Saturday, August 26, 2017

Hurricane Harvey's Real Story Begins: 40 Inches of Rain to Bring Catastrophic Flooding to Southeast Texas

Jon Erdman
Published: August 26,2017

Hurricane Harvey is now stalling over Texas, with the biggest threat of catastrophic flooding looming ahead, lasting well into next week.
Harvey made landfall Friday night near Rockport, Texas, north of Corpus Christi, as the first Category 4 hurricane to landfall in the U.S. since Charley in August 2004.
(MORE: Hurricane Central | Latest ImpactsHow You Can Help Victims)

Right Now

Harvey's center of circulation is currently located near Victoria, Texas, only roughly 50 miles from where it made landfall late Friday night.

Current Storm Information
Hurricane and tropical storm-force winds are still occurring. A sustained wind of 57 mph and a gust of 83 mph reported Saturday morning near Victoria.
A Texas Coastal Ocean Observing Network station at Aransas Pass reported sustained winds of 102 mph and a wind gust of 132 mph Friday night.
Rainfall amounts of more than 10 inches have already accumulated in southeast Texas, including 14.46 inches near Austwell.
Harvey has pushed water 2 to 7 feet above average tide levels near Corpus Christi to Lavaca Bay, and water levels remain elevated as onshore flow continues to the east of Harvey's center.

Current Radar, Winds

Current Warnings

A hurricane warning has been issued for a portion of the Texas coast, from Baffin Bay to Port O'Connor. A hurricane warning means hurricane conditions are likely within the warning area.
Sustained hurricane-force winds are affecting parts of the hurricane warning area.
Tropical storm warnings are in effect from north of Port O'Connor to High Island, Texas, including the cities of Houston and Galveston.
(MORE: First Hurricane Landfall in Texas in Almost Nine Years)

Tropical Alerts
The NHC also issued its first-ever public storm surge warning, which includes a swath of the Texas coast from Baffin Bay to High Island. This means a life-threatening storm surge is expected in the warned area in the next 36 hours. This warning does not include Galveston Bay, but does include Galveston Island and the Bolivar Peninsula.

Storm Surge Forecast
Additionally, a tornado watch is in effect until 1 p.m. CDT Saturday afternoon for southeast Texas, as tornadoes will be possible, if not common, within Harvey's outer rainbands. This watch area includes Houston, Galveston and Port Arthur, Texas.
A reported tornado damaged a McDonald's sign in northeast Galveston late Friday afternoon. Numerous other short-lived tornadoes have occurred, but damage will take time to sort out.

What's Next?

Harvey is likely to weaken to a tropical storm by later Saturday.
The circulation center is also expected to be caught in a zone of light steering winds aloft that will stall Harvey. The storm will then meander near the Texas coast for days to come, unleashing tremendous amounts of rain.
(MORE: Interactive Harvey Forecast Path)

Projected Path
In addition, the persistent onshore winds will lead to high water levels on the coast and continued high surf. Gusty winds and saturated soil conditions may lead to more downed trees in the days ahead.
(MORE: Three Reasons Slow-Moving Tropical Storms and Hurricanes Are the Worst)

Devastating Rainfall Flooding

A tropical cyclone's rainfall potential is a function of its forward speed, not its intensity.
(MORE: Water, Not Wind, the Deadliest Factor in U.S. Tropical Storms, Hurricanes)
With Harvey stalling several days, prolific rainfall, capable of catastrophic flash flooding, will result near the middle and upper Texas coast.
For now, areas near the Texas and southwest Louisiana Gulf coasts are in the biggest threat area for torrential rainfall and major flash flooding, potentially including Houston and Corpus Christi.
Local National Weather Service (NWS) offices have not minced words about the threat, warning of "some structures becoming uninhabitable or washed away" and "numerous road and bridge closures with some weakened or washed out," with record river flooding expected in some areas.
Harvey's heavy rain may not entirely exit the areas of Texas it soaks until sometime next Thursday, and may not exit the Mississippi Valley until next Friday.
(MAPS: 7-Day U.S. Daily Rainfall Forecasts)
Here are the latest rainfall forecasts from the NHC and NOAA's Weather Prediction Center through next Wednesday. Keep in mind, locally higher amounts are possible where rainbands stall.
  • Middle/upper Texas coast: 15 to 30 inches, with isolated totals up to 40 inches
  • Deep South Texas and Texas Hill Country eastward to central and southwest Louisiana: 5 to 15 inches

