Tuvalu, home to 11,500 people living on nine scattered atolls all less than 4.5 metres above sea level, will be hit on Thursday and Friday by "king tides" associated with the new moon, Hilia Vavae of the Tuvalu Meteorological Office said.
"We are not quite sure what will happen but we expect most of the areas will be flooded by the sea for an hour or so," she said.
On Thursday at 0440 GMT the tide will peak at 3.07 metres and on Friday at 0519 GMT will reach 3.1 metres.
Vavae said on the capital atoll of Funafuti homes and perhaps the airport would be flooded, along with her own office.
She did not expect the flooding to be as bad as in 2001 when most of the island disappeared. That was associated with an extreme La Nina global weather phenomena.
The extent of flooding during king tides depends also on local weather conditions at the time.
Vavae said at this point they were predicting rain bands over the islands, but no severe storms.
Over the last decade, successive Tuvalu leaders have claimed their state will be the first victim of sea level rise associated with global warming.
They have used the argument to claim special immigration access to New Zealand and recently were in discussion with another Pacific state, Niue, on moving their people there.
Niue last month was severely damaged by a cyclone.
Vavae said the local people tended not to be concerned by the king tides although successive high tides had left the island's once fresh water lens too brackish for any more use.
She said while parts of Funafuti were sinking, including an area where a University of Hawaii tide gauge stands, she added it was too soon to tell what would happen to Tuvalu in the long term.
Tuvalu, 1100km north of Fiji and about 3400km northeast of Australia, is the Polynesian or Ellice Island part of the former British colony of the Gilbert and Ellice Islands.
The Gilberts became Kiribati and both won independence in 1979.