Wait until you are at the start before applying overlays. This way you can make a decision after examining the snow. If things are what you expected, go with your original plan.
Sometimes things don't happen the way you expect them, especially with outdoor sports. The Start Pack and Troubleshooting sections seen below describe the tools and procedures for several "quick fix" wax applications that can be performed on-site when mother nature decides to surprise you.

Start pack: Bring these items to the start with you and you'll be armed to the teeth.

Overlays
Q6, 65, Race Rocket

Waxes
Butter
HyperZoom
RaceZoom New Snow
RaceZoom Old Snow

Brushes
Soft, fine steel for removing hard waxes
Brass/horsehair for 65
Brass/horsehair for base waxes
Horsehair for Q6 and general polishing
Brush cleaner (stiff brush to remove wax from other brushes)

Other items
Fiberlene to wipe base clean
Synthetic or felt cork
Nylon stockings to wrap around cork for polishing 65
Scotchbrite pad to work hot waxes into the base
Sharp scraper in case there is still extra wax left on the base

Troubleshooting at the start:
What happens when actual conditions are not as expected
Problem 1: You waxed too warm.
Problem 2: You waxed for old snow and it is snowing.
Problem 3: You waxed for new snow and it didn't fall.
Problem 4: You waxed too cold.
Problem 5: You think you waxed right, but there is snow sticking to your base and it feels kind of slow.

Problems 1-3 have the same solution.
The time required to fix them is 5-10 minutes:
Brush hard. Rub the appropriate Race Rocket bar (new snow or old snow) on the base and cork it in, then brush it out with an aggressive brush. Do this two or three times.

Problem 4 can have three different solutions depending on snow temperature.
The time required to fix it is 10 minutes:
If the snow is very wet, around 0C (32F): Brush hard. Rub 65 (or Q6) lightly on the base, then rub Butter on top. Work it into the base using a Scotchbrite pad, then brush with a brass/horsehair brush and polish with a nylon stocking.
If the snow is 0C to -8C (32F to 18F): Brush hard. Rub 65 lightly on the base, then rub HyperZoom (or the appropriate RaceZoom) on top. Work them into the base using a Scotchbrite pad, then brush and polish.
If the snow is colder than -8C (18F): Brush hard. Rub 65 lightly on the base, then rub the appropriate Race Rocket on top. Work them into the base using a synthetic cork, then brush and polish.

Problem 5 indicates that your wax or overlay is too soft for the snow conditions.
The time required to fix it is 5 minutes:
Brush hard. Rub the appropriate Race Rocket bar (new snow or old snow) on the base and cork it in. If the snow still sticks to the base, brush hard with an aggressive brush to remove as much wax as possible from the structure.