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Louisiana Floods Overtop Levee, Inundating 15,000 Homes; 98L May Develop

By: Jeff Masters 3:48 PM GMT on August 16, 2016

The highest flood crest ever observed on Louisiana’s Amite River has overtopped the Laurel Ridge Levee in Ascension Parish, about 20 miles southeast of the capital of Baton Rouge, resulting in the flooding of at least 15,000 homes—one third of the parish’s homes, reported the Baton Rouge Advocate on Tuesday morning. The river was not forecast to fall below its previous record height (set in 1983) until Wednesday. At least nine people have died in flooding in Louisiana that began last Friday, with at least 20,000 people rescued from flooded homes and vehicles; 10,000 people are in shelters due to the disaster. The federal government has declared the event a major disaster in four parishes: Tangipahoa, St. Helena, East Baton Rouge and Livingston. The governor of Louisiana, John Bel Edwards, said on CNN that he expects nearly half of all of the state’s parishes—30 out of 64—to be declared disaster areas. Update: Some 40,000 homes in all have now been affected by the Louisiana flooding, according to a Tuesday morning press conference from Governor Edwards.

Figure 1. This aerial image shows flooded areas on and near the campus of Louisiana State University (LSU), Saturday, Aug. 13, 2016, in Baton Rouge, La. (Patrick Dennis/The Advocate via AP)

Figure 2. The Amite River at Port Vincent, Louisiana crested at least 2.8 feet above its previous record level on Monday. The river was forecast to fall below its previous record height (set in 1983) by Wednesday. Image credit: NWS/AHPS.

A tropical depression-like storm with massive amounts of moisture
Multi-day rainfall amounts of 20” to 30”, produced by a slow-moving low pressure center similar to a tropical depression, hit a swath of south-central and southeast Louisiana from Friday through Sunday. Some parts of Louisiana recorded more than 20" of rain in 48 hours, which qualifies as a 1-in-1,000 year rainfall event (having a 0.1 percent chance of occurring at a particular location in any given year), according to the NWS Lower Mississippi River Forecast Center. The highest rainfall total from the storm was 31.39” in Watson, Louisiana. The storm system carried near-record amounts of atmospheric moisture, drawn from the Gulf of Mexico and northwest Atlantic, where sea-surface temperatures (SSTs) were at near-record levels. SSTs over the northern Gulf of Mexico have since cooled, due to the week-long period of cloud cover and strong winds from the storm.

Figure 3. MODIS visible satellite image of 98L, located about 500 miles west-southwest of the Cabo Verde Islands, taken on Tuesday morning, August 16, 2016. The brownish colors to the northeast of the disturbance are due to a large region of African dust. Image credit: NASA.

African tropical wave 98L may develop later this week
A strong tropical wave located about 500 miles west-southwest of the Cabo Verde Islands on Tuesday morning was headed west-northwest at 15 mph, and has the potential to develop into a tropical depression this week. This disturbance was designated Invest 98L on Sunday evening by NHC. Satellite loops on Tuesday morning showed 98L had a modest area of heavy thunderstorms which had acquired a respectable amount of spin. Wind shear was light to moderate, 5 - 15 knots, and sea surface temperatures (SSTs) were warm enough for development—28°C (83°F), which was about 1°C (1.8°F) above average. Water vapor satellite imagery showed that 98L was in a moist environment on its southern flank, but dry air from the Saharan Air Layer (SAL) was along the northern side of the disturbance, interfering with development. These conditions are marginal for development.

Forecast for 98L
Steering currents favor a west-northwesterly to northwesterly motion at about 10 - 15 mph for 98L on Tuesday, slowing to 5 - 10 mph during the remainder of the week. This track will likely take the system too far to the north for it to be a long-range threat to the Lesser Antilles Islands, though we can’t rule out a threat to Bermuda yet. The 8 am EDT Tuesday run of the SHIPS model showed moderately favorable conditions for development through Friday, with wind shear in the moderate range, 10 - 15 knots, a moist atmosphere, and SSTs near 27 - 28°C (81 - 83°F.) Working against development of 98L will be the dry air of the SAL to its north, plus large scale sinking air over the tropical Atlantic imparted by 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 Tuesday 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 into a tropical storm late this week. In their 8 am EDT Tuesday Tropical Weather Outlook, NHC gave 98L 2-day and 5-day development odds of 70% and 80%, respectively. 98L appears to be struggling with dry air today, and I think these development odds should be reduced to 50% and 70%, respectively. Should 98L become a tropical storm, the next name on the Atlantic list is Fiona.

More African waves coming
A series of tropical waves will emerge from the coast of Africa during the next week, and we will have to watch these for development as they track westwards to west-northwestwards across the tropical Atlantic. The models have been inconsistent in their handling of the track and potential development of these waves over the past few days; the Tuesday morning runs of the GFS and European models showed that the only new tropical wave that might develop in the coming week is one due to come off of Africa on Saturday.

Figure 4. In this Sunday, Aug. 14, 2016 photo, people wait for members of the Louisiana Army National Guard to rescue them from the side of the road near Walker, La., after heavy rains inundated the region. (AP Photo/Max Becherer) 

Louisiana disaster survivors with disabilities need your support after historic flooding
The Portlight.org 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 past week's floods have affected at least three times as many homes, and insurance broker Aon Benfield anticipates that the floods may end up being even more costly than the March disaster. 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!

Why have so many people been unaware of this huge disaster? Marshall Shepard has a great essay in Forbes, 5 Reasons Some Were Unaware Of One Of The Biggest Weather Disasters Since Sandy.

Bob Henson will have a post this afternoon on the Great Arctic Cyclone (2016 version) that is currently churning through the Arctic.

Jeff Masters

Flood Hurricane

The views of the author are his/her own and do not necessarily represent the position of The Weather Company or its parent, IBM.