Scotland weather presenter Gillian Smart said the heat would peak as southeasterly winds pull in intensely hot, humid air from the continent.
She said: “The highest temperatures will be in central and southern Scotland, where we’re likely to see 30 Celsius.”
However, she warned of a “thundery breakdown” on Thursday afternoon.
“A weather front to the northwest will interact with the very warm air to produce heavy, thundery showers across the Western Isles, the Northwest Highlands and the Northern Isles.
“These thundery showers will feed southwards across the country, torrential in places, and bringing a risk of hail.”
A yellow warning for thunderstorms has been issued for Highlands, Orkney, Moray and Argyll and Bute from 15:00 to 23:59, and for the rest of mainland Scotland from 00:00 to 23:59 on Friday.
Parts of England and Wales have a rare red warning for extreme heat in place and it is expected to be another scorching day for most of Europe.
Temperatures will once again be around 12 to 15C above average across northern Spain, France and into Belgium and the Netherlands.