Bob Henson ·

Above: Tropical Storm Harvey, with its center off the coast of Texas, as seen by the GOES-16 satellite at 1:27 pm CDT Tuesday, August 29, 2017. Image credit: College of DuPage. GOES-16 data are considered preliminary and non-operational.

The rains from Hurricane Harvey—which by some very preliminary estimates may be the costliest hurricane in U.S. history—are washing away meteorological and hydrological records, not only for Texas but for the United States. Although the heaviest rains were gradually shifting east of the Houston area on Tuesday, the region remained in a massive crisis, with newly overtopping levees and dams and new areas of flooding. Many thousands of residents are suffering through a third day trapped at home or marking time at a shelter. Houston’s convention center was housing more than 9000 people by Tuesday morning, while Dallas plans to open its convention center on Tuesday to another 5000 people. Also on Tuesday, President Donald Trump was visiting the Corpus Christi area, close to where Harvey made landfall on Friday.

Figure 1. A car is submerged on a freeway flooded by Tropical Storm Harvey on Sunday, Aug. 27, 2017, near downtown Houston, Texas. The remnants of Hurricane Harvey sent devastating floods pouring into Houston on Sunday as rising water chased thousands of people to rooftops or higher ground. Image credit: AP Photo/Charlie Riedel.

According to wunderground weather historian Christopher C. Burt, this is certainly the most extreme precipitation event on record to affect any major city in the United States. A multi-day rain event of this extremity has never happened before for any major city in the continental U.S. since record keeping began in 1895—and of course U.S. cities were much smaller before that point than they are now. According to Mr. Burt, who analyzed the precipitation records for 330 major U.S. cities, the records for 3-day precipitation at major U.S. cities now looks like this:

1) 32.47”, Houston Hobby Airport, TX, Aug 26 – 28, 2017

2) 30.32”, Hilo, HI, Nov 1 – 3, 2000

3) 28.44”, Houston Intercontinental Airport, TX, Aug 26 – 28, 2017

The previous continental U.S. record for 3-day precipitation in a major U.S. city was 24.11”, in Key West, Florida on Nov. 10-12, 1980.

The records for 2-day precipitation now look like this:

1) 29.28”, Hilo, HI, Nov 1 – 2, 2000

2) 24.44”, Houston Intercontinental Airport, TX, Aug 26 – 27, 2017

3) 23.06”, Houston Hobby Airport, TX, Aug 26 – 27, 2017

Figure 2. Preliminary maximum 24-hour Average Recurrence Interval (ARI) of the rainfall from August 24, 2017 through 12Z August 28, 2017. This was created using hourly Quantitative Precipitation Estimates (QPEs) from MetStormLive, a gauge-adjusted DualPol-radar based precipitation system operated by MetStat, and updated precipitation frequency grids, also produced by MetStat in 2014. Several areas experienced 24-hour rains that on average occur every 1,000+ years (have a .1% chance of occurring in any given year). Early indications suggest longer duration (48- and 72-hour) rainfalls were even rarer; MetStat is actively working on quantifying those ARIs. (Source: MetStat, Inc.) Note that a 1-in-1000 year rainfall event is not the same as a 1-in-1000 year flood, which depends on more than just rainfall.

Continental U.S. rainfall record for a tropical cyclone falls

The highest 4-day official rainfall amount from Harvey as of 10 am CDT Tuesday has been 49.32” at Marys Creek, according to NOAA/NWS/WPC. This breaks the previous all-time record for continental U.S. rainfall from a tropical cyclone or its remnants: 48.00” in the Texas hill country from Tropical Storm Amelia of 1978. We still have a few inches to go to break the record from Hawaii, though: 52.00” in Hurricane Hiki of 1950.

A number of rainfall amounts topping 45” were reported from 8:00 pm CDT Thursday through 10 am CDT Tuesday, mainly across the Houston metro area:



49.32” Marys Creek at Winding Road

48.64” Cedar Bayou at FM 1942

47.20” Clear Creek at I-45

46.08” Dayton (0.2 mi E)

45.02” Santa Fe (0.7 mi S)

As of 12 pm CDT Tuesday, at least six personal weather stations (PWSs) in the Houston area had received 84-hour rainfall totals over 45”, and four of these had rainfall amounts in excess of 50”, including one with an extraordinary 61.52”. Note: These PWS totals are unofficial and may reflect data from uncalibrated gauges, which can give erroneous readings.

61.52” Baytown, TX (Country Club Oaks)

53.64” Baytown, TX (Eastpoint)

52.30” League City, TX (South League City)

51.69” La Porte, TX (Westend LaPorte/SJJC)

47.79” Dayton, TX (Winter Valley)

45.16” Dayton, TX (Brookstone)

Figure 3. Observed precipitation from Harvey as of 9 am CDT Tuesday, August 29, 2017. The Houston area has received over 30” of rain (pink colors). Image credit: NWS.

