Floods From up to 20 Inches of Rain Create State of Emergency in Louisiana

By: Jeff Masters , 4:22PM,GMT on March 11,2016



 

A state of emergency has been declared by Governor John Bel Edwards for the entire state of Louisiana after a four-day deluge of rain dumped up to 20" of rain over northern portions of the state. The resulting record flooding has forced a call-up of the National Guard to help evacuate thousands of people from their homes. Five storm-related deaths have been reported since Monday--three in Louisiana and one each in Oklahoma and Texas. Hundreds of roads have been closed, including portions of two major interstate highways. One bridge collapsed on Louisiana Highway 557 in Ouchita Parish.


Figure 1. A bridge collapsed on Louisiana Highway 557 in Ouchita Parish, Louisiana, on March 10, 2016. Image credit: Louisiana DOTD.


Figure 2. Satellite-derived Integrated Water Vapor (the total amount of rain, in centimeters, that would result from condensing all water vapor in a column of air) as of 7 am EST March 11, 2016. Two "atmospheric rivers" of water vapor are seen affecting the U.S.--one from the Eastern Pacific and Western Caribbean flowing across Mexico into the Southern U.S., and one extending from the tropics south of Hawaii to the coast of California. Image credit: NOAA/ESRL.


Figure 3. Observed 72-hour precipitation for the period ending at 10 am EST Friday, March 11, 2016. Portions of northern Louisiana received over 20" of rain, and a large area of 8+" fell over portions of Eastern Texas, Northern Louisiana, Southeast Arkansas and Northwest Mississippi. Image credit: NOAA/NWS.

An upper-level low tapping an atmospheric river of moisture was responsible
The upper low over Mexico responsible for the deluge was able to tap into an atmospheric river of moisture from both the Western Caribbean and the Eastern Pacific, and was still bringing record amounts of water vapor to the Gulf Coast on Friday morning. The 7 am EST Friday March 11, 2016 upper-air balloon sounding from Jackson, MS set a new record for most precipitable water on record for any day during the December - March period: 1.97". New Orleans, LA set a record on Friday morning for most precipitable water of any astronomical winter date: 2.14". The record atmospheric moisture this week has led to rainfall amounts one would expect to occur only once every 200 years (a 0.5% chance of occurrence in a given year) over portions of northern Louisiana. According to the latest NOAA Storm Summary, as of 9 am EST Friday, a rain gauge two miles from the city of Monroe, Louisiana had received 20.90" of rain since Monday, and Shreveport, Louisiana had picked up 19.00" at Barksdale Air Force Base. Over a foot of rain was recorded at several locations in Southern Arkansas.


Figure 4. Total Precipitable Water over Jackson, Mississippi from 1948 - 2016, with the record amount from 7 am EST Friday March 11, 2016 shown. Image credit: NOAA/SPC.


Figure 5. The Sabine River near Burkeville, Louisiana reached its highest water level on record Friday morning, beating the previous record set in February 1999 by over five feet. Water levels for this gauge go back to 1884. Image credit: NOAA.


Figure 6. VIIRS satellite imagery from Thursday afternoon, March 10, 2016, shows the record-strength upper-level low centered over central Mexico that brought torrential rains and record flooding to Louisiana. Image credit: NASA.

Record warmth continues across the Northeast
As if Wednesday’s high of 77°F left any doubt that spring had sprung early in New York City, Thursday doubled up with a high of 79°F.  This broke the previous day’s record as the warmest temperature ever notched so early in the year in 145 years of record-keeping at Central Park. Even more impressive was Thursday’s “low” temperature: a ridiculous 63°F! That’s the warmest daily minimum Central Park has seen during any winter--or for that matter, on any date falling between November 10 and March 28--since measurements began in 1871. For maximum shock value, we can employ Thursday’s average temperature of 71°F [calculated by adding the daily high and low and dividing by 2]. A reading like this would be most likely to occur in New York during late June or late August. The latest such reading in Central Park annals is 71°F on November 2, 1971; the record-earliest value this warm was 71.5°F on March 28, 1998. Many other record highs were again set across the Northeast on Thursday, just a day after dozens of cities set records for the hottest temperatures on record for so early in the year. A weak front will push temperatures back near normal this weekend, but only briefly, before another spell of warmth kicks in next week across most of the central and eastern U.S.

The forecast: more flooding in the South
Thankfully, the stalled-out record-strength upper-level low pressure system responsible for the historic flooding is finally on the move, headed northwards across Texas. The low will still be tapping into the atmospheric river of moisture responsible for this week's heavy rains, and will bring additional widespread heavy rains of 2 - 5" over the Central Gulf Coast states through Saturday, with a few areas up to 10" possible. By Sunday, the heavy rains should end as the upper-level low weakens and the flow of moisture from the Gulf gets cut off.


Figure 7. Predicted precipitation for the 2-day period ending 7 am EST Sunday, March 13, 2016. Heavy rains in excess of 5" are expected over coastal Mississippi and Southeast Louisiana, including New Orleans. Image credit: National Weather Service.

Heavy rains and high winds hit U.S. West Coast
Meanwhile, another storm system with its own atmospheric river of moisture has been hitting the U.S. West Coast, bringing flooding rains and heavy mountain snows to California. Wind gusts in excess of 50 mph swept across much of Washington State on Thursday, causing some impressive damage and knocking out power to 100,000 customers. According to the NWS in Seattle, the top wind gust was 109 mph on top of Mt. Baker.

Jeff Masters and Bob Henson