It issued a yellow - "be aware" - severe weather warning from 3am tomorrow until 6am on Thursday for much of Lothian and the Borders, Dumfries and Galloway, and for England and Wales as far south as the Midlands .
A spokeswoman said: "Bouts of rain, heavy and thundery at times, will move in from the west to affect parts of southern Scotland and northern England from the early hours of Wednesday.
"More extensive thunderstorms may then break out over a larger part of England and Wales from late Wednesday afternoon onwards into Wednesday night, leading to torrential downpours, frequent lightning and a chance of hail.
"This could result in some disruption, more likely from late Wednesday afternoon, which may include sudden localised flooding of transport routes, homes and businesses.
"Frequent lightning in association with the heaviest rain may also temporarily disrupt power supplies.
"However, many places will likely see little or no impacts. Indeed many parts of the warning area will see dry and sunny weather through much of the daytime on Wednesday.
"Areas of severe weather are not expected to occur at the same time across the whole warning area.
"Instead, the threat of heavy rain and thunder is initially confined to parts of northern England before the threat of severe weather becomes more extensive late Wednesday."