Outlook for Harvey and its rains

As of 1:00 pm CDT, Harvey’s center was located about 80 miles south-southwest of Port Arthur, TX, or roughly 100 miles southeast of Houston. Top sustained winds are 45 mph and not expected to increase. Harvey will finally pick up its pace starting Wednesday, accelerating toward the north-northeast to the Louisiana coast by Wednesday and the Mississippi Valley by Friday, with heavy rains likely extending into western Tennessee and Kentucky. The increased steering flow and wind shear at upper levels will also tamp down any strengthening of Harvey, despite its location over warm water. A Tropical Storm Warning is in effect from north of Port O’Connor, TX, to Morgan City, LA, with a Tropical Storm Watch eastward to Grand Isle. Storm surge is expected to range from 1 to 3 feet in the Storm Surge Warning area from Port Bolivar, TX, to Morgan City, LA.

Harvey is now much more reminiscent of a subtropical or extratropical storm than a tropical cyclone, with a classic comma-shaped cloud pattern (see image at top). The distinct line of showers and thunderstorms (convection) that materialized late Sunday off the Texas coast—analogous to a cold front in a winter storm—had progressed to southeast Louisiana by Tuesday morning, dumping heavy rains in New Orleans and Biloxi. Meanwhile, relentless rains continue to be clustered around and just east of Harvey’s core, across far southeast TX and southwest LA. These will continue into early Wednesday, progressing very slowly eastward along a weak front located along the TX/LA coast. These core rains are likely to stay focused toward the eastern half of the Houston metropolitan area, extending east toward Beaumont/Port Arthur and northward from Galveston Bay. This is good news for western parts of the metro area, although it would only take a minor westward redevelopment to bring additional rain to western parts of the Houston area.

Figure 4. Jimmie Bradley speaks about the flooding in his neighborhood in Moss Bluff, a Lake Charles, La., suburb in Calcasieu Parish, on Monday, Aug. 28, 2017. Bradley, 78, and his wife Brenda, had stacked sandbags at their doors, but the rising water was lapping at the steps to their back porch and had overtaken their front yard. Virtually every neighbor on Crawford Drive has at least a foot of water in their yards. Image credit: AP Photo/Rogelio V. Solis.

Across the Gulf Coast from far southeast TX across southern Louisiana, the rains from Harvey are far from over. Rich Gulf moisture will continue to stream north across a frontal zone that stretches along the coastline. The front will enhance rainfall that may organize into slow-moving bands of “training” echoes. Totals of 5” – 10” or more could fall with the current cluster of storms moving through southeast LA and coastal MS, and similar amounts may occur by this evening across other parts of southern LA. Even larger amounts may fall between Galveston Bay and the TX/LA border, exacerbating flood concerns in those areas.

Figure 5. Rainfall predicted by the HRRR mesoscale model run at 11:00 am CDT Tuesday, August 29, 2017, for the 18-hour period through 5:00 am CDT Wednesday. The heaviest rains will extend along the Gulf Coast from southwest Louisiana to the western Florida Panhandle, although another 2” – 4” could still fall in the Houston area. Image credit: tropicaltidbits.com.

Figure 6. Businesses and cars are flooded near the Addicks Reservoir as floodwaters from Tropical Storm Harvey rise on Tuesday, Aug. 29, 2017, in Houston. Image credit: AP Photo/David J. Phillip.

New flood worries across Houston area, as levees and spillways are overtopped

The rains may be on their way out of Houston, but the city’s flood worries are anything but over. The massive amounts of rainfall over the last four days are still working their way through mainsteam rivers and into reservoirs, and even smaller-scale channels such as bayous are only slowly receding or holding steady.

The biggest concern for central Houston is now the dual Addicks and Barker Reservoir system, which was designed and built in the 1940s for water levels expected to occur on average once every 1,000 years. Water began flowing around the north side of the Addicks dam and onto a spillway on Tuesday morning, the first uncontrolled release of water in the reservoir’s history. The Addicks spillway is at 108 feet above mean sea level (MSL), about 14.7 feet below the top of the dam itself. The uncontrolled flow will continue along and around the north side of the earthen dam and onward into Buffalo Bayou, eventually spiking the bayou’s flood level as it heads toward downtown Houston. The bayou at West Bend has remained at record levels since Monday, although its height had receded somewhat by Tuesday in western and downtown Houston. “We have never faced this before. We have uncertainty in how the water is going to react as it moves out of the spillway and into the surrounding area,” said Harris County Flood Control District meteorologist Jeff Lindner at a news conference on Tuesday. “We are trying to wrap our heads around what this water will do.”

