The power of precipitable water
By Jared Shelton News-Press NOW meteorologist
When it comes to forecasting warm season showers and thunderstorms, meteorologists must consider several factors in order to best answer a number of key questions: What are the chances of showers and storms? When will they start and stop? Will rain be light or heavy? Is there a threat for severe weather? How much rain is expected? The list goes on.
Although nuanced and in some cases complex, the basis of forecasting to answer these rain-related questions ultimately lies in the supply of three main ingredients — moisture, lift and instability. Of these three variables, lift and instability tend to be the most fickle and can mean the difference between clear skies or a heavy thunderstorm. Depending on the path of a front or low-pressure system (lifting mechanism) and local maxima of hot air near the surface (source of instability), determining exactly when and where a summer storm will flare up can be a difficult task, if not impossible.
In contrast to the chaotic nature of lift and instability on a local scale, moisture tends to be a more straightforward variable to forecast. An integral ingredient of all precipitation types, moisture is more or less an ambient characteristic of a given air mass, one that can be quantified before clouds even start to form. Relative humidity (RH) is one of the most common measures of surface moisture, a familiar number to most, expressed as a percentage.
Another commonly referenced figure for moisture content in the air is dew point, expressed in degrees Fahrenheit. Both measures of moisture in the air can be used to calculate the heat index or feels-like temperature and also aid in forecasting the likelihood of summer showers and thunderstorms in a given area.
A lesser-known, yet robust forecasting tool for quantifying moisture in the atmosphere is precipitable water, abbreviated as PWAT. Expressed in inches, precipitable water is defined by the National Weather Service as “a measure of the depth of liquid water at the surface that would result after precipitating all of the water vapor in a vertical column over a given location, usually extending from the surface to 300 millibars.” To translate, precipitable water provides a decent estimate of how much water could be squeezed out of the atmosphere over one particular location … in other words, how much it could rain under the right circumstance.
PWATs are particularly useful when forecasting a risk for heavy rain and flooding when storms are in the forecast but not the likelihood of rain or thunderstorms themselves. For example, PWATs can be very high in the presence of a tropical air mass, even on a clear day. If clouds grow to precipitate in the presence of high precipitable water, they have the potential to dump copious amounts of rain, which may or may not be realized.
The next chance of showers and thunderstorms across the mid-Missouri river valley is set to arrive Friday night. With high PWATs forecasts for the area due to an influx of deep tropical moisture, the National Weather Service is warning of the potential for high rainfall rates with any storms that develop. An early warning for the possibility of localized flash flooding, thanks to the power of precipitable water.
