Weather 101: Precipitable water

Weather 101

With summer in full swing, precipitable water becomes an important variable to monitor. Why you might ask? Precipitable water can give us an idea of how moisture-rich an air mass is. An air mass with a high moisture content can produce very heavy rain across the area. Prolonged heavy rain events can lead to instances of flooding.

Precipitable water is the amount of water that would gather at the surface if the entire column of air were to condense and fall to the ground as rain.

Precipitable Water Value (inches)Moisture Content of Air Mass
0.5 inch or lessvery low moisture content
greater than 0.5 inches up to 1.25 incheslow moisture content
greater than 1.25 inches up to 1.75 inchesmoderate moisture content
greater than 1.75 inches up to 2.0 incheshigh moisture content
greater than 2.0 inchesvery high moisture content (tropical)
Precipitable Water with Associated Moisture Content

A good forecast tool for detecting the boundary of air masses (fronts) is precipitable water. This chart can help you to easily find the dry line or cold front indicated by the sharp contrast in precipitable water values. Of all the seasons it is most useful in the summer months as we tend to see the most moisture return from the Gulf of Mexico during that time.

Influences of Precipitable Water on Severe Weather

While moisture content can play a big role in how it feels outside, it can also affect what types of severe weather we see.

In supercell thunderstorms, the higher precipitable water values can dictate what the visibility of the storm will look like. A higher amount of moisture in the atmosphere can lead to a greater amount of rain being produced. This rain can then be ingested into the rotating storms. The extreme reduced visibility of these high precipitation (hp) supercells can make chasing them very dangerous.

High Precipitation (HP) Supercell Moving into Fayetteville, AR on July 21, 2018. Photo taken by Mike Price.

In addition to the reduced visibility of a supercell, the presence of higher amounts of moisture can cause the storms to become downdraft dominant. This means that the sinking air in the storm often overtakes the rising air in a storm. Without the warm moist air feeding into the updraft, the storm often dies out.

Speaking of the rising motion within a storm, high amounts of moisture can also limit the updraft strength due to the force of gravity pulling the water drops downward. With a weaker updraft and a dominant downdraft, high precipitation supercells tend to be disorganized. This of course does not mean that they don’t produce tornadoes, they definitely do.

The weaker updrafts of the storms also tend to limit the production of large hail. The high moisture along with the higher levels of (CAPE) also create efficient lightning-producing storms.

Lightning Over Northwest Arkansas. Photo taken by Douglas Keck.

For other exciting and interesting digital weather content, check out other Weather 101 and Weather Blog pieces.

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