After a spell of unusually warm weather, UK temperatures are set to tumble with the coldest night of autumn so far arriving by midweek.

Wednesday night will bring the first frost of the season to some places, as colder air arrives from the Arctic.

In some areas, minimum temperatures could be just a degree or two above freezing by dawn on Thursday.

Along with the plummeting temperatures, some much-needed rainfall is on the way for England and Wales.

A southerly shift in the jet stream will allow colder air to flood across the country through the middle of the week

So far this autumn, the jet stream - which determines what sort of air mass affects the UK - has been sitting to the north.

This has allowed mild south or south-westerly winds to flow, particularly across the southern half of the British Isles.

This week, however, the jet stream will start to sink further south, allowing the winds to turn more northerly and introduce a colder air mass.

While it will feel significantly colder compared to recent weeks, especially in the south, temperatures will actually be near normal for the time of year or perhaps just a little below.

Afternoon highs on Wednesday will be around 12C to 14C in Northern Ireland and Scotland and 14C to 16C in England and Wales.

Temperatures by Thursday dawn will be between 3 and 9 Celsius in most towns and cities although rural places could be several degrees lower.

Whilst the northern half of the UK has already seen temperatures near the seasonal average this month, it has consistently been several degrees above average in the south, with somewhere reaching above 20C every day since 11 June.

By the overnight hours, temperatures will be unseasonably low with a noticeable chill first thing on Thursday.

Minimum temperatures at dawn will be around 5 to 7C for most places, but the coolest areas in northern and eastern England as well as eastern Scotland could be just a degree or two above freezing.

A touch of frost is likely on grass and car windscreens here.

Whilst scattered showers will affect the north-west on Wednesday, a spell of more prolonged rain is expected in eastern England - good news for areas that saw very little rain over the past month.

In addition to the autumn chill, much-needed rain will spread across England and Wales from midweek.

Wednesday will start cloudy and wet as a frontal system slowly drifts eastwards.

Whilst sunshine will return to many places in the afternoon, rain is expected to linger for most of the day in East Anglia and the far south-east.

Southern and eastern England, where it has been especially dry lately, could receive around 20 to 30mm.

This would equate to up to half a month's rainfall in just 24 hours.

Nearly three quarters of England and all of Wales are still in drought.

The Environment Agency has previously said that 20% more rainfall than average is needed for at least seven months before all areas will be out of drought status.

The chilly feel to the weather will continue into Thursday, with maximum temperatures between around 11 to 16C north to south.

With plenty of dry and bright weather though, it may well feel pleasant in the afternoon sunshine for many.

By Friday, temperatures will be on the rise once more.

Westerly winds will push milder air across England and Wales, where 20C could again be reached in the east.

Further north, no frost is expected by Friday dawn and afternoon highs will peak in the mid teens Celsius.

Keep up to date with the longer range forecast in BBC Weather's Monthly Outlook.

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