The warning on 3 October follows the driest September since records began in 1910.
September was also among the top five warmest on record, but it follows the wettest January-August period recorded.
This means water resources are "normal", said the Met.
Nursery consultant John Adlam advised outdoor growers to batten down the hatches and get fleecing.
The five day forecast sees rain spreading south, and "generally windy" and cooling weather.