The Met Office said the warm air brought a risk of lightning, hail, storms and flash flooding later on Tuesday and into Wednesday morning.
A Yellow "be aware" warning is in place from 17:00 until 18:00 on Wednesday.
The extent of the thunderstorms is uncertain, but some places could see over 30mm of rain in an hour.
Prestwick in Ayrshire had the day's highest recorded temperature at 29C, followed by nearby Auchnicruvie at 28C.
Shetland was one of the only places in Scotland not to enjoy the sunshine - with the weather largely cloudy with patchy rain and drizzle.
Aberdeen - which was sitting at around 19C at lunchtime - was hotter than Nairobi, Kenya.
Heavy, thundery showers are forecast for Tuesday night.
The adverse weather could cause disruption on Wednesday morning. Frequent lightning is expected to continue along with high temperatures of 22 or 23C in the afternoon.
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Temperatures topped 29C on Scotland's hottest day of the year so far - sparking a warning of overnight downpours and thunderstorms.