Make the most of any sunshine this week as wind, rain and dismal temperatures threaten to blight the rest of summer.
Storms lined up in the Atlantic will be steered towards Britain by an unusually strong jet stream over the coming days.
Chilly northerly winds due at the weekend will push temperatures well below those seen earlier this summer.
The entire country faces an unsettled spell ahead with long-range forecasts not boding well for the rest of August.
The best of any warm weather this week will be in the south although thermometers will still struggle to ruse much above 21C.
Scotland and the north will feel much cooler with daytime highs of around 17C or 18C dropping to single figures overnight.
The weather will take a downward turn tomorrow with heavy rain, gales and thunder expected in parts.
Most places will get a downpour with further unsettled conditions forecast until the weekend, the Met Office warned.
Spokesman Grahame Madge said: “Low pressure moves over the UK on Wednesday bringing heavy rain through the rest of the week.
“Everywhere will get showers which could be heavy, persistent and thundery at times while winds could reach gale-force in exposed regions.
“As low pressure moves away later this week it will pull cooler winds in from the north resulting in lower temperatures.”
“The good news is much of the country could see a brighter picture at the weekend although there will be showers around”, he added.
Met Office forecaster Emma Sharples blamed the autumnal conditions on an unusually strong jet stream over the UK.
She said: “We have a low-latitude and fairly powerful jet stream at the moment steering these lows in from the Atlantic.
“It is further south than we would usually expect at this time of year.”
The outlook for the rest of August, though mixed, is largely unsettled with the risk of further spells of heavy rain and wind.
Warmer waters across the North Atlantic will lead to a more active hurricane season and greater risk of stormy weather in the UK, experts warn.
Todd Crawford, chief meteorologist at The Weather Company, said: “There has been a clear trend over the past month towards warmer North Atlantic ocean temperatures which favour a more active 2017 Atlantic tropical season.”
Weather Channel UK forecaster Amy Hodgson said Britain faces at least another two weeks of wet and windy weather.
She said: “The next couple of weeks are forecast to remain unsettled in a mainly westerly flow with temperatures around normal to slightly below-normal for the time of year and above normal precipitation amounts.
“Our sub-seasonal forecast predicts low pressure continuing to dominate particularly across the north of the UK.
“There is a risk that if a negative North Atlantic Oscillation develops temperatures will be cooler more widely across Europe.”
The Met Office’s 30-day outlook warned of a mixed bag ahead for the rest of August and meteorological summer.
It warns the north will remain cool and wet through the month with the best chance of seeing any decent weather confined to the south.
It states: “Friday and Saturday [will] see a continuation of sunshine and showers for many areas, with the showers heaviest and most frequent in the northwest with the risk of thunder.
“Thereafter it looks likely to remain fairly changeable across the UK, with spells of rain or showers mixed with some drier and brighter interludes.
“As we head into the middle of August, there are some signs that southern areas may see a greater frequency of drier and sunnier spells, but elsewhere is likely to remain on the changeable side with bands of rain interspersed with sunshine and showers.
“Temperatures are likely to remain on the cool side for many, but southern areas are likely to feel warmer in the drier and sunnier spells.
“Later in August, confidence reduces significantly, but a northwest-southeast split is probable, with the northwest seeing the most frequent wet weather.”