Rain is likely to hit most of the country during the day and although many areas will clear up in time for midnight, the likelihood in London is that as Big Ben strikes so will the rain.
The rain will take the shine off the chain of beacons that will straddle the country and the 200m high river of fire planned for the Thames, which is expected to draw thousands of revellers along the riverbank.
While organisers expect their pyrotechnics to survive the worst the weather is expected to bring tonight, it will pose a headache for the many open- air street events including theatre and dance.
London and the South-east will fare poorly tonight but the rest of the country looks likely to escape the worst, according to forecasters. Temperatures will range from 4C (39F) in Edinburgh to 6C (43F) in London, Birmingham and Cardiff.
"Scotland and Northern Ireland will be windy with a spell of rain during the afternoon and the evening but it should clear up before midnight," one forecaster said. "Wales and the West of England will be breezy and damp with some rain but it should hopefully clear up before midnight."
In eastern and south-east England, including London, it will be cloudy and breezy with rain coming in from the west during the evening. "Although temperatures are relatively mild, because of the breeze it will feel chilly."
Meanwhile, heavy rain has continued to plague the South-west where hundreds of flood warnings have been issued for rivers. Engineers were pressing on with plans to protect the Somerset town of Bridgwater after repeated rain and flooded meadows left the town's 14th-century defences, the Baltmoor Wall, at near-breaking point. The nearby village of Muchelney is surrounded by water at least 2ft deep and accessible only by four-wheel-drive vehicles.
While the weather poses a nuisance factor for many revellers, it has taken its toll on some of the wackier celebration plans including those of the British climber Paul Bailes who was planning to host a mountain-top dinner party.
Mr Bailes, 40, organised a seven-strong party to scale the 22,834ft peak of Mount Aconcagua in the Andes and set up a dinner table where they would have sampled a three- course meal with wine.
After last-minute hitches he swapped the location to the 4,400ft peak of Ben Nevis in Scotland. But because of treacherous wind, rain and snow the team has been advised not to undertake the task. It is now being rescheduled for March 2000.