Precipitable water (PWAT) is the depth of liquid water that would result if all of the water vapor in a vertical column of the atmosphere condensed and fell as rain, usually reported in inches or millimeters.
What the column measurement captures
PWAT is computed by integrating the mixing ratio through the depth of the atmosphere above a location. A column with a shallow but very moist surface layer and a dry middle troposphere can carry the same PWAT as a column with modest moisture spread through a much deeper layer, so the single number hides the vertical distribution.
Because it is a bulk quantity, PWAT is well suited to comparisons. Forecasters compare the current PWAT at a site against the climatological mean and standard deviation for that date. Values well above the local climatology, particularly above the 90th percentile, are often flagged as a heavy rainfall or flash flood signal.
How forecasters use it
For severe weather, PWAT gives a quick read of moisture available to storms and how much rain a slow-moving cell could produce. A rich column supports high rainfall rates when storms fire and can favor water-loaded downdrafts. A dry column tends to produce more evaporation of falling precipitation and stronger, cooler outflows.
For heavy rain and flash flood forecasting, PWAT is one of the ingredients in the ingredients-based method used by NWS forecasters. It is paired with storm motion, warm cloud depth, and instability to judge whether repeated cells over the same area could bring enough rain to threaten flooding.
Important limits
A high PWAT does not tell you where or when it will rain. Lift, forcing, and storm mode still decide whether the moisture is tapped. Values well above normal with no lift can sit for a day producing only isolated showers.
PWAT also does not describe rainfall efficiency. Two storms in the same PWAT environment can produce very different totals depending on their motion, their depth, and whether entrainment of drier air aloft reduces how much of the column moisture actually reaches the ground.