Although hurricane season has not yet arrived, Florida Panhandle residents and parts of coastal Alabama saw disastrous flooding from record rainfall Monday and Tuesday more akin to amounts seen in tropical storms than a spring weather system.
According to data from the weather station at Pensacola Regional Airport, 15.55 inches of rain -- the greatest rainfall amount from any calendar day on record—fell Tuesday. Data from the station go back to 1879.
"The 24-hour amount is between a 1 in 50 and 1 in 100 year event," forecasters from the National Weather Service Mobile/Pensacola Forecast Office wrote in a report on the storm. The two-day total, at 20.47 inches, is between a one-in-100- and one-in-200-year event.
The reason for such heavy rainfall totals is that the storm system, which caused deadly tornadoes across the southern and south-central United States over the weekend, stayed put for a long time over coastal Alabama and the Florida Panhandle, said Dan Petersen, a meteorologist at the National Weather Service's Weather Prediction Center in College Park, Md.
"The system was so slow-moving that the showers and thunderstorms persisted for a couple-day period," he said.
This rainfall led to flash flooding in the region, with one reported fatality when a woman became trapped in her car in floodwaters.
A one-hour wonder
Images of floating cars and submerged buildings filled news reports as the storm system made its way northward, where it dumped heavy rains across the mid-Atlantic and parts of the Northeast, but far less than the extent seen in Florida and Alabama.
The amount of rain that fell in one hour at the Pensacola Regional Airport station—5.68 inches—is a one-in-200- to one-in-500-year event, forecasters reported.The heavy rains wiped out bridges and formed sinkholes in roads, as well, enough to cripple a region accustomed to the heavy rains that often accompany tropical storms and hurricanes.
In an event sometimes referred to as "convective robbing," the severity of the storm in the Florida Panhandle and Alabama served to decrease its severity as it moved northward.
"Tuesday was a day where the morning storms, because they were so intense, actually helped to diminish the intensity of the storms that formed later in the afternoon," said Bill Bunting, operations chief at the National Weather Service's Storm Prediction Center in Norman, Okla.
Sometimes storm behavior leads to equally intense follow-up storms, because the earlier storm clears out in time, allowing the sun to heat up the atmosphere, and the southerly winds bring up new storms from the Gulf of Mexico, Bunting said.
Other times, prior storms have a "weakening effect," so the storms that follow them are less severe, he added.
That happened in this case, to the benefit of residents north of the Panhandle. Those in the area with the strong storms, however, saw heavier rains because the system lingered over them.
It is hard to predict in advance which way the prior storm will affect future storms, though, whether they will strengthen or weaken subsequent systems, Bunting said.
'Turn Around, Don't Drown!'
"You never really know which is going to win," he said. Forecasters use satellite images to track whether cumulus clouds are forming, which indicates another round of severe weather.
Flooding is an often underrated hazard when it comes to severe weather, Bunting noted. It causes more property damage in the United States than any other weather-related event.
On average, flooding leads to 89 deaths and $8.3 billion in damage annually, and a large number of those deaths come from people driving into floodwaters. The National Weather Service has a marketing campaign called "Turn Around, Don't Drown" that aims to teach people about the risks of driving into floodwaters.
Although the Southeast has overall seen less rainfall over the last 110 years, an increase in extreme precipitation events is predicted to occur as the climate changes and the Earth's atmosphere warms and holds more water vapor.
The National Oceanic and Atmospheric Administration has tracked an increase in extremes of one-day precipitation in the United States, a change that is in line with how climate change is projected to affect severe weather.
Reprinted from Climatewire with permission from Environment & Energy Publishing, LLC. www.eenews.net, 202-628-6500