AIR: East Coast storm system responsible for flash flooding

Torrential rains from the same storm system that has spawned multiple tornadoes and severe thunderstorms across the Midwest and Southeastern US have caused flash flooding across the Florida Panhandle and Southern Alabama, says modelling firm AIR Worldwide.

Parts of the region were inundated with as much as 22 to 26 inches of rain.

According to NWS meteorologists, this extreme rainfall has led to the worst flooding in 30 years on the Florida Panhandle. In Mobile, Alabama, Wednesday, April 30thwas the 5th wettest day recorded for that city over the past 143 years.

“The storm system that has produced multiple damaging tornadoes across the American Midwest and Southeast over the past several days was also responsible for the record-breaking rains seen on Tuesday and Wednesday in eastern Florida and southern Alabama,” says Dr. Eric Robinson, scientist at AIR. “The heaviest rains associated with this system totalled 22 to 26 inches in the worst-affected regions.”

The system is expected to proceed northward along the east coast. In fact, it is already bringing heavy rain and thunderstorms to the Mid-Atlantic and Northeast, prompting river flood warnings in parts of New Jersey. According to the NWS, showers and thunderstorms may continue over parts of Florida through Friday evening.

Robinson observes, “The Pensacola region – located east of Tallahassee along the Florida Panhandle – was particularly hard-hit by flash flooding, with portions of the downtown under four feet of water. Two sections of the scenic highway in Pensacola have collapsed, as the ocean-facing bluff underlying the road was weakened by rainfall. Three heavily used bridges in Escambia County, Florida, have reportedly been badly damaged or destroyed.”

A state of emergency has been declared for 26 counties in Florida. Along Alabama’s Gulf Coast, county roads were flooded and rivers overtopped their banks after 22 to 26 inches of rain fell in some regions. On Wednesday, 200 people were reported to have been evacuated in the Florida Panhandle.

According to AIR, although the current flooding has inflicted property damage in Southern Alabama and the Florida Panhandle, it is important to recognise that the majority of exposures in the region have been designed to protect against flood damage.

AIR claims that more than 60-70 percent of buildings in the US Gulf Coast region meet Flood Insurance Rate Map (FIRM) standards set in 1980. In the Florida Panhandle, an even higher proportion – between 70 percent and 80 percent of buildings meet FIRM standards. While the specifics of the FIRM standards vary somewhat by building construction and occupancy class, these standards dictate that buildings must be elevated above base flood elevation, as defined by FEMA, and that any parts of a building that remain below base flood elevation must be constructed of flood-resistant materials.

In the upcoming AIR inland flood model for the US, set for release this summer, both first floor elevation and basements are included as secondary modifiers, which will allow these important determinants of building vulnerability to be included in analyses of flood risk.

Robinson concludes, “Further heavy rain is expected as this storm system moves northward along the US east coast. Indeed, heavy rainfall from the system has already prompted river flood warnings in parts of New Jersey, according to the NWS. As of Thursday morning, heavy rains from this system have reportedly caused a street to collapse in Baltimore, Maryland, when a retaining wall failed, spilling parked cars downslope and onto railroad tracks.”

Urban flooding may be a concern in several large cities, including New York City, Washington, D.C., and Philadelphia, for which 4-6 inches of rain are forecast to fall over the course of Friday.