Harvey Threatens Texas with Major Flooding

Harvey has now re-formed into a tropical depression off the northwest coast of the Yucatan Peninsula. This means the center of circulation has been located, which will improve forecasts. The official National Hurricane Center track and intensity forecast (below) depicts Harvey tracking towards the Texas Coast by Friday evening, and then stalling for several days.

National Hurricane Center intensity and track map for Harvey.
Link: http://www.nhc.noaa.gov/storm_graphics/AT09/AL092017_5day_cone_no_line.png

MAIN POINTS

-Harvey likely to develop into a tropical storm today or tonight;

-All major models bring Harvey towards the Texas coast;

-Harvey may stall out near the coast or slightly inland;

-The most certain impact is major flooding from days of torrential rain and compound rain/ storm surge

Visible satellite image shows deep convection in the remnants of former Tropical Storm Harvey to the northwest of the Yucatan Peninsula. 

DETAILS

Timing: For the Houston/ Galveston/ Bolivar Peninsula region- expect wind and rain squalls to arrive by late in the day on Friday, however, there is too much uncertainty right now to pinpoint a time. Storm surge/ waves may start increasing in the Gulf and Galveston Bay by late Friday.

Duration: Most models show Harvey slowing down/ stalling near the Texas Coast or slightly inland. This could mean that rain, wind and surge impacts are felt from late Friday until Tuesday.

Impacts:

            Risk of Damaging Winds: Medium

            Risk of Power Outage: Medium

            Risk of Substantial Storm Surge (5 feet or higher): Medium-High

            Risk of Heavy Rainfall Flooding: High

The Quantitative Precipitation Forecast (QPF) from the National Weather Service, issued at on Wed Aug 23 at 2:57AM Central Time. 

The Quantitative Precipitation Forecast (QPF) from the National Weather Service, shown above, depicts 7-day rainfall potential exceeding 10" in a widespread area, including metro Houston, Freeport, Galveston, Beaumont and Lake Charles. This model shows a peak value of 16.5" near Galveston Bay.

AVOID LOOKING AT THE CATEGORY/ NUMBER

A lot of people are talking about what “category” Harvey will be. They are referring to the Saffir-Simpson Hurricane Wind Scale, which classifies hurricanes on a scale of 1-5. This category system is misleading and people who make decisions based on the category are often shocked by unexpected storm impacts.

The category is ONLY based on maximum sustained winds. It says nothing about heavy rainfall potential. It says nothing about storm surge potential.

Hurricanes and tropical storms throw three hazards at us- wind, rainfall and storm surge. Think of the impacts separately. Storms with weaker winds are more likely to stall and dump heavier rainfall. This shocks people, as it would seem intuitive that a cat-5 hurricane would tend to dump more rain than a cat-1 hurricane. But the opposite is true.

The category is only for one hazard- maximum sustain wind. Storms are too complex to define them by one number.

Deterministic "snapshot" forecast valid for 7:00PM Central Time on Friday, August 25, from the ECMWF model. Red shows highest levels of vorticity, or spin, in the atmosphere. Image:https://www.tropicaltidbits.com/analysis/models/

Deterministic "snapshot" forecast valid for 7:00PM Central Time on Monday, August 28, from the ECMWF model. Red shows highest levels of vorticity, or spin, in the atmosphere. Image:https://www.tropicaltidbits.com/analysis/models/

DURATION

Harvey's duration could be truly astronomical. The two maps above show a deterministic, or "snapshot" of Harvey's potential position on Friday and Monday evening, Central Time. Note how little the center of circulation has moved in a 3-day period. This potential scenario is a major concern for Texas and Louisiana.

