Revellers across the UK are being warned to pack umbrellas for New Year's Eve events and avoid floodwater, as 2012 ends on a damp note.
Forecasters say there is a risk of showers in many places, particularly the north and west of England.
The weather has caused travel chaos in parts of Wales.
Up to an inch of rain (20 to 25mm) fell on higher ground in south Wales on Monday morning.
Arriva Trains Wales said it was forced to cancel services and use buses to transport passengers between Llanrwst and Blaenau Ffestiniog, Conwy, and between Newtown and Machynlleth, Powys.
Flooding is also affecting the line at Ferryside, Carmarthenshire.
During the afternoon and evening of Monday, a band of rain will push eastwards across the UK, followed by a band of showers, BBC forecaster Holly Green said.
These are particularly likely to affect the north and west, but the South East could also see some showers.
It is possible 2012 will be the wettest year since records began.
But 2013 will start on a brighter note, with many places staying dry on New Year's Day.
Hundreds of thousands of people are expected to line the banks of the Thames in London on Monday evening to watch the New Year's Eve fireworks display.
Much of central London will be closed to traffic, but the Tube will run all night and most public transport will be free from 23:45 GMT until 04:30 GMT.
Mayor Boris Johnson said the fireworks would round off a year that "glimmered with excitement and enthusiasm".
Tens of thousands of people are expected to descend on the streets of Edinburgh for the world-famous Hogmanay celebrations.
Scottish rock band Simple Minds will headline the Concert in the Gardens, where they will be joined by The View and Bwani Junction against the backdrop of Edinburgh Castle.
And the organisers of the Derry-Londonderry 2013 City of Culture year are planning to kick off the year-long celebrations with a spectacular fireworks display.