Carol Kirkwood reported the BBC Breakfast weather forecast live from the Garden Museum in Lambeth, south London, where she told viewers it would be a “muggy” day.
She said: “It’s quite a muggy start in the south, the temperatures here are about 19 degrees Celsius, but at the moment between 15 and 18 degrees Celsius across England and Wales.
“For Northern Ireland and Scotland it’s much fresher for you this morning; we’re looking at around five or six or maybe 10 degrees Celsius.”
A weather front gaining in strength throughout the day will cause downpours and patchy rain in the west.
It will be a cloudy start for England and Wales with a few light drizzles, but some sunshine in the east later.
Temperatures will drop across the country with highs of 18 degrees Celsius in London this morning and down into single figures in the countryside in Scotland.
It should be cooler and fresher further north, but brighter, despite rain spreading into Northern Ireland and most of Scotland in the early afternoon.
In England and Wales the cloud will break up to leave behind some sunshine and largely fine weather.
A risk of rain and showers will exist mainly across western parts of the UK, whereas areas in the south east could see temperatures soar above 20 degrees.
Further north it will be milder with 14 and 15 degrees in Glasgow and Belfast, respectively, and by the end of the afternoon Northern Ireland and Scotland turning wet.
Heavy rain throughout the night will spread across north England and it will feel cooler in west Scotland and Northern Ireland.
It will be warm and pleasant tomorrow, especially in the south east where most of the sunshine will remain.
Some outbreaks of rain over northern England and north Wales, meanwhile showers in Wales, the Midlands and East Anglia.
Conditions are set to improve later in the day, with the best of the weather in London where temperature could top 25 degrees Celsius.
Elsewhere it will be in the low 20s or high teens, but some sunshine away from the showers in the Hebrides, the Scottish highlands and islands.
As temperatures sizzle this week the bookies have been forced to slash the chances of 30C being hit to just 4/7.
It is also only a 6/4 shot that this month goes down in history as the warmest June on record.
Ladbrokes spokeswoman Jessica Bridge said: “It’s going to be a hot and sweaty week thanks to the mercury rocketing, meaning there could be a lot of sleepless nights on the cards.”
She added: “Storm Hector seems like a long time ago now and it looks like the thermometer will nudge 30C before the end of the month.
“The scorching temperatures over the next few days could also see the hottest ever Royal Ascot meeting.”
BBC Breakfast airs weekdays on BBC One at 6am.
