“Hurricane” Hal Needham is a storm surge scientist who specializes in data-driven storm surge analysis. He is the founder and president of Marine Weather and Climate.

A Complex Flood Event Threatens Texas and Louisiana This Weekend

  • Oct 24, 2015

A complex flood event threatens Texas and Louisiana over the next several days. A combination of a slow-moving front, the availability of ample moisture, and prolonged onshore winds will put southeast Texas and south Louisiana on alert through the weekend.

Low pressure is forming along a slow moving front in Texas, which is serving to pump tremendous amounts of moisture up from the Gulf of Mexico. The remnants of Hurricane Patricia — which struck western Mexico on Friday as a Category 5 hurricane — will add even more moisture to this developing flood event in Texas and Louisiana.

Texas Rain Radar

National Weather Service radar depicts ample moisture streaming into Texas from the south and southwest as of Saturday morning. Moisture will pump into coastal Texas for at least two more days.

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Patricia became the most intense hurricane in the history of the western hemisphere, producing 200 mph winds, before rapidly weakening as it neared the coast. The storm generated severe wind and flood damage, particularly west of Manzanillo, Mexico.

Although Patricia’s winds will not directly impact Texas, its moisture will help create phenomenal rains, particularly near the Gulf Coast. The National Oceanic and Atmospheric Administration’s Quantitative Precipitation Forecast map depicts a widespread area of rain will exceed 6 inches, particularly along and to the south of the I-10 corridor in Texas and southwest Louisiana over the next two days. This forecast predicts a maximum value more than 11 inches in the Houston-Galveston corridor.

Rainfall forecast Texas

NOAA's Quantitative Precipitation Forecast predicts a widespread area will receive more than 6" of rain along coastal Texas and Louisiana, with maximum amounts exceeding 11" near Houston- Galveston.

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An extended storm surge event will produce water levels noticeably above normal high tide levels and will exacerbate flooding near the coast and even inland. Water levels at Galveston’s Pier 21 have been running more than a foot above normal over the past several days.

High water levels and surf were noticeable already on Friday morning, as waves reached the seawall. I took the photo below on the Galveston Seawall on Friday morning. Several locals at the Living on the Edge Conference told me that water gets that high about one time per year.

Galveston seawall

Onshore winds drove sea water to the base of the Galveston Seawall on Friday morning. Locals said this happens about one time

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The Houston-Galveston National Weather Service Forecast Office predicts that water levels could reach as high as 4.8 feet above a datum called Mean Lower Low Water (MLLW), particularly on Sunday. This level would be approach 3 feet above predicted tide levels (if we take away the storm). The highest water over the past several days was approximately 3 feet above MLLW at Galveston Pier 21, or about 1.5 feet above normal.

Although a 3-foot storm surge is not phenomenal for Galveston, a prolonged 2-3 foot surge could have dire impacts if accompanied by 10 or more inches of rain. Most drainage around the Houston-Galveston area is gravity-fed, meaning the drainage depends on a noticeable slope between the ground and the water body into which the rainwater is discharged like a bayou, channel, Galveston Bay or the Gulf of Mexico.

If Galveston Bay is elevated 2-3 feet above normal, the slope between land and water is reduced considerably, making rainfall drainage less efficient. This becomes a major concern when looking at the extraordinary rainfall in the forecast.

Galveston tide gauge

Water levels at Galveston's Pier 21 have been running around 1.3-1.5 feet above normal over the past few days. This has produced maximum storm tides of around 3 feet above MLLW. The National Weather Service predicts tide levels could reach 4.8 feet above MLLW this weekend, which would be as much as 3 feet above normal.

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The combined flood risk from heavy rainfall and heightened seas has been a focus of new research in the past several years, and certainly relates to climate change, as melting glaciers and thermal expansion push sea levels upward. Dr. Thomas Wahl and colleagues recently published a paper showing that combined flooding from heavy rainfall and heightened sea levels is becoming more common along the U.S. Gulf and Atlantic Coasts.

I will be in the Houston-Galveston area through this weekend, as I attempt to document this complex flood event. Feel free to shoot me an email or contact me on Twitter with any questions or comments. Also, always feel free to send in pics or videos from coastal flood events around the world.