“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.

Tropical Depression Harvey Is Threatening Texas with Major Flooding

  • Aug 23, 2017

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 depicts Harvey tracking towards the Texas coast by Friday evening, and then stalling for several days.

tropical depression harvey

National Hurricane Center intensity and track map for Harvey.

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

tropical depression harvey

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

Credit:

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 and waves may start increasing in the Gulf of Mexico and Galveston Bay by late Friday.

Duration: Most models show Harvey slowing down and 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 outages: Medium
  • Risk of substantial storm surge (5 feet or higher): Medium-high
  • Risk of heavy rainfall flooding: High

Quantitative Precipitation Forecast (QPF)

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 inches in a widespread area, including metro Houston, Freeport, Galveston, Beaumont and Lake Charles. This model shows a peak value of 16.5 inches near Galveston Bay.

Avoid Looking at the Category

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 Category 5 hurricane would tend to dump more rain than a Category 1 hurricane. But the opposite is true.

The category is only for one hazard: maximum sustained wind. Storms are too complex to define by one number.

snapshot forecast for harvey

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

snapshot forecast for harvey

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

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. 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.

Comparing Harvey to Ike

The last hurricane to make landfall on the Texas coast was Hurricane Ike. It reached shore on September 13, 2008. Ike was very different than Harvey. Here are some comparisons:

Ike brought catastrophic storm surge to the Bolivar Peninsula, with communities completely washed away and major storm surge on Galveston Island, most communities of Galveston Bay, and northeast to Port Arthur, Orange and Lake Charles. There were maximum sustained winds around 110 mph, near the Category 3 threshold.

Harvey is likely to bring a moderate to high storm surge with peak levels that may reach 6-9 feet. Most places will see 4-7 feet. Its maximum sustained winds will most likely reach tropical storm force (less than 74 mph) or possibly Category 1 hurricane (74-95 mph) speed.

Widespread rainfall of 10+ inches along Texas coast is like, with some places that could see 20+ inches rain. Harvey could impact coastal Texas for four or more days.

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?

Consider that Hurricane Isaac (2012), which was "only" a Category 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, 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 of Mexico and bays or inlets. The closer you are to saltwater, the more likely you will flood.

This originally appeared on Hurricane Hal's Storm Surge Blog.