Training can be done in all weather. One just needs to adjust their methods.
When it rains or snows I don't do long precisions. I make sure the distances I jump are realistically safe. If it is icy or a huge downpour I usually don't do anything except small things like drilling jumps from no heights and practice rolls.
Harsh conditions is the time where your fear is a good thing. Black ice and heavy rain isn't a joke when training. If you feel uncomfortable with the height, distance, or movement you are thinking of preforming don't do it.

Wear warm clothing and keep hydrated.

my opinon would be do whatever you feel comfratable with if you want to do a 5 foot precicion jump go for it one other thing is dont over do your self


be safe 

I actually jsut recently made the mistake of misjudging the weather and my move. Parkour, in my opinion, is like driving. With a car, never drive faster than is safe for current conditions. I attempted a cat leap onto a SLEEK/WET railing that looked like so: IIIIII and I've done this before. I would catch my hands on the top of the rail and land my balls of my feet onto the middle. Jumped, got the hand placement, HAD the foot placement, feet slipped down, smashed both my shins and knees into the railing. I have been inactive since Thanksgiving *did this stunt the day before* but now have recovered mostly from it. Might wait a day or two more before I do anything really 'cool.'

This is a grand example of rest. Love it or hate it every single free runner, parkour'er, or tracour'er has HAD to do it. Know your limits, can you do a successful precision jump 5 feet onto a possibly iced rail/cinderblock/whatever you are jumping on?

this also brings up something that I do. I like to bring a towel or two with me. I can use it to either:
1. wipe off/dry up a rail/surface
2. wipe off/dry up me