Eric Berger

Harvey almost certainly the biggest US flood-producing storm

Monday, 1pm CT—Houston remains in the midst of Hurricane Harvey’s devastating aftermath, and we are weeks if not months away from a full accounting of its toll on the fourth largest city in the United States. And while a definitive account of its epic proportions will have to wait, we can at least begin to make some preliminary assessments.

In this post we will discuss how Harvey compares to other major flood storms in US history, and also how it stacks up to the region’s previous flooding event of record, Tropical Storm Allison. (Spoiler alert: Allison had a good run, but Harvey has forever dethroned her).

Mississippi discharges

The Texas state climatologist, John Nielsen-Gammon, has been closely tracking Harvey and its attendant flooding. To gauge the overall impact of a rain event, he uses a metric that defines a target zone of 20,000 square miles (about the size of West Virginia, or about 30 times the size of the city of Houston), and then measures the rainfall rate over a 120-hour period in comparison to the discharge rate of the Mississippi River. Here are the top 10 Gulf Coast flood events, as measured in “Mississippi River discharges” over that time:

  • 3.6 Hurricane Harvey 2017 (estimate)
  • 3.4 Hurricane Beulah, 1967
  • 3.1 Brazos River flooding, 1899
  • 2.8 Hurricane Georges, 1998
  • 2.6 Southeast Texas flooding, 1994
  • 2.5 Louisiana floods, June 1940
  • 2.4 Tropical Storm Alberto, 1994
  • 2.3 November Texas floods, 1940
  • 2.3 Louisiana floods, 1953
  • 2.3 Tropical Storm Allison, 2001

Note that the initial estimate for Harvey may be too low. As of Monday morning, Neilsen-Gammon’s hand estimate for Harvey to date was 3.1, but more heavy rains were falling in the target area and a full five days not reached. Already, for 72 hours, Nielsen-Gammon said Harvey is the greatest event over this shorter time scale.

Under this metric of rainfall over a large area, then, Harvey will very likely produce the greatest total amount of rainfall in the United States from any single storm, at least during the last 120 years. (the Gulf coast is by far the most susceptible US regions to such events due to its proximity to the warm, humid body of tropical water).

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Harvey late Monday morning: Rains remain, but storm finally moving

Monday, 10:30am CT—There is no way to escape the reality of the situation on the ground this morning: Houston bayous are flooding, and even after the region has received 30 inches of rain during the last two days, more is now falling over the central Houston area. It is a catastrophic situation. Finally, however, the end is in sight. Another day or two of this …

Harvey

Harvey remains barely a tropical storm, but its center moved into the Gulf of Mexico near Matagorda Bay this morning. Presently moving to the southeast at about 5 mph, some slight strengthening is possible over the next day or two as Harvey turns east, then northeast, but as discussed yesterday we are not too concerned about this possibility due to the lack of an organized core, and additionally some drier air is working into the storm. (See official track forecast).

Tropical Storm Harvey at 10am CT on Monday. (NOAA)

Overall, Harvey’s movement into the Gulf may be a positive development, because the storm is getting closer to being picked up by lower pressure in the Midwestern United States, which should lift it north. By later Tuesday, or Wednesday, we should hopefully be on the back side of the storm, with northerly winds helping to push water from inland bayous out to the Gulf of Mexico. Heavy rains should end, hopefully, by Wednesday or Wednesday night.

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Harvey late night: Water levels rising across Houston metro area

Monday, 12:30am CT—Nearly every bayou, river, stream, and waterway in and around Harris County has reached the top of its banks, or exceeded them. Flooding is occurring in multiple neighborhoods across the region, and worse is still to come. It is anything but hyperbole to say this is the most catastrophic flooding event in the city of Houston’s history. Tropical Storm Allison, a 500-year-flood, held the title for just 16 years.

This radar image (below) from around midnight shows what the region is up against tonight, and in the coming days. As the center of Tropical Storm Harvey drifts toward Matagorda Bay, it is producing a huge, 400-mile arc of showers that stretches all the way from south of the Texas-Mexico border into The Woodlands. This is an awesome, terrifying feat of nature—a freight train laden with moisture bound for the upper Texas coast.

Radar image at midnight, Sunday. (Intellicast)

Truthfully, we don’t really know what will happen next. Some drier air has moved into the southern half of the region from the west tonight, and this has led to some drying behind the line of showers, from El Campo to Galveston. Some of the short-term, high resolution models suggest the heavier activity will now slowly move off to the northeast of Houston tonight, toward Beaumont and Western Louisiana. (Beaumont is having an historic rain event too, by the way). This may offer a short reprieve for parts of Houston, and truthfully anything is welcome.

But with Harvey’s center so close by, we can almost certainly expect more to come. Tonight, western areas of Houston, including Katy, Cinco Ranch and other communities along the Sam Houston Tollway, picked up 7 to 10 inches of rain in just six hours. Who will be hit so hard next? We probably will have to worry about these mega-rain-bombs for another two or three days, perhaps into Wednesday, or Wednesday night. And what will that mean for Houston, its bayous, and its increasingly beleaguered citizenry?

Truly, we don’t know what happens next. This is uncharted territory for Houston in terms of flooding and potential disruption to our communities. Certainly, some help is on the way from the state and federal government. But in the meantime, be kind to your neighbors. We have seen incredible acts of valor and bravery and heroism during these last 36 hours from common, ordinary people. We have saved lives, and made the lives of others just that tiniest bit easier. We will need much more of this to make it out the other side.

Posted by Eric at 12:30am CT on Monday by Eric

Another very wet night on tap for Houston

Sunday, 8:40pm CT—As darkness descends upon Houston, another very wet night has already begun for the region. So far this evening, the heaviest rains have fallen across the western part of the region, including the Katy area. As much as 6.5 inches of rain have fallen near Cinco Ranch from 5pm to 8pm. A flash flood emergency is continuing for pretty much the entire Houston area tonight.

Tropical Storm Harvey, at sunset over Houston. Sunday. (NOAA)

Addicks and Barker

These rains have occurred near, and over the massive Addicks and Barker reservoirs on the west side of the city. Earlier today, the US Army Corps of Engineers said it would have to make controlled releases into Buffalo Bayou (which already is beyond flood stage) in order to prevent a more catastrophic release later next week. “This flood event will exceed the 2016 Tax Day Flood elevations,” said Col. Lars Zetterstrom, Galveston District commander.

It seems probable that some homes near Clay Road and Eldridge, and in the vicinity of the Westpark Toll Road and Highway 99 will see water remain in homes for up to two months. That is not a misprint.

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