning strikes can also be expected.
People are advised to keep clear of flooded creeks and drains, not to walk, cycle, or drive through floodwaters, unplug electrical appliances and computers if lightning is around or thunder is heard, if caught in an open field or playground during lightning, minimise body height and move quickly to a safe shelter such as a building or house, not to shelter under trees during lightning, stay indoors, away from windows, and keep children indoors.
The Weather Office says people should stay vigilant and continue to monitor weather conditions.
Thunderstorm activity with intense heavy rainfall and lightning may continue until late afternoon or evening, due to uncertainty in thunderstorm movement and development, not all areas may be affected, and impacts may vary from place to place.
