More Flooding Rains for Houston and Lake Ontario; Bonnet Carre Spillway to Reopen

The western Gulf Coast states are bracing for more rounds of torrential rain and potential flooding into this weekend, after parts of the Houston area were inundated by 10” to 14” of rain on Tuesday. Flash flood watches were hoisted Thursday from Austin-San Antonio and Corpus Christi east and north to the lower Mississippi Valley. Most of the watch areas can expect at least 3” to 7” of rain through the weekend, and localized pockets could easily top 10”.

“This event will be a marathon, not a sprint,” warned forecasters from the Houston/Galveston office of the National Weather Service.

Figure 1.  Multi-sensor rainfall estimates for the week ending at 7 am CDT Thursday, May 9, 2019. Image credit: NOAA/NWS Advanced Hydrologic Prediction Service.

The tail end of the cold front that brought severe weather to the central U.S. on Tuesday and Wednesday will slow to a halt along the western Gulf Coast late Thursday, lodging in place till at least Saturday as a coastal front. Rich moisture will flow northward from the Gulf atop the frontal zone, providing ample fuel for heavy showers and thunderstorms. At upper levels, low pressure will be reorganizing in the West, and a strong jet streak will push across southern Texas over the weekend, fostering upward motion and more periods of heavy rain. When all is said and done, it’s quite possible that some parts of southeast Texas will end up having gotten 20” or more of rain in a week-long period.

Many Houstonians are no doubt experiencing an unwelcome sense of déjà vu. Even before 2017’s catastrophic Hurricane Harvey—whose widespread 30” to 40” totals from Houston to Port Arthur caused massive damage and dislocation—the city had to contend with the Memorial Day spring floods of 2015, which dropped a foot of rain on parts of the metro area in less than 12 hours. More than 500 water rescues were conducted, according to the Houston Chronicle, and seven people were killed. The inundation came near the end of Texas’s wettest month on record.

These are among the 14 rescue boats and 11 high-water vehicles we have stationed throughout the county in anticipation of flooding. Please follow @ReadyHarris and @NWSHouston to stay informed and safe during severe weather. #HouNews pic.twitter.com/mY9AylBow7

— HCSOTexas (@HCSOTexas) May 9, 2019

“Precipice of a disaster”: Eastern Great Lakes socked with flooding

Months of wetter-than-usual conditions are culminating in record- or near-record water levels across the Great Lakes. Lake Ontario is at the greatest risk of creating flood problems over the next several days. Almost a million sandbags have been deployed across the lake’s New York shoreline, and the state’s governor, Andrew Cuomo, said areas along the shoreline are "at the precipice of a disaster”, the Associated Press reported. More rainfall is in the forecast. On the other side of the lake, the Toronto Region Conservation Authority (TRCA) has issued a shoreline hazard warning until further notice. Both the New York and Ontario coastlines of Lake Ontario suffered major spring flooding in 2017.

As reported by weather.com's Brian Donegan, flooding is already occurring in northwestern Ohio and southeastern Michigan along western Lake Erie, which is in "uncharted territory with near-record-high levels," according to the National Weather Service office in Cleveland.

Downtown #PortClinton getting the brunt of the flooding from #LakeErie. Looks like the only way to get around is by boat or paddle board. Be careful out there and let us know about your flood reports and pictures! Image credit: Michelle W. #OHwx #PAwx #ThisIsCLE #CLEwx #NWS pic.twitter.com/H8R0UwsuRf

— NWS Cleveland (@NWSCLE) May 8, 2019

Multiyear swings in Great Lakes water levels are not uncommon.  The lake levels were unusually low from the late 1990s to the early 2010s before increasing sharply in the last few years. Over the coming century, as climate change leads to warmer temperatures and more evaporation, the average lake level is projected to decline slightly, according to the 2018 U.S. National Climate Assessment. However, variability will continue. Since the most extreme precipitation events are getting more intense in many areas, it's possible that the peak periods of high water on the Great Lakes will intensify even as long-term levels change little or drop slightly.

