Flood alerts have been put in place for some parts of the UK - while other regions are braced for the arrival of scorching temperatures with highs in the 30sC.
The Met Office has issued a yellow weather warning with some torrential downpours possible in Scotland and Northern Ireland.
Elsewhere it should remain unsettled until warm air arrives from the south during Wednesday with the mercury soaring above 30C in many regions by the end of the week.
Met Office forecaster Alex Deakin predicted rain would start across Northern Ireland early on, but "really gets going in western Scotland where it lasts for most of the day," reports MirrorOnline.
There is also the risk of "drizzly" rain in northwest England, particularly in Cumbria - as well as on the hills in Wales.
"So, we do actually have a yellow warning in force - that rain could really mount up through the day, 40, 60mm - more than that on the hills, possibly as much as 100mm, which could make for some pretty bleak traveling conditions."
The warning is in place from 6am until midnight with flooding of a few homes and businesses a possibility. The Scottish Environment Protection Agency lists six flood alerts in place for Tuesday.
Mr Deakin suggested the rain will spread to eastern and northern Scotland, before potentially returning to Northern Ireland with some falling in the north-west of England and the Welsh hills.
"For a good part of England, it's a dry day and the further south east you are the better chance of seeing lengthy spells of sunshine, which will again lift those temperatures," he added.
Forecast site The Weather Outlook also predicted heavy downpours in places.
It said: "Wet weather affects much of the north today. In the south it remains dry with bright spells.
"Tomorrow further spells of rain are likely in the north and west but it will be turning warmer in the south.
"Through Thursday and Friday temperatures soar with 35C (95F) possible in the south. There is uncertainty about the weekend but cooler air from the Atlantic may gradually return southeastwards.
"There is a risk of thundery showers breaking out."
Met Office five-day weather forecast
Today
Outbreaks of rain spreading across northern UK and turning heavy and persistent across parts of western Scotland. Mainly dry elsewhere with some sunshine in southeast. Windy in northwest.
Tonight
Mild and humid for most with rain sinking south into central UK, and turning heavy over hills of northwest England. Slightly cooler and fresher in north, mostly dry in south.
Wednesday
Cloudy in southern UK with some warm sunshine during the afternoon. Dry and bright in north, but elsewhere rather cloudy with rain at times.
Outlook for Thursday to Saturday
Mainly dry with sunny spells, although some parts of the west, especially far north-west, will see some showery rain at times too. Widely warm, becoming very hot in the southeast.
