10 hours ago
Record amounts of atmospheric moisture fueling heavy rainfall in western Pennsylvania, northern West Virginia
While the Upper Ohio Valley is no stranger to heavy rainfall and flooding, the conditions that have been in place over the past couple of days have supercharged the atmosphere to produce some localized deluges in portions of the area.
What makes this latest round of heavy rain and flooding unique is the amount of moisture that has been observed in the atmosphere leading up to and during the height of the worst flooding from Saturday night through Wednesday morning.
(Photo: KDKA Weather Center)
What is precipitable water?
We can measure the vertical depth of moisture in the atmosphere through the launch of weather balloons looking at a value called precipitable water. Precipitable water is a measure of the amount of moisture in the column of air between two different levels in the atmosphere.
While high values of precipitable water don't guarantee heavy rain will occur at any moment, it does indicate an exceptionally moist atmosphere that is more efficient at producing and packing in more raindrops per unit volume in precipitating clouds. This is especially true if there is a front or low-pressure area moving through that can lift the moisture and cause clouds to form.
(Photo: KDKA Weather Center)
Precipitable water values break records
On Saturday night before the heavy rain that led to deadly West Virginia Panhandle floods, precipitable water values measured in Pittsburgh were within the top 95% of available records. On Sunday morning, before a flash flood emergency was declared south of Morgantown near the Fairmont, West Virginia, area, moisture levels remained anomalously high, although not record-breaking.
The area eventually moved into record-breaking levels of moisture starting the morning of June 17. On June 17 at 8 a.m., 1.83 inches of precipitable water was observed breaking the 1984 8 a.m. record of 1.82 inches.
At 8 p.m. on June 17, Pittsburgh smashed the precipitable water record of 1.77 inches in 1984 (the 8 p.m. record) with 1.89 inches of precipitable water.
This morning, June 18, we broke another precipitable water record with 1.85 inches observed, breaking the old record of 1.83 inches set in 2024.
(Photo: KDKA Weather Center)
A warmer atmosphere has more room to hold moisture
The weather pattern that led to these unusually high levels of moisture involved a strong ridge of high pressure across the southeast U.S. and semi-persistent troughing in the Midwest and Central Plains.
A persistent southwest flow near the surface and aloft led to rich moisture flowing north from the Gulf. Water temperatures in the Gulf are also much warmer than normal, which allows for greater rates of evaporation and more moisture transport inland to the U.S. mainland whenever the weather pattern is favorable.
(Photo: KDKA Weather Center)
As earth's atmosphere continues to warm thanks to a warming climate, there is growing concern that these exceptionally moist environments will continue to become more frequent.
A warmer atmosphere expands and has more room to hold more moisture. This means precipitating systems like thunderstorms and landfalling tropical systems will produce more extreme and damaging amounts of precipitation. This raises the risk of damaging and deadly flooding, especially in vulnerable areas with poor or dilapidated infrastructure.
