The Environment Agency and Met Office said heavy rain could affect the county between 23:00 BST on Wednesday and midday on Thursday.
There is a 30% chance of more than 50mm of rainfall in six hours in some areas.
Last Thursday, vehicles broke down, homes were flooded to "waist deep" and roads were closed after flash floods hit Dorset.
Dorset Police warned residents and motorists to take extra care and to seek regular weather updates from the Met Office.
A spokesman said that if localised flooding happened in urban areas, there was "potential for a number of surface water flooding incidents".
The worst-hit areas last week were Lulworth to Bournemouth and on to Christchurch.
Patients had to be evacuated from a private hospital after a ceiling was damaged because of the water.
Several people were rescued from vehicles stuck in flood water. Bournemouth's Lower and Central Gardens were also submerged.