Floodwaters have finally crested across most of southern Louisiana after a harrowing weekend of record-high water that left at least six people dead, pushed at least 10,000 people into shelters, and prompted the rescues of more than 20,000 people
. The federal government has declared the event a major disaster in four parishes
: Tangipahoa, St. Helena, East Baton Rouge and Livingston. According to the insurance broker Aon Benfield, “The major flood and thunderstorm event that impacted parts of Louisiana, Texas, and Arkansas during March 2016 caused roughly USD1.5 billion in economic damage. It is currently anticipated that the August 2016 event will approach and possibly exceed this cost once all damage incurred to homes, businesses, public facilities, vehicles, infrastructure, and agriculture is taken into account.”Figure 1.
In this aerial photo over Hammond, La., flooded homes are seen off of LA-1064 after heavy rains inundated the region, Saturday, Aug. 13, 2016. (AP Photo/Max Becherer)
Multi-day rainfall of 10” to 20”, produced by a very slow-moving low pressure center, covered a large swath of south-central and southeast Louisiana (see Figure 2 below). Bands of heavy rain also extended northeastward as far as Ohio along a preexisting frontal boundary, as moisture was funneled northward from the Gulf Coast low. (Parts of the St. Louis area received 4” to 6” of rain on Sunday night.) The storm system carried near-record amounts of atmospheric moisture
, drawn from the Gulf of Mexico and northwest Atlantic, where sea-surface temperatures are well above average. Climate change has already been shown to increase the amounts of rain falling in the most intense events across many parts of the world, and extreme rainfall events like this week's Louisiana storm are expected to grow increasingly common in the coming years.Figure 2
. Rainfall amounts analyzed by data from multiple sensors for the week extending from 12Z (8:00 am) Monday, August 8, 2016, to 12Z Monday, August 15. Most of the rain in Louisiana has fallen since Thursday, August 11. Image credit: NOAA/NWS Advanced Hydrologic Prediction Service
Some parts of Louisiana recorded more than 20" of rain in 48 hours, which qualifies as a 1-in-1,000 year rainfall event. In other words, an event of this magnitude has a 0.1 percent chance of occurring at a particular location in any given year. Image credit: NWS Lower Mississippi River Forecast Center.The numbers
Some local rainfall totals during this event were truly astounding, as catalogued in a NOAA/NWS precipitation summary
issued on Monday morning. The top multiday amounts observed in each state through 10 am EDT Monday, August 15, include:Watson, LA
: 31.39”Gloster, MS
: 22.84”Panama City Beach, FL
: 14.43”Ellsinore, MO
: 12.10”Gloster, AL
: 9.94”Tomball, TX
: 8.82”Makanda, IL
: 8.05”Vincennes, IN
: 5.52”Pocahontas, AR
Flooding over the weekend was most intense across the southeast corner of Louisiana, especially the region east of Baton Rouge and north of Lake Ponchartrain. Among the all-time record crests observed on Sunday: Amite River at Denham Springs
: 46.2’ (old record 41.5’ on April 8, 1983)Comite River at Comite Joor Rd.
: 34.22’ (old record 30.99’ on June 9, 2001) Tickfaw River at Holden
: 22.16’ (old record 21.04’ on April 7, 1983)Tangipahoa River at Robert
: 27.33’ (old record 27.10’ on March 14, 1921)
The previous record crests shown above are a telltale sign that most of the biggest flood events in southeastern Louisiana occur following large-scale winter and spring rainfall events rather than landfalling tropical cyclones. The latter typically (but not always) come and go more quickly, dumping heavy rain but not persisting long enough to cause widespread river flooding.An odd-duck storm with tropical-cyclone-like impacts
The low pressure center that generated the past week of torrential rain along the Gulf Coast was a strange one indeed. Because surface winds were light and the surface low stayed generally onshore (see embedded video at bottom), the system was never declared a tropical cyclone by the National Hurricane Center. At the same time, for much of its life the storm was a symmetric warm-core low, the same type of structure associated with tropical cyclones. Regardless of its classification, the storm behaved much like other tropical depressions, tropical storms, and hurricanes that have taken their time moving through the Gulf Coast region. Two analogs that come to mind for the past week’s events are:Figure 4
. Analysis of total rainfall amounts (in inches) produced in Louisiana from the August 1940 hurricane. The hurricane’s path is indicated by the arrow. Image credit: David Roth, “Louisiana Hurricane History
,” NOAA.Hurricane 2 (unnamed), August 1940
: Also called the 1940 Louisiana hurricane
, this compact cyclone (hurricane-force winds extended just ten miles from its center) stayed just off the central Gulf Coast for most of its lifespan, intensifying as it nudged toward the coast of southwest Louisiana and coming ashore at Sabine Pass, TX, with top sustained winds of 100 mph. The hurricane’s slow motion both offshore and onshore resulted in mammoth, widespread rains, making it the wettest tropical cyclone in Louisiana history. The town of Crowley reported a storm total of 33.71”
, including 19.76” in 24 hours. Almost 2 million acres of land reportedly went underwater by at least a foot. Six lives were lost, and damages totaled $9 million (1940 dollars), according to a NOAA report
.Tropical Storm Allison, June 2001
: Allison moved onto the Texas coast as a tropical storm, then made a languid loop through the eastern part of the state over several days, depositing colossal amounts of rain. Houston was the hardest-hit metro area, as more than 70,000 homes
and hundreds of businesses were flooded, including much of the Texas Medical Center, leaving the city virutally paralyzed for days. The port of Houston received a storm total of 36.99” of rain
, with many other reports of 20” to 38” across Harris County. Allison caused 41 direct and 14 indirect fatalities, with damages totalling $9 billion (2001 dollars). Figure 5.
