Your author seems to remember that the weather was pretty awful at the start of last years parade, and then the rain stopped and it was jolly nice, so you shouldn't let a bit of rain put you off. The show takes place under the watchful eye of the Lord Mayor of the City of London and is one of the longest established and annual events in London which dates back to 1535 (or 1215, depending on who you listen to). One interesting aspect of the show is that it is the origin of the word "floats", which is often used in the context of carnivals. The Lord Mayor used to arrive by boat, apparently, and was followed by a procession of "floats".
The parade includes all sorts of military, business and community organisations, and the Great Twelve Livery Companies (the Mercers, Grocers, Drapers, Fishmongers, Goldsmiths, Merchant Taylors, Skinners, Haberdashers, Salters, Ironmongers, Vintners and Clothworkers) also take part, as does the Lord Mayor himself, in his special coach. If you're still enjoying it when dusk falls, don't forget the annual firework display, which begins at 5pm.
For more on all aspects of today's activities, visit http://www.lordmayorsshow.org/
But don't hang about. It starts at 11am.