According to ancient folklore, rain on July 15 heralds 40 days of downpours - and forecasters have not dismissed the possibility.
Legend has it that showers on St Swithin's day are an omen for more than a month of wet weather.
But though deluges will not blight the entire summer, experts did not rule out the potential for some rain every day.
Forecasters said many parts of the UK would be deluged with showers in the morning following heavy rain in the previous 12 hours.
Met Office forecaster Charles Powell said 25mm to 30mm fell overnight on Wednesday and warned bad weather and unseasonably strong winds will continue into Friday.
Gusts of up to 60mph are predicted in the South West, hitting coastal areas with some force.
This could result in tiles being whipped from roofs, with the additional risk of trees being uprooted in exposed areas, he said.
He said that though the unsettled weather is set to continue "for the foreseeable future", it will be broken up by sporadic sunny spells.
Saint Swithin was a ninth century Saxon bishop.
Legend has it that the removal of the saint's bones from his preferred burial place outside Winchester Cathedral to another location coincided with 40 days of continuous rain and storms.