A flash flood is a sudden surge of water that can be particularly dangerous for canyoneers. They are usually caused by rainfall which may occur well upstream and out of sight of the affected canyon. The time between normal conditions and extreme, unsurvivable amounts of water can be very small, so the best protection against flash floods in canyons is to stay out of the canyon whenever conditions favor a flash flood.
The most important factor for a flash flood is how much rain the watershed above the canyon receives over a relatively short amount of time. Flash floods can occur even when rain clouds are not visible as it may be raining many miles away and still be within the same drainage. Rain directly on your team while in a canyon is mostly irrelevant except that it serves as an indication of how much rain the upstream watershed may be receiving.
When reading forecasts for precipitation, a 40% chance of rain (for instance) means that if you chose a location within the forecast area at random, you would have a 40% chance of collecting some rain over the forecast period. That may mean the forecaster believes half of the forecast area has an 80% chance of receiving some rain while the other half has no chance, or it may mean the forecaster believes the entire area has a 40% chance of receiving some rain (or some other variation). This is contrary to videos and publications from Zion National Park where they claim a 40% chance of rain means that 40% of the forecast area will certainly receive some rain. Confusion on this point is common.
A sudden rise in flow rate or sudden change in the water clarity (usually suddenly muddy) are strong indicators that a flash flood is imminent and your team needs to get to safety immediately; finishing the canyon, or even getting the remainder of the team down a rappel, is usually not an option at this point.