A devastating flood event was unfolding over southeast Louisiana on Friday, and conditions may get worse yet, as an extremely slow-moving center of low pressure is dumping colossal amounts of rain on the region. This sprawling, “stacked” low is carrying more water vapor than many tropical cyclones, and its slow motion is leading to persistent rains that could add up to all-time record totals in some places.
Multi-sensor analyses indicate that several areas in southeast Louisiana and southermost Mississippi racked up more than 6” of rain from 7:00 am CDT Thursday, August 11, to 7:00 am Friday (see Figure 1). More than 10” of rain was analyzed just northeast of Baton Rouge, the hardest-hit area thus far. In the 24 hours from 2:00 pm CDT Thursday to 2:00 pm Friday, Baton Rouge Metropolitan Airport recorded a preliminary total of 8.49” of rain. Since records began in 1892, the city’s largest calendar day total is 11.99” (set on April 14, 1967), and the largest two-day calendar total is 14.03” (June 6-7, 2001). Given the very slow motion of the stacked low, these all-time records are conceivably within reach. A cooperative observer in Livingston, LA, reported 17.09” of rain
from midnight to 3:00 pm CDT Friday. The state’s official 24-hour record is 22 inches
, reported near Hackberry on August 28-29, 1962.Figure 1.
Multi-sensor rainfall analysis for the period from 7:00 am CDT Thursday, August 11, to 7:00 am Friday shows a gyre-like pattern of torrential rains spinning around a low in southern Mississippi. Image credit: NOAA/NWS Advanced Hydrologic Prediction Service
As the low edges westward over the next 24-48 hours, the zone of heaviest rain potential will shift toward west Louisiana and east Texas, but southeast Louisiana will remain under the gun for more downpours at least into early Saturday. The short-range HRRR model produces another 2”-6” of widespread rain over southeast Louisiana through Saturday morning, with localized totals of 8-12” not out of the question.Figure 2.
Enhanced infrared satellite image for the central Gulf Coast reveals the vast scope of the area of low pressure generating torrential rains in southeast Louisiana. Image credit: NOAA/NESDIS
.Severe flood threat for Baton Rouge area
Both flash flooding and river flooding threats are looming large for southeast Louisiana, where flash flood warnings were in place on Friday afternoon. Major flooding has already occurred
throughout the day Friday, and a flash flood emergency (the most urgent type of flash flood warning) was in effect Friday afternoon for parts of Feliciana, West Feliciana, St. Helena, and East Baton Rouge parishes, which extend roughly from Baton Rouge northward. Water rescues and evacuations were under way in this region, according to the NWS. Even if the rains ease during the weekend, the area faces a major flood threat. The Tickfaw River at Montpelier, LA, hit a record crest of 22.75 feet
at 1:30 pm CDT Friday, with several more feet expected this weekend. A number of other rivers across southeast Louisiana are projected to reach all-time crests, including the Amite River, where record levels of flooding can be expected to inundate many homes and roadways on the eastern side of the Baton Rouge metro area for an extended period.Figure 3.
Forecasts issued on Friday morning, August 12, 2016, were calling for an all-time record flood crest of 42.5 feet late Sunday on the Amite River at Denham Springs, just east of Baton Rouge, LA. The forecast keeps waters above the previous record of 41.5 feet (April 8, 1983) for a full 24 hours. These projections could be boosted further in light of the heavy rains persisting in the area on Friday. The last major crest in this region was 36.09 feet on March 13, 2016. Image credit: NOAA/NWS Advanced Hydrologic Prediction Service
.Tropical cyclone or not? Does it matter?
Although this system does not qualify as a tropical cyclone--its center has remained just inland--the point is moot in terms of impact, as the torrential rains and flooding from this low could end up ranking among some of the more damaging tropical depressions and tropical storms on record. The low’s rainmaking power is a combination of its extremely slow motion and the astoundingly moist air mass feeding into it. The upper air sounding launched from Slidell, LA, at 12Z Friday (7:00 am CDT) showed that the atmosphere was carrying 2.85” of precipitable water (the amount of water in a column of air over a given point). This is the second-highest amount of water measured in any sounding since records began in the New Orleans area in 1948, and just 0.03” below the record of 2.88”. In Jackson, MS, only two other dates have seen more precipitable water than the 2.74” measured on Friday morning, with the record being just 0.02” higher (2.76”). These values may seem puzzlingly low compared to the amounts of rain occurring. This is because showers and thunderstorms can concentrate the amount of moisture present in the atmosphere throughout a region, so they can produce much higher local totals than the precipitable water values would suggest.
The New Orleans Times-Picayune is providing live updates
on the situation in southeast Louisiana. Governor John Bel Edwards has declared a state of emergency for the entire state through at least Saturday.
Bob HensonFigure 4.
An inundated boat launch ramp in New Iberia, LA, on Friday, August 12, 2016. Image credit: wunderphotographer kaiju76