|Above: Natural-color satellite image of Tropical Storm Barry at 2155Z (5:55 pm EDT) Thursday, July 11, 2019. Image credit: tropicaltidbits.com.
Most of the southeast Louisiana coast was under a hurricane warning Thursday night ahead of Tropical Storm Barry, which is predicted to intensify from Friday up through its expected landfall on Saturday. Whether or not Barry becomes a hurricane, it is on track to dump enormous amounts of rain in southeast Louisiana—a vulnerable region pummeled by multiple flood disasters over the last few years. Widespread and severe flooding is a strong possibility.
As of 5 pm EDT, Barry was still a minimal-strength tropical storm, with top sustained winds of 40 mph. Northerly wind shear was pushing dry air into the storm, keeping the northern half of Barry mostly free of convection. There were signs that Barry was starting to form a more “stacked” circulation, as a low-level vortex moved closer to the mid-level circulation. Data from a hurricane-hunter flight in progress Thursday evening suggested that modest strengthening was under way. More sustained strengthening of Barry will not happen until convection wraps around the entire core, and this may not occur in time to give Barry a runway to become a hurricane.
Most of the model guidance now brings Barry onshore in southeast Louisiana, between Friday night and midday Saturday, although the 12Z run of the UKMET model still insisted on a landfall in far southeast Texas late Saturday. The 12Z runs of the GFS, HMON, and HWRF tracked Barry onshore and northward near or just west of New Orleans. The official NHC forecast calls for a landfall in central Louisiana, closest to the 12Z European model solution, with a northward track arcing close to the Mississippi Delta near Memphis by Monday.
|Figure 1. Three-day rainfall forecast for Tropical Storm Barry from 7 pm EDT Thursday, July 11, 2019, through 7 pm Sunday. Image credit: NOAA/NWS/WPC.
Water still the big threat from Barry
The NOAA Weather Prediction Center (WPC) upped its rainfall forecast for Barry on Thursday afternoon, calling for a pocket of 20-25” amounts near Barry’s track between Thursday and Sunday evening. It’s very unusual for a NOAA/WPC forecast to depict amounts above 20”, which testifies to the center's high confidence in this extreme rainfall event.
Widespread and locally severe flash flooding can be expected as Barry’s rains push onshore, especially from Saturday into Sunday. WPC placed most of southeast Louisiana under a high-risk designation for excessive rains from Saturday morning to Sunday morning. A high-risk rainfall outlook on Day 3 has only been issued in two other cases: with Hurricanes Harvey (2017) and Florence (2018), both of which produced multi-billion-dollar flooding. WPC’s comments on Thursday afternoon are well worth noting:
“This scenario is not quite as certain as other storms of recent memory like Florence last year. Barry has not yet become very well organized. But we have seen in numerous examples, including the unnamed system that struck Louisiana in August 2016, that when the global models unanimously begin depicting these high-end rainfall amounts they are usually correct in forecasting a strongly anomalous event with potential to produce substantial impacts. For now we have trended the WPC QPF up to indicate areal average 10 to 20 inch amounts, with isolated 25 inch amounts, and with much of the bulk of this occurring on Day 3.”
|Figure 2. St. Bernard Parish Sheriff's Office inmate workers move free sandbags for residents in Chalmette, La., Thursday, July 11, 2019 ahead of ahead of Tropical Storm Barry from the Gulf of Mexico. Image credit: AP Photo/Matthew Hinton.
Storm surge threat from Barry
The 5 pm Thursday outlook from NHC continued to call for the potential of 3-6’ of inundation at high tide in a storm surge warning area that extended along the Louisiana coast from the mouth of the Atchafalaya River to Shell Beach. If it becomes more clear that Barry is unlikely to reach hurricane strength, these numbers may be reduced, but coastal residents should err on the side of prudence for the time being.
There remains the potential for some amount of storm surge to work its way up the Mississippi toward New Orleans, although this threat is diminishing as Barry struggles to organize. Still, the Mississippi has experienced record-prolonged flooding this year, and the arrival of an early-season tropical cyclone atop late-season flooding is an unusual and concerning prospect. See Jeff Masters’ post from Wednesday night on how even a Category 1 hurricane could produce a damaging storm surge in the New Orleans area.
As of Thursday morning, NOAA was predicting the Mississippi to crest at the New Orleans Carrollton Gage on Saturday at 19’, whereas the levees protecting the city are at least 20’ high (as confirmed Thursday by the U.S. Corps of Engineers). This outlook takes into account a potential storm surge as well as rainfall. Some of the river levees south of Oakville and Caernarvon in Plaquemines Parish are at higher risk of being overtopped, according to the Corps, and intermittent overtopping could happen closer to the city due to wave action. With this in mind, it wouldn’t be a shock to see waters lapping over the levees at times even if no breaching or major overtopping were to occur—something to keep in mind as pictures and videos start to appear in social media this weekend.
93L trundles across the Eastern Atlantic
The system dubbed Invest 93L continued to move across the eastern tropical Atlantic late Thursday with little change in strength. See this morning’s post for a discussion from Dr. Jeff Masters on the potential evolution of 93L.