JOINT BASE SAN ANTONIO-RANDOLPH, Texas –
Stormy weather, a topic covered in this year's Air Force-wide Critical Days of Summer campaign, includes the usual suspects of lightning, hail and tornadoes, but it's flash flooding that especially concerns safety and weather officials from Joint Base San Antonio-Randolph.
"Flooding can occur with as little as a quarter-inch of rain," Wes Fillmore, 12th Operations Support Squadron lead meteorological technician, said. "In many local areas, stone - mostly limestone - is directly beneath the soil, so when it rains, water may only sink a few inches and then build on top of the stone."
In other words, "dry ground doesn't accept moisture as quickly as it rains," Fillmore said.
Rapid water buildup coupled with a topography that descends from north to south, producing sloped paths and low water crossings, means "south central Texas is very prone to flash flooding," Fillmore added.
A recent example of deadly flash flooding in San Antonio occurred during Memorial Day weekend when a storm dispensed 10 inches of rain, killing three people and putting some parts of the city underwater. The city's all-time average precipitation for the month of May is 3.59 inches; in May 2013, 13.19 inches of rain fell, according to the National Weather Service.
Record South Texas flooding during an October 1998 weekend killed 31 people, 26 of which had drowned. Seventeen of the drowning victims were in vehicles driving into water or swept away by rising water, according to a report from the U.S. Commerce Department.
The Texas Department of Public Safety reported that more than 50 percent of flood-related drownings happen in vehicles.
"Nearly eight out of 10 vehicle-related flood fatalities in Texas occur in the dark, between 6 p.m. and 6 a.m.," Marvin Joyce, 502nd Air Base Wing safety and occupational health specialist, said. "Driving at night makes these situations even worse because visibility is limited and it is difficult to judge how deep the water may be."
Never attempt to drive or walk through flooded low water crossings.
"Stop, turn around and find a safer route," Joyce said.
When it comes to bad weather in Texas, storm systems called stationary fronts, which result from the collision of two different air masses, are commonplace.
"Stationary fronts brew thunderstorms that don't move," Fillmore said. "They persist and linger, and are able to produce prolonged precipitation and storm trains."
The "training effect" occurs when storms develop and move through a region over and over again, following a single-path pattern like train cars on a track, Fillmore said.
For San Antonio, stationary fronts are the product of warm moisture from the Gulf of Mexico colliding with cold air from the north, which may not only cause flash flooding, but numerous lightning strikes.
In Texas, lightning strikes killed 24 people from 2003-2012, but many were avoidable.
"Most of the lightning deaths occur when people are caught outside in a thunderstorm," Joyce said. "Be cautious of dark clouds. Never hide under trees and stay indoors if possible."
If a tornado forms and touches down, stay off the roads and seek shelter in a basement or in the center of a sturdy building, away from doors and windows.
"In South Texas, rapid weather changes are common; it can go from being a nice, sunny day to severe weather conditions," Joyce said. "When it comes to severe weather, early preparation and awareness are important to help ensure everyone's safety."