Britain is braced for the tail end of a violent Atlantic storm to unleash "tropical" downpours this week.
Tropical storm Bill will start to shake things up on Thursday (June 25) with sultry humid air pushing temperatures towards 28C.
Wind and heavy rain threaten to lash the UK with this year's Glastonbury music festival looking like another mudbath.
Forecasters say the remnants of Bill, which has unleashed chaos in the United States, will collide with another stormy system heading this way.
It will "buckle the jet stream" allowing monsoon conditions to sweep in from the southwest bringing steamy temperatures and heavy showers.
Although parts of the country will enjoy a dry and fine few days, unsettled conditions will spread from the north towards the weekend.
Chris Burton, forecaster for The Weather Network, said after a cool start to the week things will warm up from Thursday.
He said: "It will turn cloudier across north-western parts of the country through Wednesday with the risk of some patchy rain."
"As we go into Friday and towards the weekend, the unsettled conditions further north and west look like slowly edging further south and east, bringing the risk of showers to all parts of the country by the weekend," he continued.
"Ex-tropical storm Bill could influence the UK's later this week, the remnants of the storm will track across the North Atlantic over the next few days and become absorbed into a large area of low pressure to the west of the UK.
"This will buckle the jet stream, allowing warm south-westerly winds to drag in rather humid air across the country.
"By Wednesday and Thursday, temperatures will get into the mid-20s across parts of England and Wales, with 25-26C possible in places."
He said although Glastonbury will start off dry and warm, rain is forecast in Somerset as the party gets going.
"The festival looks like getting off to a dry and warm start with spells of sunshine through Wednesday and Thursday and temperatures around the 20C mark," he said.
"Through Friday and into the weekend there will be an increased risk of showers."
Jim Dale, forecaster for British Weather Services, said the remnants of the storm will bring a blast of warmth before heavy downpours at the weekend.
He said: "The remnants of tropical depression Bill will create a battle ground over the country bringing very warm temperatures towards the end of the week.
"This will clash with cooler air over Scotland, there is a chance of heavy rain into the weekend.
"However it is going to be much warmer than recently with highs of 28C possible in the southeast, definitely t-shirt weather."
The full force of Bill will be felt on Friday with warm tropical air pushing thermometers up into the weekend.
Met Office forecaster Helen Roberts said: "The remnants of Bill will head into the UK just before Friday bringing breezy conditions to the north with heavy and persistent rain.
"Rainfall in the south could be moderate or heavy by the end of the week and tropical maritime air will see temperatures rise.
"Although we are not expecting the hottest day of the year just yet, it is something we are keeping our eyes on.
"The rainfall is not technically what we call 'monsoon', a European monsoon describes a change of airflow back to westerly at this time of year."
The turbulent outlook comes after the summer solstice marked a wet and miserable start to the season.
So far June has seen below-average temperatures and grey skies with the forecast for the start of July not much better.
The Met Office said showers will turn heavy into the weekend with average temperatures and rain expected into next month.
Its 30-day outlooks states: "Rain will become persistent and perhaps heavy at times through the weekend, especially in the northwest where the strongest winds are likely to be.
"The final days of June and early July will probably see further unsettled weather, with the south and east most likely to see some longer, drier spells, although thundery outbreaks are possible.
"Temperatures are expected to be around normal for the time of year, but above average at times in the southeast and generally rather cool in the northwest."
Bookies have slashed odds on a record low temperature this month after a chilly and miserable June so far.
Alex Donohue of Ladbrokes said: "Sadly the odds of June being anything other than cold, wet and miserable are looking pretty long indeed.
"The betting suggests weather records for freezing temperatures could easily be broken before the month is out."