Total amount of water computed to be in a column of air that could precipitate out as rain/snow in inches of water. In weather models, it is referred to as PWAT.
As a general rule, PWATs over 1.7″ can lead to heavy rains. PWATs greater than 2″ can lead to areas of flooding.
Tropical systems that make it into our area can have PWATs as high 2.5″