Heavy, thundery showers on Saturday afternoon and evening have the potential to bring localised flooding, hail and frequent lightning.
A spokesman for the Met Office said: “Skies will brighten following a band of showery rain, though rising temperatures will cause heavy showers to develop.
“Whilst many will miss the worst, some places could see 20-30 mm in an hour or less.
“The risk of this looks slightly higher across Scotland than across England or Northern Ireland.”
A spokesman for the Environment Agency said: “There is a potential thunderstorm risk from tomorrow afternoon through until tomorrow evening (Saturday).
“As thunderstorms are extremely hard to predict, there is much uncertainty as to where these may strike but the Met Office have indicated NW England is most at risk.
“Should a thunderstorm strike, there is the potential for localised torrential rain and hail which may lead to surface water flooding. We do not expect rivers to react however smaller becks and streams may be more vulnerable to the intense rainfall.
“We also have spring tides over the weekend and we have Flood Alerts in force for North Morecambe Bay. The highest tide is at lunch time tomorrow and we expect flooding to low lying land and roads in the Sandside and Arnside area.”
For more information on tidal warnings in force and river levels visit https://flood-warning-information.service.gov.uk