Rainfall Outlook
To put this into perspective, these peak rain totals are roughly the average yearly precipitation in Austin (34.29 inches) and Corpus Christi, Texas (31.73 inches). So, a year's worth of rain may fall in the span of a few days near the Texas Gulf Coast.
This forecast is subject to change depending on the exact path of Harvey, locations of rainbands and how long it stalls. Generally, areas along and east of Harvey's path are in the greatest threat of flooding rainfall.
(MORE: Your Vehicle Can Be the Biggest Danger in a Flood)
Among the biggest uncertainties is the heavy rain potential in central Texas, including for the flood-prone cities of Austin and San Antonio. That all depends on how far inland and to the west Harvey tracks and how long it stalls in that area.
Flash flood watches have been issued for much of southeast, southern and parts of central Texas.
Rainfall so far, through 9 a.m. CDT Saturday:
  • 14.46 inches near Austwell
  • 13.82 inches San Antonio River near McFadden
  • 10.54 inches in Aransas
  • 10.06 inches in Edna
  • 9.91 inches near Fulshear
  • 8.17 inches near Bay City
  • 8.42 inches near Eagle Lake
  • 8.08 inches in East Bernard
  • 8.12 inches in Greatwood
  • 7.89 inches near Danbury
  • 7.50 inches near Columbus
  • 7.41 inches near Meyersville
  • 6.28 inches near Victoria
  • 5.39 inches at Houston Southwest Airport
  • 3.54 inches near Corpus Christi
  • 3.02 inches in Galveston

Storm Surge Threat

Here are the latest storm surge forecasts, according to the NHC. Note that these inundations above ground level are worst-case scenarios along the immediate coast if the peak surge coincides with high tide. Due to Harvey's earlier than anticipated landfall, worst-case scenario surge values were avoided, but periods of high tide may bring additional surge and waves ashore over the next few days.
A storm surge of more than 6.6 feet has been recorded at Port Lavaca, Texas.

Coastal Flood, Waves, Wind Setup
According to the NWS, structural damage to buildings near the coast is expected, with "many washing away." Battering waves riding atop the surge will lead to "extreme beach erosion," "massive damage to marinas, docks, boardwalks and piers" in the areas of highest storm surge.
Importantly, rising water will cut off near-shore escape routes and secondary routes before peak surge arrives.
At least some coastal flooding, along with battering waves, could persist for several days over multiple tide cycles along the Texas and southwest Louisiana coasts into next week.
This coastal flooding and wave action could increase if Harvey's center re-emerges over the Gulf and intensifies, potentially leading to a second storm surge along parts of the Louisiana or upper Texas coast next week. Again, this part of the forecast is highly uncertain at this time.
Furthermore, this water rise from the Gulf of Mexico won't allow rain-swollen rivers and bayous to drain, compounding the inland flood threat.

Destructive Wind Threat

If that wasn't enough, there's the winds.
The NWS warns of "structural damage to sturdy buildings" and "complete destruction of mobile homes" where the eyewall of Harvey tracks.
Roads not already impassable by flooding may become blocked from downed trees or other debris. Power and communication outages will be widespread near and inland of the landfall in central and southeast Texas.

Potential Power Outages
Furthermore, persistent winds, even if they are not particularly high-end when Harvey is over land, could down more trees than they otherwise would given the rain-soaked or flooded ground, possibly for several days as Harvey lingers.
Highest winds so far (gusts unless otherwise noted):
  • Port Aransas: 132 mph, sustained to 110 mph
  • Near Copano Village: 125 mph
  • Near Lamar: 110 mph
  • Near Port Aransas: 109 mph
  • Rockport: 108 mph
  • Near Taft: 90 mph
  • Near Magnolia Beach: 79 mph
  • Edna: 73 mph
  • Near Clear Lake Shores: 71 mph
  • Corpus Christi NAS: 65 mph

A Truly Historic Hurricane

Harvey made landfall Friday night near Rockport, Texas, a town of less than 10,000 people and about 30 miles up the Texas coast from Corpus Christi.
Harvey is this nation's first major (Category 3 or stronger) hurricane landfall since Hurricane Wilma struck South Florida in October 2005, an almost 12-year run. A multi-day deluge of the Texas Gulf Coast with catastrophic and life-threatening flooding and destructive winds through could leave areas uninhabitable for an extended period of time, the National Weather Service has warned.
Harvey went from a tropical depression to a major hurricane in 56 hours. It intensified rapidly to a Category 4 hurricane with winds up to 130 mph after moving over a pocket of hot water in the Gulf.
Harvey is also be the strongest landfall in this area, known as the Texas Coastal Bend, since the infamous Category 3 Hurricane Celia hammered the Corpus Christi area in August 1970 with wind gusts up to 161 mph, damaging almost 90 percent of the city's businesses and 70 percent of its residences and destroying two hangars at the city's airport.
The Texas Coastal Bend hadn't seen a Category 4 landfall since Hurricane Carla, in September 1961, produced catastrophic damage from storm surge and high winds in Port O'Connor and Palacios, Texas, among other locations.
The only other Category 4 landfall of record near the Texas Coastal Bend was the infamous Indianola hurricane of August 1886, which devastated the town of Indianola just 11 years after another Category 3 hurricane, eventually turning the former bustling port into a ghost town.
The Federal Alliance for Safe Homes can help you plan for a hurricane. NOAA also has excellent resources to plan for flooding.
Check back with for updates on Harvey.
(MORE: The Weather Company Forecasts More Active Hurricane Season Than First Predicted)

The Weather Company’s primary journalistic mission is to report on breaking weather news, the environment and the importance of science to our lives. This story does not necessarily represent the position of our parent company, IBM.

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