Figure 2. The Bonnet Carré Spillway during its opening in January 2016. Image credit: Bob Henson.

Mississippi River flood heights force an unprecedented double opening of the Bonnet Carré Spillway

This month’s heavy rains in Arkansas and surrounding states have swollen the Lower Mississippi River to near-record heights. The flood heights at Natchez, Mississippi, Red River Landing, Louisiana, and Baton Rouge, Louisiana and expected to be among the top seven on record by May 23. Because of the high water levels on the river, the U.S. Army Corps of Engineers announced on Thursday that it planned to open the gates on the Bonnet Carré Spillway in St. Charles Parish, Louisiana on Tuesday, May 14, in order to allow flood waters from the Mississippi River to flow into Lake Pontchartrain. The spillway has already been opened once this year: a 43-day period from February 27 – April 11. Next week’s opening marks the first time in the 88 years of the spillway's existence for a double opening in one year. This year also marks the first time the spillway has been operated in consecutive years; it was also opened in March 2018, when it was operated at approximately half of its capacity.

Opening of the spillway is expected to keep the Mississippi River below its 17-foot flood stage in New Orleans; levees there protect the city against floods 3 - 5 feet higher than that. The river was just below flood stage on Thursday afternoon, at 16.8’, and the latest National Weather Service forecast predicts that the river will maintain a level of 16.9' into the first week of June. Let’s hope the city doesn’t get an early-season Category 1 hurricane in June. With the Mississippi River running just three feet below the tops of the city’s levees, the storm surge from a Category 1 hurricane travelling up the river could overtop the levees and flood the city. Five hurricanes over the past 55 years--Betsy (1965), Katrina (2005), Georges (1998), Isaac (2012), and Gustav (2008)--have pushed storms surges of seven feet or higher up the Mississippi River to New Orleans. Fortunately, all of these storms occurred in the late summer/early fall, when the river was well below flood stage. The $20 billion upgrade to New Orleans’ levee and flood defense system constructed in the wake of Hurricane Katrina in 2005 did not address the vulnerability of the city to storm surges traveling up the Mississippi River from the Gulf of Mexico.

When all 350 bays are opened, the spillway can divert one-fifth of the Mississippi River’s water into Lake Pontchartrain. The last time all 350 bays were opened was in 1983. During the opening earlier this year, a maximum of 206 bays were opened. The Army Corps has not yet decided how many bays will be opened this May, nor how long the spillway will be open.

Next week’s opening will be just the fourteenth time the spillway has been opened since 1937. These openings have been occurring with increasing frequency in recent years—six of the fourteen openings, including the two this year, have occurred since 2008. The spillway has opened in 2019 (twice), 2018, 2016, 2011, 2008, 1997, 1983, 1979, 1975, 1973, 1950, 1945, and 1937.

Although the spillway drains water into Lake Pontchartrain through uninhabited land that is primarily used for recreation, operation of the spillway does come with a cost: the excess fresh water that drains into the Gulf of Mexico is harmful to the oyster industry. The prolonged opening of the spillway in 2011 likely cost the Mississippi oyster industry $22 - $46 million and the loss of several hundred jobs during the period 2011 – 2013, according to an analysis by Mississippi State University. Water levels on the Mississippi River are not high enough to force the Corps to open the Morganza Floodway in Pointe Coupee Parish northwest of Baton Rouge, which would divert water from the Mississippi River down the Atchafalaya River. That floodway has been opened only twice--in 1973 and 2011--and has a much higher cost to open compared to the Bonnet Carré Spillway. The value of property at risk of flooding in the Morganza Floodway is up to $2.2 billion, including over 21,000 homes.

Weather.com published an excellent long-form piece on the Bonnet Carré Spillway back in 2016, which has a very detailed look at its history, function and ecological impacts.