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More generally, a 30% chance of precipitation means there is, in the opinion of the forecaster, a 30% chance of measurable precipitation in the time period to which it applies ( measurable precipitation is 0.01 inch of precipitation or more). ...

(M)ost of the time, the forecaster is expressing a combination of degree of confidence and areal coverage. If the forecaster is only 50% sure that precipitation will occur, and expects that, if it does occur, it will produce measurable rain over about 80 percent of the area, the PoP (chance of rain) is 40%.

1/4 (0.25) of an inch of rain A light rain for 2-3 hours, moderate rain for 30-60 minutes or heavy rain for 15 minutes. ... 3/4 (0.75) of an inch of rain A light moderate rain never reaches this amount, heavy rain lasting for 2-4 hours. There would be deep standing water for long periods of time.

Moderate rainfall measures 0.10 to 0.30 inches of rain per hour. Heavy rainfall is more than 0.30 inches of rain per hour. Rainfall amount is described as the depth of water reaching the ground, typically in inches or millimeters (25 mm equals one inch). An inch of rain is exactly that, water that is one inch deep.

The forecast of a 30 percent chance of rain can mean all of the area will get rain if it rains, but the forecaster has only a 30 percent confidence that rain will indeed arrive. It can also mean that scattered rain is a certainty, but it will affect only 30 percent of the area.

On average, thirteen inches of snow equals one inch of rain in the US, although this ratio can vary from two inches for sleet to nearly fifty inches for very dry, powdery snow under certain conditions.

So, even with a 30 percent chance of rain, you could still see flooding or a severe storm. Probability of precipitation is just one tool to explain rain chances, so as forecasters we use other tools to try to explain to you how the weather will develop throughout the day.

For the entire United States, excluding Hawaii and Alaska, the average amount of moisture falling as rain and snow is 30.21 inches (767 millimetres).

An 80 percent chance of rain (or of any other kind of precipitation) means the weather forecaster believes there will be an eight in ten chance (or 80 chances out of 100) of measurable precipitation (0.01 inch or more) in the area under consideration during the time interval that is specified in the weather forecast ( ...