Barry’s Rains: Slow to Arrive, Still a Threat by Bob Henson

Long-awaited rainbands from Tropical Storm Barry were streaming northward across the lower Mississippi Valley on Sunday. The region was plastered with flash flood watches. Barry’s rains are now unlikely to match the catastrophic levels originally predicted and feared, but widespread flash flooding is still expected. The rains will also help extend the most prolonged flood on record across parts of the mid- and lower Mississippi River.  

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One of the most relieving yet puzzling aspects of Barry is how little rain the storm has produced onshore. Up through Sunday morning, Barry’s heaviest rains were limited to pockets of coastal Louisiana, Mississippi, and Alabama, where 5” to as much as 8” fell (see Figure 2). Amounts were mostly 1” to 3” elsewhere across eastern Louisiana and southern Mississippi. More rain is still expected, of course, as detailed above.

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We don’t yet know exactly why Barry’s inland rains have been later and weaker than expected, but one piece of the puzzle is the strong northerly winds that have impeded Barry from the outset. The northerly winds created strong wind shear and pushed dry air into Barry’s north side and led to a tilted system, with a broad, complex surface low near the coast and a mid- and upper-level circulation well to the south (further offshore)

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It’s surprising that even our best computer models failed to pick up on how much this evolution would cut back on potential inland rainfall through at least early Sunday. The models did convey that wind shear could be an issue, and for several days model ensembles called for a sharp west-to-east dropoff in rainfall close to the New Orleans area. Tropical meteorology researchers will have much to learn from this fascinating case study.

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Overall, the storm surge from Barry came in much closer than rainfall to forecast expectations. Barry’s peak storm surge hit on Saturday, with the highest storm surge of 7.0 feet recorded midday Saturday, July 13 at Amerada Pass in south-central Louisiana. A 5-foot storm surge moved up the Atchafalaya River at Berwick, south of Morgan City. The surge was nearly 3 feet at Morgan City, pushing the river to its third highest level on record. The only higher floods occurred when the Morganza floodway was activated to alleviate Mississippi River flooding in 1973 and 2011. Note that spring flooding also sent the river to its fifth highest level on record back on March 15.

Barry’s peak surge of 3 - 4 feet in southeast Louisiana resulted in multiple levee overtopping events. One caused a mandatory evacuation for about 400 residents in Terrebonne Parish along Louisiana 315 and Brady Road south of the Salgout Canal Road, due to extended overtopping of a four-mile section of the Lower Dularge East Levee. Another levee overtopped in several places in Plaquemines Parish. It is one of two in the parish not reinforced in the wake of Hurricane Katrina, although funding was allocated for the levee. High water also overtopped levees on the north side of Lake Pontchartrain, in the Mandeville area, resulting in some localized flooding.