MODIS visible satellite image of 98L south of the Cabo Verde Islands taken on Monday morning, August 15, 2016. The brownish colors to the north of the disturbance are due to a large region of African dust. Image credit: NASA.African tropical wave may develop late this week
A strong tropical wave located a few hundred miles south of the Cabo Verde Islands is headed west-northwest at 15 mph, and has the potential to develop into a tropical depression late this week. This disturbance was designated Invest 98L
on Sunday evening by NHC. Satellite loops
on Monday morning showed 98L had a modest area of heavy thunderstorms which had acquired a moderate amount of spin. Wind shear
was light to moderate, 5 - 15 knots, and sea surface temperatures (SSTs) were warm enough for development--27.5°C (82°F), which was about 1°C (1.8°F) above average. Water vapor satellite imagery
showed that 98L was mostly in a moist environment, with the dry air from the Saharan Air Layer (SAL)
along the northern side of the disturbance. These conditions are generally favorable for development.Forecast for 98L
Steering currents favor a west-northwesterly to northwesterly motion at about 15 mph for 98L this week, which will likely take the system too far to the north for it to be a long-range threat to the Lesser Antilles Islands. The 8 am EDT Monday run of the SHIPS model
showed moderately favorable conditions for development through Thursday, with wind shear in the light to moderate range, 5 - 15 knots, a moist atmosphere, and SSTs near 26.5 - 27.5°C (80 - 82°F.) Working against development of 98L will be the Madden-Julian Oscillation (MJO),
a pattern of increased thunderstorm activity near the Equator that moves around the globe in 30 - 60 days. The active portion of the MJO is currently located in the Western Pacific, which leads to 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.
The Monday morning operational runs of our three reliable models for predicting tropical cyclone genesis, the European, GFS and UKMET models, had two of the three--the European and GFS--showing development of 98L. The 00Z Monday runs of the GFS and European model ensemble forecasts, done by taking the operational high-resolution version of the model and running it at lower resolution with slight perturbations to the initial conditions in order to generate a range of possible outcomes, had more than 50% of their ensemble members predict that 98L would become a tropical depression. In their 8 am EDT Monday Tropical Weather Outlook,
NHC gave 98L 2-day and 5-day development odds of 20% and 30%, respectively. Should 98L become a tropical storm, the next name on the Atlantic list is Fiona. What's next?
We’re not particularly concerned about 98L's potential to cause trouble, since atmospheric steering currents are currently expected to take the storm far enough to the north that it will have difficulty making the long crossing to North America without recurvature. However, the next wave to come off the coast of Africa--due to emerge on Thursday--is likely to experience steering currents that will keep it farther to the south, on a course that could potentially bring it into the Caribbean by the middle of next week. We are now entering the peak part of hurricane season, and tropical waves like this one can become dangerous hurricanes that do not recurve harmlessly out to sea if atmospheric conditions come into alignment.Figure 6.
Danielle Blount kisses her 3-month-old baby Ember as she feeds her while they wait to be evacuated by members of the Louisiana Army National Guard near Walker, La., after heavy rains inundating the region, Sunday, Aug. 14, 2016. (AP Photo/Max Becherer) Louisiana disaster survivors with disabilities need your support after historic flooding
disaster relief charity, founded and staffed by members of the wunderground community, is responding to this week's devastating floods in Louisiana. The disaster is particularly troublesome for a state that is still in recovery from major flooding just last March, and many resources are completely depleted because of the March flooding. That storm left more than 5,000 homes damaged or destroyed and cost $1.5 billion across a three-state area. This week's flooding may end up being even more costly, according to insurance broker Aon Benfield. There is an urgent need for durable and consumable medical supplies as well as housing. Portlight will be working with the American Red Cross, local stakeholder organizations, and federal partners to respond to this historic flooding event. Your support is needed to make this happen! Please consider making a donation to Portlight's disaster relief fund at the portlight.org website
to further their reach and response in the state of Louisiana. Thank you for any support you can offer!
Bob Henson and Jeff Masters