The Met Office this afternoon upgraded the yellow weather warning to an amber warning.
What to expect
The Met Office says to prepare for:
* Flooding of homes and businesses is likely and could happen quickly, with damage to some buildings from floodwater, lightning strikes, hail or strong winds
* Fast flowing or deep floodwater is likely, causing danger to life
* Where flooding or lightning strikes occur, delays and some cancellations to train and bus services are likely
* Spray and sudden flooding probably leading to difficult driving conditions and some road closures
* Power cuts likely to occur and other services to some homes and businesses could be lost
* Some communities may become cut off if roads flood
A Met Office forecaster said: "The very high afternoon temperatures mean that it is likely that some heavy thunderstorms will develop across parts of East Anglia and Lincolnshire on Friday afternoon.
"Where the thunderstorms occur torrential downpours are possible with as much as 30 mm of rainfall in 1 hour and 60 mm in 3 hours. Large hail, frequent lightning and strong, gusty winds will be additional hazards.
"However, many places will see much lower rainfall amounts. This amber warning covers the period of highest risk into this evening. Further thunderstorms are possible until late evening, however, and this risk is addressed in the yellow warning already in force."