Historic Flood Event in Louisiana From 20-30 Inches of Rain

A historic flooding event continues over southern Louisiana, where widespread rainfall amounts in excess of twenty inches since Friday have brought all ten river gauges on the Amite, Tickfaw, and Comite Rivers to record flood crests, flooded thousands of homes, and caused over 1,000 water rescues. The most extreme floods have occurred on the Amite River, which flows along the east side of the Baton Rouge metropolitan area. Flood waters stranded hundreds of cars on Interstate 12 just east of Baton Rouge for more than 24 hours, Saturday through Sunday. About 25 miles east-southeast of Baton Rouge, the flood crest on the Amite River appears likely to overtop the levee system built at Port Vincent after the destructive floods of April 1983 (at the tail end of the 1982-83 “super” El Niño). The flood control system was designed to handle a recurrence of the 14.6-foot crest observed in that record event. However, the Amite at Port Vincent had already reached 14.91 feet as of 9:15 am CDT Sunday, and it is projected to hit a crest of 16.5 feet early Monday, remaining above the previous record until Tuesday. Major flooding can be expected to the south of Port Vincent in southern parts of Ascension Parish, where voluntary evacuations are already in effect. “If you can get out, get out now,” said parish president Kenny Matassa on Sunday morning.


Figure 1. Major flooding in Prarieville, Louisiana on Friday, August 12, 2016. (@presleygroupmk/twitter.com) 


Figure 2. The Amite River at Denham Springs, just east of Baton Rouge, was at 46.2 feet on Sunday morning, August 14, 2016, nearly five feet above its previous record crest of 41.5 feet on March 8, 1983. Records there date back to at least 1921, making this an impressive feat. Today's crest is the only one of the river’s top 80 historic crests to occur during August, since the worst floods in the region are more commonly associated with winter and spring rainfall than with landfalling tropical cyclones. The Amite crested at 58.56 feet in Magnolia, Louisiana, topping the old record at that location by more than six feet set on April 23, 1977. Image credit: NOAA/NWS Advanced Hydrologic Prediction Service.


Figure 3. Aerial view of flooding in Hammond, Louisiana on August 13, 2016. AP Photo/Max Becherer.

Some of the 24-hour rains that fell on Friday in Louisiana (ending at 11AM CDT/16UTC) had a recurrence interval at over 500 years, according to Metstat. Topping the list of phenomenal rainfall amounts catalogued by the NWS Weather Prediction Center for the period 6:00 am CDT Tuesday, August 9, 2016, through 9:00 am CDT Sunday was 31.39” near Watson, Louisiana. Other impressive amounts:

27.47”  Brownfields, LA
22.84”  Gloster, MS
14.43”  Panama City Beach, FL
8.97”  Fairhope, AL
8.11”  Williamsville, MO
7.90”  Cobden, IL


Figure 4. In a dramatic rescue on Saturday near Baton Rouge, a woman and her dog were pulled to safety just as their car went under water. See the powerful video here. Image credit: WAFB.

A tropical depression-like storm with tropical depression-like impacts
The storm system responsible for the record rains formed a distinct surface low just inland along the Alabama coast on August 11, with a central pressure of 1013 mb. By August 13, the low had drifted over northwest Louisiana, and intensified to a central pressure of 1007 mb. Like a tropical depression, the low had a warm core, and the counter-clockwise flow of air around the storm brought huge amounts of tropical moisture from the near record-warm waters of the Gulf of Mexico and northwest Atlantic northwards over land. The amount of moisture in the atmosphere over the Gulf Coast region over the past week has been nothing short of phenomenal. Over multiple days, soundings of the atmosphere collected by weather balloon from locations such as New Orleans have measured record or near-record amounts of precipitable water (the amount of moisture in the atmosphere over a given point), often in the 2.5” to 2.75” range; sounding data extends back to 1948 in most cases. Sunday morning’s precipitable water of 2.61” in Lake Charles, LA, was among the top-ten values on record for that station.


Figure 5. Three-day precipitation totals ending at 10 EDT Sunday, August 14, 2016 showed several areas of 20+ inches had fallen over portions of Louisiana. Image credit: NOAA/NWS.


Figure 6. Projected rainfall from 12Z (8:00 am EDT) Sunday, August 14, 2016, through 12Z Wednesday. Image credit: NOAA/NWS/WPC.

The northern flank of this historic flood event
Separate from the heavy rain along the central Gulf Coast, a band of sometimes-torrential rain has pulsated over the last several days along a pre-existing frontal zone along and north of the Ohio River, stretching roughly from Arkansas to Ohio. This pattern bears some of the fingerprints of a PRE—a “predecessor rain event.” As we noted in a post last October, PREs tend to develop along preexisting frontal boundaries a few hundred miles north of landfalling tropical cyclones, as prevailing winds funnel huge amounts of moisture northward from the cyclone and concentrate it along the frontal zone. One challenge with such events is nailing down the location of the frontal zone, which can oscillate north or south as a multi-day PRE unfolds. Late Sunday into Monday, the Gulf Coast low itself will begin migrating northward along the frontal zone, further raising the possibility of flooding rains. Flash flood watches extended on Sunday morning along a belt from northern Arkansas to extreme northwest Pennsylvania.

On Friday, torrential rains put a damper on the Illinois State Fair at the state capital, Springfield, where an all-time calendar-day rainfall record was set with 5.59” (beating 5.44” from September 8, 1926). Of that total, 3.44” fell in just one hour. Although some events had to be cancelled or rescheduled, no injuries to people or livestock were reported at the fair. Parts of the southwestern Chicago suburbs received 4-5” of rain on Friday.


Figure 7. MODIS image of a strong tropical wave off the coast of Africa south of Cabo Verde as seen on Sunday morning, August 14, 2014. A large region of African dust is visible from the coast of Africa extending over Cabo Verde. Image credit: NASA.

African tropical wave may develop late this week
NHC was not highlighting any Atlantic tropical weather threat areas in their 5-day Tropical Weather Outlook on Sunday morning. However, two of our reliable models for predicting tropical cyclone genesis--the European and GFS models--showed that a strong tropical wave that moved off the coast of Africa on Saturday night does have the potential to develop into a tropical depression late this week as it moves west to west-northwest at 15 mph into the central Atlantic. In their 00Z Sunday runs, about 30 - 50% of the members of the European and GFS model ensemble forecasts predicted development of this system into a tropical depression late this week.

Working against development of this wave will be the fact that the Madden-Julian Oscillation (MJO), a pattern of increased thunderstorm activity near the Equator that moves around the globe in 30 - 60 days, is currently located in the Western Pacific. When the MJO is located there, we can expect to see increased typhoon activity in the Northwest Pacific, but compensating sinking air and surface high pressure over the tropical Atlantic, with reduced chances of tropical cyclone development there. Dry air from the Saharan Air Layer (SAL) will also likely interfere with Atlantic development this week, though the SAL is currently less prominent over the tropical Atlantic than it was early in August.

The Amite River Basin between Watson and Central thanks to the @uscoastguard #laflood #lawx pic.twitter.com/jKYZzpScZh

— Louisiana GOHSEP (@GOHSEP) August 13, 2016

Video 1. The Amite River Basin between Watson and Central, as videotaped by the U.S. Coast Guard on Saturday. Image credit: Louisiana GOHSEP, via NWS New Orleans.

Jeff Masters and Bob Henson