The warning area covers all but extreme eastern and western parts of the region, and some local torrential rain has been forecast, with 20-30mm possible in one or two hours.
This will be accompanied by hail and lightning in a few places.
A Met Office spokesperson said: “Where flooding or lightning strikes occur, there is a chance of delays and some cancellations to train and bus services.
“Spray and sudden flooding are likely to lead to difficult driving conditions and some temporary road closures
“There is a slight chance that power cuts could occur and other services to some homes and businesses could temporarily be lost.”