Mourners watching the Queen's coffin procession in central London are set for sunshine and 23C highs on Wednesday afternoon - but they are likely to face heavy rain if they are queuing in the morning.
A low pressure that has been over the country in recent days bringing warmer weather and heavy showers is gradually leaving with a high pressure system moving in from the north.
It means that as the week progresses there will be increasingly cooler weather and it will feel notably colder by the weekend with widespread temperatures in the mid to high teens.
For much of the UK it will start brightly but in the south east there is set to be heavy rain and mourners lining the streets of London to see the Queen's coffin are facing the strong possibility of heavy rain.
While high pressure from the Atlantic was trying to nudge its way in to potentially throw the rain off course of the capital, forecasters said the morning was still likely to be a wet one.
"(It) will be a rather wet start to the day on Wednesday - especially so in London where people may be queuing to see the lying in state throughout the day," BBC Weather's Simon King said.
That rain was expected to clear before midday though, and by the afternoon temperatures could rise to 23C.
Met Office forecaster Alex Burkill said: "For many a bright and a little fresh start to Wednesday morning but a wet picture across the southern counties and the rain could be heavy at times as we head through morning rush hour and it will take a little time to clear away so around the far south east I think it will be well into the afternoon before things turn fully dry and bright.
"Elsewhere across much of the UK it is going to be a fine day with just some showers across northern parts of Scotland. There will be some sunshine around but despite this temperatures in the north set to be a little bit lower than we have seen of late getting into the mid to high teens at best.
"In the south the temperatures perhaps a bit higher than on Tuesday because the wet weather will have cleared away and there will be some sunshine around but it may feel a touch more comfortable or fresher for some because of the humidity that we have had recently will also have cleared away."
The Queen's coffin will leave Buckingham Palace at 2.22pm on Wednesday afternoon and make its way to Westminster Hall.
There will be a short service conducted by the Archbishop of Canterbury, ahead of four days where the Queen's body will lie in state.
Those wishing to pay their respects will be able to file solemnly past the Queen’s coffin 24 hours a day from 5pm on Wednesday until 6.30am on September 19.
UK forecast for the next 5 days
Rain in far south. Otherwise mostly dry.
Today:
Rain, some heavy, in far south slowly clearing to the southeast whilst weakening. Elsewhere mostly dry with good sunny spells though light showers affecting west and north Scotland and perhaps Northern Ireland. Some heavier showers later for the Northern Isles.
Tonight:
Dry for most with clear spells. Chilly for far north England and much of Scotland. Showers continuing to feed into parts of Northern Ireland and north Scotland, some heavy here.
Thursday:
Dry for most with good sunny spells. Perhaps an isolated shower east England during the day. More frequent showers feeding into Northern Ireland and north Scotland, some heavy here.
Outlook for Friday to Sunday:
Generally fine and dry, variable cloud and sunny spells. Isolated showers possible, mainly the northern and eastern areas, where it will also be breezy. Turning cooler with some chilly nights.
