Predictions that the sun would shine on the royal newlyweds, the most recent forecast suggests hundreds of thousands of people lining the streets on Friday will get drenched.
If it does rain, it also means the public will be denied the chance to see the bride and groom in an open carriage on their way back from Westminster Abbey, as a closed-in carriage is on standby to be used in wet weather.
Temperatures will probably be somewhere in the high teens but it is difficult to pin down so far ahead.
"The weather is always changing, there is that risk that we are seeing on the current outlook that we have at the moment, but nearer the time the models might change a little bit.
Sunday saw cooler weather with a top temperature of 25.3C (around 78F) in Lee-on-the-Solent, Hampshire, and a maximum of 24C (75F) across London.
A weak cold front over Scotland and Northern Ireland will push south today bringing cloud and some drizzle to areas such as the Pennines and mountain areas of Wales.
Temperatures will rise to a high of 22C (72F) in London and the south of the country, and between 11C and 14C (52F to 57F) in Scotland and Northern Ireland.
Lindsay Dove, forecaster with MeteoGroup, said: "Today will be noticeably cooler, across the south it will be maybe two or three degrees cooler than yesterday.