Calculating precipitable water in Texas thunderstorms

AUSTIN, Texas - Central Texas is no stranger to very heavy rain.  It’s an important part of the climatology and ecology of the environment but can have many big impacts.  

Flooding being the most obvious of them all. 

With this in mind, it’s important to be able to forecast extreme rain events and have an idea of how much rain could be expected in the forecast period.  That’s where the parameter precipitable water comes in. 

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Precipitable is a measure of the total atmospheric water vapor contained in a column of air.  That column usually starts at the surface and goes to about 24000 feet above the ground.  That amount of area is enough to approximate the top end of rainfall the area could see.   

This metric is used to predict the highest rainfall the atmosphere can create through one thunderstorm.  In general, a number above 1.4 is considered very high though in extreme situations, such as hurricanes or tropical waves, that number can go as high as 3.   

The numbers may seem low but that is the total rainfall in that specific, stationary column of air.  And the atmosphere doesn’t stay still for very long at all so the storms dumping very heavy rain will sometimes drop that rain over the same area for several hours.   

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Precipitable water values are a very useful tool when forecasting extreme rainfall potential.