There will be ponchos and Pimm's at the ready on The Mall later when 10,000 picnicking revellers celebrate the Queen's birthday beneath storm clouds.
Thundery showers were on the march towards central London as an army of organisers completed the final preparations on Sunday morning for a street party of epic proportions.
But with umbrellas banned on site, thousands of plastic macs are on standby alongside the pork pies and cake.
The Met Office has forecast rain for central London on Sunday morning before scattered heavy and thundery showers move through the area in the afternoon.
The best of the weather will be in the far north of the country, with sunshine predicted all day for street parties on Shetland.
A yellow warning of rain covering an area stretching from the South East to the North West of England is in place from 11am to 9pm, with those downpours moving slowly eastwards bringing a risk of localised surface flooding.
However forecaster Mark Wilson offered a glimmer of hope for royalists hopping on the Tube in their glad rags and galoshes.
"There is an 80 to 90% chance of rain through the morning from around 8am which will break up in the afternoon, although when the sun shines through and the temperature goes up it can spark showers.
"But they will be very hit and miss and if they are lucky they could miss London."
Temperatures are expected to reach 22C (71F) in London, while it will be a warm and muggy day with showers for most.
Two bands of rain will merge in the afternoon bringing rain across the country.
Low pressure is forecast to dominate over the British Isles far into the week ahead bringing unsettled conditions, while low wind speeds will mean rain showers will clear slowly.