Heavy showers and thunderstorms are expected across northeast England and southeast Scotland on Wednesday, the Met Office said.
The warning, which stretches from north of Edinburgh to south of Durham, comes into force at 11am and runs until 6pm.
Some areas could see 15-20mm of rain in less than an hour, which could bring flooding, forecasters added.
There could also be damage to buildings and other structures from lightning strikes.
Train and road networks could also see delays, while there could be a “short term loss” to power in some areas.
Find out the forecast for your area
It comes after the UK recorded its hottest day of the year so far.
On Tuesday, a temperature of 34.7C (94.4F) was recorded at St James’s Park in central London, the Met Office said.
Cooler conditions have been sweeping across the country from northern parts overnight.
Forecasters said despite the possibility of thundery downpours, eastern areas should become drier and sunnier as evening approaches.
England and Wales is expected to be dry overnight, but it will be cloudier in Scotland and Northern Ireland with showers spreading eastwards.
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