There are also major flood concerns behind the Addicks and Barker reservoirs, where backwater is pushing into neighborhoods (all outside the 100-year flood plain) that were built on private land within the reservoirs’ physical basin. According to the U.S. Army Corps of Engineers, more than 3,000 structures behind the reservoirs are at risk of being inundated for as long as a month or more, as it will take time to empty the reservoirs safely.

Figure 7. The Addicks and Barker Reservoirs are “dry” reservoirs, normally filled with grasses and trees. They were built in the 1940s to keep flood waters from devastating central Houston, as occurred in 1929 and 1935. Water release from the reservoirs (in either a controlled or uncontrolled fashion) flows downstream into Buffalo Bayou, which winds from west to east into downtown Houston. On Tuesday, water was flowing around the north side of the Addicks dam in the first such uncontrolled release on record. Base image © Google.

Dam operators take great pains to avoid uncontrolled releases, but in this case the Addicks outlet channels were already discharging water as fast as they safely could. The top of the Addicks Dam is at 122.7 feet elevation, and water is currently not expected to reach that level. However, water will continue to flow in an uncontrolled fashion over the spillway until inflow from this week’s heavy rains begins to decrease. The U.S. Army Corps of Engineers is monitoring the state of the Addicks and Barker system around the clock. Thus far, they report no present concerns about the stability of the dams—which is a very good thing, since we are entering uncharted territory in terms of stress on the reservoir system and the dams themselves. This ongoing Twitter thread from Eric Holthaus (Grist) hits many of the key points.

More than 40% of land in the Addicks watershed—originally rural—has now been developed, which reduces the ability of the land to absorb water and exacerbates flooding. In 2009, the USACE rated the Addicks and Barker dams as being at “extremely high risk of catastrophic failure,” which put them among just six dams in the nation with that designation. As reported in the Houston Chronicle last year, “If the dams failed, half of Houston would be underwater. Under the worst scenario at Addicks, property damages could reach $22.7 billion and 6,928 people could die.” The Houston Press, which published a harrowing in-depth report on the state of the dams in 2012, filed an update on Sunday. Video from a Tuesday morning news briefing with USACE staff can be viewed on the KHOU-TV website.

In the exurbs just west and southwest of Houston, levees are being breached along the Brazos River, raising the stakes on mandatory evacuations. Hundreds of homes in the Columbia Lakes resort community are at risk from an overtopping levee in Brazoria County. Elsewhere south of Houston, the city of Dickinson—one of the area’s hardest hit over the weekend—remained under mandatory evacuation Tuesday, while the mandatory evacuation for Bay City was lifted.

A new service that maps flooded streets in real time

On Tuesday, Galveston-based Tailwind Labs and Marine Weather and Climate launched the U-Flood Project to provide near real-time mapping of flooded streets during Hurricane Harvey. The project provides a crowdsourced mapping platform for the Houston metro area, as well as the I-45 corridor, Galveston, Beaumont/ Port Arthur, Lake Charles, Baton Rouge and New Orleans. “Real-time mapping is crucial right now, as first responders are still in rescue mode trying to locate and save flood victims,” says surge expert Hal Needham. You can help in several ways:

Map it: Click on a city image and start mapping flooded streets. Even if you just map the street in front of your house, the collective effort of thousands of users will construct a near-real time map of current flooding!

Tell others: Spread the word about this service.

Donate: The creators of U-Flood Project are seeking donations on Patreon in order to expand resources as the catastrophe unfolds. “We have a donate button on the bottom of the U-Flood homepage," says Needham. "Even a $10 gift will help so much!”

Figure 8. Volunteer rescue boats make their way into a flooded subdivision to rescue stranded residents as floodwaters from Tropical Storm Harvey rise Monday, Aug. 28, 2017, in Spring, Texas. Image credit: AP Photo/David J. Phillip.

Portlight disaster relief charity responding to Hurricane Harvey

The Portlight.org disaster relief charity, founded and staffed by members of the wunderground community, is responding to Hurricane Harvey. They need your help!! The flooding caused by Harvey has left many people stranded, and experience shows that a disproportionate number of them will be people with disabilities and older adults. During Harvey, Portlight’s Partnership for Inclusive Disaster Strategies Disability Hotline (800) 626-4959 has already assisted hundreds of older adults and people with disabilities. They’re rallying stakeholders, working to get people to safety, providing for any immediate needs for durable medical equipment and other assistive technology, and responding to evacuation and sheltering issues and problem-solving for a variety of immediate disability accessibility issues. Portlight has already heard from people being turned away from shelters, denied sign language interpreters, and many of the other issues the disabled have suffered from throughout past disasters. We hope you'll consider supporting Portlight's work with a donation.



Jeff Masters co-wrote this article.

