Many places should enjoy some blue skies and warmth over the next 12 hours before temperatures plunge as the weather takes another turn for the worse.
Hail, thunder, gales and torrential downpours threaten to keep summer on hold at least into the start of next month, long-range experts say.
Government forecasters predict unsettled conditions past the first week of August before any hint of a change to better weather.
A fierce low pressure system lurking in the Atlantic will smash into Britain later this evening bringing a wet and windy rush hour tomorrow.
Western regions will take the brunt of the foul weather first thing before rain moves through the UK during the day.
The outlook for the rest of the week is for below-par temperatures, wind rain and the risk of thunder in parts.
AccuWeather forecaster Eric Leister warned to batten down the hatches for torrential downpours, wind and possible flooding.
He said: “A potent storm system will lash the United Kingdom with soaking rain and gusty winds on Wednesday.
“Northern England and Scotland will face the brunt of the storm with spells of rain and occasional downpours.
“This amount of rain falling in a short period can result in some localised flooding and slow travel.
“While the heaviest downpours will affect northern parts of the country, a few showers will also dampen southern England including Greater London.”
The Met Office said today will be the best day of the week with the mercury possibly touching 25C (77F) in the south.
The north will be slightly cooler at around 21C (70F) although for most of the UK it will be a dry and bright day.
However it will turn cooler from tomorrow as unsettled conditions take hold with no sign of a return to summer glory.
Met Office forecaster Chris Page said only parts of eastern Britain will escape the grim conditions tomorrow.
He said: “Today is actually not going to be too bad a day with some sunshine for most people and only a few isolated showers.
“But there is a low pressure system over the Atlantic and this moves in tonight, winds will strengthen and by dawn tomorrow we expect rain across the southwest, Wales, northwest England and Western Scotland.
“It will be quite a wet start and not good for commuters, rain will quickly and erratically spread eastwards through the morning.
“Cooler conditions will follow behind this system by the afternoon and into early evening we expect blustery showers in the north.”
Temperatures will nosedive to around 16C (61F) or 17C (63F) in the north with the south lucky to get much above 20C (68F), he added.
He said the rest of the week and the rest of July will bring a mixture of blustery showers and cooler temperatures with only the odd blip of sunshine.
He said: “Thursday and Friday will see persistent rain in the north while further south it will be drier with a mixture of sunshine and scattered showers.
“The next five days are looking unsettled.
“However it is only July and August is just around the corner.”
But the Met Office’s longer range outlook warns stormy conditions will persist into the start of next month with temperatures hovering between “normal to cool”.
It said “strong winds” will hammer parts of the country tonight with further gusty downpours due this weekend.
It states: “Strong winds and heavy rain will spread across all parts on Wednesday.
“The Outlook for Thursday to Saturday is unsettled, windy and fresher with sunshine and blustery showers throughout this period, these heaviest in the northwest.
“A generally unsettled spell of weather is expected over the weekend with a mixture of sunshine and showers for most. “Heavy showers and strong winds are most likely in the north west [this weekend] with a risk of hail and thunder, as well as a possibility of gales in exposed areas.
“The unsettled regime looks set to continue until early August with further frontal systems affecting the UK, interspersed with clearer or perhaps showery weather and some unseasonably windy conditions across many areas.
“Temperatures should be around normal to cool, with perhaps only the far south east seeing some warmer conditions at times, although there is low confidence on this.”
Weather models predict a 40 per cent chance of stormy conditions building in western Britain by tomorrow evening.
The rest of the country is at a 10 to 40 per cent risk of seeing thunder and torrential downpours through the rest of the week and into the weekend.