What does precipitable water (PWAT) mean for intensity of storms?

Photo: Average precipitable water for ice-free oceans from 1988 to 2016 worldwide.  (Courtesy of Willis Eschenbach of WUWT)

​Precipitable water is a term (often stylized as PWAT) that is not often heard through media outlets when referring to the intensity of a storm.  This is a term best defined as: if all of the moisture in a specific column of the atmosphere was to be in one container, that amount measures an approximation of how much water can precipitate from a storm. This value could determine the strength of a storm system in accumulation and intensity.  For example, desert and high drought areas have low precipitable water rates, while moist environments (generally near the equator or near a body of water) see higher rates.  While these are the most common criteria for determining a rate, there can be fluctuations globally and seasonally.  The value for precipitable water is read in millimeters or inches and can be shown on a map similar to accumulation forecasts.

To better define the concept of precipitable water, here’s an example provided by the GFS (Global Forecast System) model for Australia.  The first example depicts the 12-hour precipitation that fell in comparison to the higher rate for precipitable water:

Photo: This is a precipitable water map for Australia on the morning of 11/3 showing values between 1.5 and 2 inches highlighted in purple. (Courtesy of Pivotal Weather)

Photo: This is a map showing accumulated rain 12 hours following the high PWAT in the area showing a concentration of the highest rainfall in Eastern Australia.  (Courtesy of Pivotal Weather)

​These photos depict an obvious correlation between the higher precipitable water rates and higher amount of rainfall.  A higher influx of moisture from open waters that becomes paired with hot and dense air can hold more water in the atmosphere, thus resulting in a larger chance of accumulating rainfall at the surface.

Since precipitable water is measured in a column of air in a given area, it can change frequently, vary over a short distance and can depend on the current season.  The most noticeable difference is during the winter when air is colder and has a decreased ability to hold large amounts of water, thus making the precipitable water rate much lower than summer.  While strong snow storms may seem like a significant producer of precipitation, a ratio of 10 inches of snow to 1 inch of water is the average, and this warrants quite a low rate of precipitable water.

The main concerns of focusing on precipitable water in a forecast is the possibility of flooding.  Precipitable water rates that are higher than average can cause trouble for those living near areas that do not commonly see high amounts of water.  Some examples include those living near rivers that don’t regularly experience water levels rising and farmers with large fields of crops.  Overall, we see precipitable water as a tool that can aid in the forecast for any form of precipitation and will give further insight into explaining expected totals.  The impacts can be avoided if there is added confidence to a forecast that allows for an audience to prepare.

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© 2019 Meteorologist Jason Maska