Jubilee revellers heading to London on Sunday to watch the one of the largest flotillas ever assembled on the River Thames should brace themselves for wet weather.
Royal watchers will be huddling under umbrellas rather than basking in glorious sunshine as 1,000 boats sail down the river to mark the Queen's 60th year on the throne.
Andy Ratcliff, a forecaster at MeteoGroup, the weather division of the Press Association, said: "London will see rain at times and there could be the odd heavy burst. It will be a chilly day so people should dress accordingly.
Later on Sunday, the River Thames will host a spectacle not seen on its waters for more than 300 years on Sunday - a River Pageant in honour of the Queen's Diamond Jubilee.
Up to one million people are expected to watch from the river banks to watch a procession of 1,000 ships, with the royal barge at its heart, carrying the monarch and her husband, the Duke of Edinburgh.
The lavishly decorated royal barge will also carry the Prince of Wales, the pageant's patron, the Duchess of Cornwall, the Duke and Duchess of Cambridge and Prince Harry.
Every conceivable vessel will be afloat on the Thames from kayaks, skiffs, Dunkirk little ships and dragon boats to tugs, barges, rowed shallops and passenger vessels.
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At its head will be a floating belfry with each bell named after a senior member of the royal family.
Amongst the celebrities taking part in the pageant will be Olympic gold medallists Sir Matthew Pinsent and Sir Steve Redgrave, rowing with 16 others in the million-pound row barge, Gloriana.
Kate's parents Michael and Carole Middleton will be on the 19th century paddle steamer the Elizabethan along with London 2012 Olympic chief Lord Coe, as well as the BBC's director-general Mark Thompson.
Prime Minister David Cameron and his wife Samantha will watch the pageant from the moored Corvette HMS President in the City.
Near Tower Bridge, the royal barge will moor and the Royal Family will watch as the flotilla passes by.
The event will close when the final music barge, carrying members of the London Philharmonic Orchestra and the Royal College of Music Chamber Choir creates a Last Night of the Proms-style atmosphere, playing Land of Hope and Glory, Rule Britannia and the national anthem.
The pageant is one of the highlights of the four-day Diamond Jubilee weekend, which also features a Jubilee concert outside of Buckingham Palace, multiple lunches and receptions, and a procession through London.