Explaining precipitable water and how it is used in severe weather forecasting
When we talk about severe weather setups in Central Missouri, the discussion naturally gravitates towards familiar terms: high temperatures, humidity, and the track of a cold front. But behind the scenes, meteorologists are also tracking other metrics to gauge the intensity of these downpours that inevitably accompany storms. One of these metrics is called precipitable water, or PWAT for short, and it is the exact reason localized flash flooding is a concern for the severe weather potential this afternoon and evening.
To understand why tonight's thunderstorm potential could lead to rapidly rising waters across Columbia and surrounding communities, we have to pull back the curtain on how atmospheric moisture is actually measured.
If you look at the standard daily humidity metrics, they only tell you what the air feels like right at the surface where you are standing. PWAT, however, takes a three-dimensional look at the entire environment.
To contextualize PWAT, imagine taking a giant, vertical straw stretching from the pavement in Mid-Missouri all the way up to the edge of space. If you were to grab that entire column of air and completely wring it dry like a wet sponge, the depth of the liquid water pooling at the bottom is your precipitable water value.
On a standard, comfortable day in Missouri, that sponge is relatively dry, yielding a PWAT value under half an inch. There simply isn't much total moisture available for clouds to utilize. But when deep, sub-tropical moisture surges northward, that value can soar well past two inches. When that happens, the atmosphere essentially becomes an incredibly heavy, fully saturated sponge waiting for something to wring it out.
Meteorologists track these shifts using weather balloons that send back vertical data profiles known as soundings. When we plot this data on a Skew-T diagram, we focus entirely on two primary lines: the temperature and the dew point.
When the air column is dry, these two lines are spread far apart across the chart. This indicates a large dewpoint depression. Even if a storm manages to form, a lot of its rainfall evaporates into the dry air layers before ever hitting your backyard.
When severe weather or heavy rain setups loom, these two lines closely hug each other from the ground all the way up through the mid-levels of the atmosphere. This tight convergence means the air column is heavily saturated. Any thunderstorm tapping into a juiced profile like this becomes a hyper-efficient rainmaker, dropping impressive amounts of water in minutes because none of it is lost to evaporation on the way down.
This exact saturated profile is what is locking into place over Central Missouri this afternoon and evening. Persistent, strong southwesterly winds have established a pipeline, continuously dragging an expansive reservoir of moisture straight out of the Gulf and dumping it into our region.
With daytime temperatures hovering in the mid to upper 80s and a high amount of ultraviolet radiation coming from the sun, the low-level atmosphere has turned into a very favorable setup. Humidity values are also high, feeding that invisible atmospheric sponge until it is ready to burst.
Since PWAT values are climbing into the 1.5-2+" range this afternoon and evening, any thunderstorm that develops along the oncoming boundary will have immediate access to a high supply of water vapor. Not only will these storms bring the potential for lightning, damaging wind, large hail, and tornadoes, but they will also be capable of dumping high amounts of rain in a short period of time. In urban spots or low-lying rural areas where drainage is slow, the water has nowhere to go. Keep a close eye on the roads tonight, and remember: if you encounter a flooded localized path, turn around, don't drown.
