The alert covers most of the UK, the Met Office said: causing confusion everywhere.
Thunderstorms will be avoided in some locations, but thunderstorms can become increasingly widespread after Monday.
The storm is likely already underway in parts of Northern Ireland and Scotland by early Monday morning and will then develop over parts of England and Wales later in the morning and into the afternoon.
Storms from the west should begin to ease later. Very locally, there can be 20-30 mm of precipitation per hour and up to about 50 mm of rain in 3-6 hours.
Hail and frequent lightning are also possible.
Thunderstorms can affect the following areas: Central, Tayside & Fife, South West England, North East England, Northern Ireland, Grampian, Wales, West Midlands, Highlands & Eileen Shere, London & South East England, North West England, East England, Dumfries, Galloway, Lothian & Borders, Yorkshire & Humber, Essat Midlands, Strathclyde.
Extreme weather conditions can quickly flood homes and businesses, and floods, lightning, hail and high winds can damage some buildings.
The Japan Meteorological Agency warned that train and bus services could be delayed or canceled in the event of flooding or lightning strikes.
Sprays and sudden floods can also make driving difficult and close some roads.
Finally, there is a small chance that power outages will occur and other services to some homes and businesses will be lost.