COMPARISON TO IKE

The last hurricane to make landfall on the Texas Coast was Hurricane Ike, on September 13, 2008. Ike was very different than Harvey. Here are some comparisions:

IKE

Catastrophic storm surge on the Bolivar Peninsula (communities completely washed away);

Major storm surge on Galveston Island, most communities of Galveston Bay, and northeast to Port Author, Orange and Lake Charles;

Maximum sustained winds near Cat-2/ Cat-3 threshold (around 110 mph)

HARVEY

Most likely a moderate to high storm surge- Peak levels may reach 6-9 feet, but most places will see 4-7 feet;

Maximum sustained winds most likely tropical storm force (less than 74mph), or cat-1 hurricane (74-95 mph);

Torrential rain: Widespread rainfall of 10”+ along Texas Coast- some places could see 20”+ rain

Long-duration: Harvey could impact coastal Texas for four or more days (Late Friday - Tuesday)

COMPOUND FLOODING

Harvey will likely be a prime example of compound flooding that comes from both storm surge and heavy rainfall. Prolonged onshore winds will raise the level of the Gulf of Mexico, as well as bays, harbors and estuaries. Expect places like Galveston Bay to be elevated by several feet for as long as 60+ hours.

This creates a problem because most rainfall drainage is gravity-fed. The larger the slope between the land and water, the faster the water drains. The problem with a 2-3 day storm surge event is that the water from torrential rains cannot drain efficiently. If the Bay is elevated several feet, where will the rain water drain?

Hurricane Isaac's floodwaters near Laplace, Louisiana, in August, 2012. Compound storm surge/ rainfall flooding was substantial in this region.

Consider that Hurricane Isaac (2012), which was "only" a cat-1 hurricane, flooded some communities near New Orleans that did not flood in Hurricane Katrina. Isaac's main weapon was compound flooding, which inundated many communities around Lake Pontchartrain, like LaPlace (see pic above), forcing the rescue of nearly 4,000 people.

Harvey has the potential to set up a scenario like this in coastal Texas.

This is the big concern with Harvey. I’m particularly concerned about places like Galveston Island, north of Broadway, and Pasadena, Baytown, Laporte, Clear Lake, Kemah and Nassau Bay. If these areas get torrential rains for days and the Bay is elevated, I just don’t see where the water can go except into neighborhoods.

For this reason, some neighborhoods may flood from just five or six inches of rain, when normally that much rain is not a problem. Consider your elevation and proximity to the Gulf/ bays/ inlets. The closer you are to salt water the more likely you will flood.

TAKE ACTION

1.     Eat frozen food now (I never turn down an invite if you’re grilling) =)

2.     Gas car now;

3.     Buy extra gas now for generator, if you have one;

4.     Buy supplies now;

5.     Have evacuation plan in place;

6.     Have a plan to park your car on higher ground if in flood-prone area;

7.     If you have important documents in your home, try to put them in one box/ container and put most valuable in sealed zip-lock bags;

8.     If you plan on staying, or evacuating to a place that could lose power, have cash on hand. If power goes out, stores may still be open but take cash only;

9.     I have worked in flood zones for around a decade. If you have valuables at home that you cannot take to another site, consider putting them “up”. Keep in mind that 2-3 feet of flood water can float/ flip tables and other furniture- in case you put valuables on top of furniture.

ABOUT THIS BLOG

I’ve been running this blog for eight years now. It started out as a way to share storm surge information while I was a doctoral student at Louisiana State University.

I am not affiliated with the National Weather Service (NWS), National Hurricane Center (NHC), or a private forecasting group. I run a company called Marine Weather and Climate, that focuses on data-driven flood risk analysis. I also run the U-Surge project and just launched Galveston Hurricane Tours.

I am a hurricane/ storm surge scientist with a Ph.D. in geography/ climate science and I have researched Gulf Coast hurricanes for around a decade now.

As the NWS and NHC make forecasts available, consult what they are saying for decision making purposes. The purpose of these blog posts is not to replace the official forecast, but to give my friends on the Texas coast more lead-time for decision making. The NHC will issue hurricane watches when tropical storm conditions are expected within 48 hours. But many of my friends that have homes in Galveston or Bolivar Peninsula live in Houston or Beaumont and they need more time to secure their property, boats, etc.

I will try to update this blog every morning. Please feel free to send me any questions and I will do my best to answer them.

REFERENCES
Berg, 2013: