The Harris County Flood Control District (HCFCD) will hold a community engagement meeting to share Phase 2 results for the Feasibility Study of Stormwater Conveyance Tunnels. The meeting’s purpose: to inform residents about the status of the Tunnels study and share study information.
At the highest level, the study looks at the potential to reduce flooding risks in Harris County via large-diameter, deep underground tunnels that convey stormwater.
The study includes three phases:
Phase 1 examined the feasibility of tunnels in this area.
Phase 2 looked at potential routes and alignment concepts for areas with unmet needs.
Phase 3, if needed, will include a preliminary design to validate assumptions.
Stock photo from Phase 1 report shows tunneling machinery (cutterhead and shield) being lowered into a launch shaft.
Tunnel Tradeoffs
The primary benefit of tunnels: they add stormwater conveyance without disturbing development on the surface. In highly developed or environmentally sensitive areas, this is important. But tunnels also come with technical and financial challenges. For instance, you must route them around oil wells, water wells, and geologic faults. And the cost can be considerable: up to $150 million per mile for a 40-foot-wide tunnel.
More about Phase 2
In Spring 2022, HCFCD completed Phase 2 of its feasibility investigation. The purpose of Phase 2 was to identify unmet flood mitigation needs in Harris County’s watersheds. Phase 2 also developed distinct tunnel concepts to meet those needs.
This phase of the study focused on identifying:
Watersheds that met the criteria for a tunnel
Flood damage centers that presented the highest risk and determining whether the tunnels would be more cost-effective over traditional flood control measures (e.g. stormwater detention, channelization, or buyouts)
Potential strategic locations for intakes and outfalls
Opportunities to integrate tunnels with existing and proposed flood damage reduction systems
Geologic and man-made hazards.
Phase 2 found that a tunnel SYSTEM, rather than one or more individual tunnel alignments, should be the focus of further study. Thus, we would need additional study before a final decision on whether to move forward with tunnels.
Phase 2 received funding from the U.S. Department of Housing and Urban Development’s Community Development Block Grant Disaster Recovery program.
Meeting Details: How to Register
Community engagement is an important component of this study. So, HCFCD invites your participation.
The Virtual Community Engagement Meeting will be held on: Thursday, June 16, 2022 6:30 p.m. – 7:30 p.m. Join online at: PublicInput.com/Tunnels Or by phone* at 855-925-2801 with Meeting Code: 9622
*If you attend by phone only, maps and other exhibits will not be visible. However, information will be available after the meeting on the project webpage at hcfcd.org/tunnels.
The meeting will begin with a brief presentation to share project updates. A moderated Q&A session will follow. You can submit questions, comments and input before, during and after the meeting. Any comments not addressed during the Q&A session will receive a response after the conclusion of the public comment period.
Even if you can’t attend the live meeting, still register to receive project updates. Video of the meeting will be available on the Flood Control District’s website and YouTube channel after the event.
Accommodations can be made for those with disabilities. If needed, please contact 346-286-4040 at least three business days prior to the meeting. For questions, please contact the Flood Control District at 346-286-4000, or fill out the comment form online at hcfcd.org/tunnels.
Phase 1 took a high-level look into the feasibility of constructing large-diameter deep tunnels to help move stormwater out of Harris County. It considered soil types, geotechnical challenges, hydraulic capacity, impacts, scheduling, and cost projections. Phase 1 was not watershed specific. Nor did it focus on any particular alignment/location.
Phase 1 findings include:
Geotechnical conditions do not appear to present any remarkable, nor non-negotiable concerns.
Geologic faults may require special design and construction considerations if crossed by the tunnel; not considered fatal flaws.
Tunnels can move a significant rate of stormwater operating by gravity as an inverted siphon.
Tunnel cost, including a 50 percent contingency, for a representative 10-mile long, 25- and 40-foot diameter tunnel is approximately $1 billion and $1.5 billion respectively.
For the complete 1700-page, 300-megabyte final report, click here.
Phase 3 will include preliminary design. The purpose:
Prove project benefits and costs
Select locations
Investigate geologic faults
Validate assumptions made during Phase 1 and 2
Identify internal and external sources of funding.
https://i0.wp.com/reduceflooding.com/wp-content/uploads/2022/06/20220612-Screen-Shot-2022-06-12-at-2.18.38-PM.jpg?fit=1200%2C900&ssl=19001200adminadmin2022-06-12 14:52:002022-06-12 14:58:41Virtual Meeting on Flood Tunnels Thursday Night
The damages phase of the Upstream Addicks-Barker class-action lawsuit over Hurricane Harvey flooding concluded Friday, 6/11/2022. Earlier, Judge Charles F. Lettow ruled that the Army Corps was liable for damages. The question being decided now is “How much will they get?” We don’t yet have that answer, but should before the end of the year.
Flooded homes inside Addicks Reservoir during Harvey but still outside even today’s 100-year floodplain.
Basis for Claims
After Hurricane Harvey, people and businesses both upstream and downstream of the Addicks and Barker Reservoirs on Houston’s west side sued the Army Corps. Plaintiffs in both cases alleged that the Army Corps’ operation of the dams flooded their homes and constituted a taking of their property without compensation. The Fifth Amendment of the U.S. Constitution prohibits that.
“No person shall be … deprived of … property, without due process of law; nor shall private property be taken for public use, without just compensation.”
However, the Upstream and Downstream cases also have important differences. Upstream, the Corps did not own all the land inside the U-shaped reservoirs. Worse, the Corps permitted developers to build homes and businesses inside the reservoirs on land that remained in private hands. The Corps did not anticipate it all flooding based on storms they had studied going back to the 1890s. Yet the Corps still built the walls taller and longer than it needed to hold anticipated floods.
When Harvey came along, the water in the reservoir backed up onto that private property and flooded hundreds of homes.
Lawyers for the flooded property owners asserted that the federal government cannot use private property to store federal floodwaters without providing compensation. The judge agreed.
Although the trial portion of the damage phase just concluded, the case is not yet over. McGhee, Chang, Landgraf & Feiler, one of the law firms representing plaintiffs in the class-action suit, said they must still submit post-trial legal briefings. Then they will make final closing arguments in Washington D.C. in a few months. “We expect a decision to be rendered by the Court thereafter – probably sometime in late fall/winter,” said a press release by the firm.
Exponential Growth, Larger Storms, But No Mitigation
After reading the 46-page decision, I gained a better grasp of the history of the dams and the nature of the claims.
The Corps built the dams much higher than they needed to hold a 100-year flood based on what they knew at the time.
But the Corps did not purchase all land inside the reservoirs. They left private property outside the area expected to flood. At the time the dams were constructed, that land was used for ranching and rice farming.
If the land flooded, reasoned the Corps, not much damage would result. But then came Houston’s exponential growth in the 1950s. Those ranchers and rice farmers sold their land to developers. And developers started to build inside the reservoir.
Then the Corps realized that the storms on which it based the reservoirs’ designs (including a storm from the 1890s) were smaller than storms hitting the Houston area in the modern era. But by then, it was too late.
When the Corps realized future floods would likely invade homes, it launched an awareness program and held some public meetings. But the judge felt that information didn’t filter down to most homebuyers.
Also, the Corps took no concrete steps to reduce flood risk when it realized the severity of the problem. Worse, the Corps continued to issue permits and authorizations for more developments.
To sum up 46 pages in a sentence, “The Corps knew it had a problem and did nothing to fix it.” (That’s my takeaway, not the judge’s language.) The Corps remained focused on its primary objective – preventing downstream flooding.
Downstream Focus Looms Large in Upstream Decision
Said Judge Lettow, “Equipped with the knowledge that storms of the design-storm magnitude were probable, the Corps did not stray from its primary objective to prevent downstream flooding (indeed, it probably could not), even when it knew that could well mean impounding water on private property.”
Lettow cites a 2012 Water Control Manual which the Corps followed during Harvey. It instructs the Corps to operate the dams in a manner consistent with their original purpose: to protect downstream property by impounding water in upstream reservoirs. It states “…operate the reservoirs in a manner that will utilize to the maximum extent possible [Emphasis added] the available storage to prevent the occurrence of damaging stages on Buffalo Bayou.”
Knew Larger Floods Probable
According to the judge, the Corps continued to follow that policy even though it understood that rainfall events – larger than ones they designed the dams around – were “probable, rather than merely possible.”
Lettow also found it “undisputed that plaintiffs did not know their properties were located within the reservoirs and subject to attendant government-induced flooding.”
Government Planned for Years to Impound Floodwater on Private Property
Said one hydrologist who reviewed a detailed history of the Corps’ decision making, “The Corps of Engineers did NOT buy the entire area they knew would be inundated if Addicks and Barker reservoirs were at peak storage capacity.”
Judge Lettow said, “The government had made a calculated decision to allow for flooding these lands years before Harvey, when it designed, modified, and maintained the dams in such a way that would flood private properties during severe storms. Defendant cannot now claim that this harm was unavoidable when it planned for years to impound floodwaters onto plaintiffs’ properties.”
The Corps made the best decisions it could with the information AVAILABLE at the time. But as we all know, things change! And that’s what worries me most about this case.
Right now, developers are building projects all around the region based on flood maps that will soon be replaced.
The thoughts expressed in this post represent opinions on matters of public concern and safety. They are protected by the First Amendment of the US Constitution and the Anti-SLAPP Statute of the Great State of Texas.
https://i0.wp.com/reduceflooding.com/wp-content/uploads/2022/06/Flooded-Homes-Addicks-Reservoir.jpg?fit=1200%2C765&ssl=17651200adminadmin2022-06-11 16:03:182022-06-11 21:44:24Upstream Addicks-Barker Trial Concludes, But No Ruling Yet on Damages
The factors that create hurricane strength may not be the same factors that create intense tropical rainfall. According to NOAA, warm sea surface temperatures can increase storm intensity. Meanwhile, the absence of steering currents and wind sheer can cause even weak storms to stall over an area and dump huge amounts of rainfall.
Two things happened this week to bring these factors into focus.
First, sea surface temperatures in June have already reached those not usually observed until late July or August in Galveston.
Second, this week marks the anniversary of Tropical Storm Allison, which set rainfall records for its era and caused all the flood maps to be redrawn (until Harvey). That prompted more research into meteorological factors that affect hurricanes, their formation, and their destructiveness.
Record Heat Tied to Higher than Usual Sea Surface Temperatures
Jeff Lindner, Harris County’s meteorologist, released a report this morning that said, recently all along the Texas coast, the nighttime lows have reached near record highs. Galveston, for instance, has failed to fall below 83 degrees for the last 72 hours and the low yesterday was only 84 degrees which is 1 degree shy of the all-time high “record low” of 85 from last summer.
These extremely high “low temps,” says Lindner, are more typical of August than June and directly tied to the nearshore water temperature which is already 83-86 degrees along the Texas coast.
28-30 degrees Celsius translates to 83-86 degrees Fahrenheit.Source: NOAA.
That raised two questions for me:
Are sea surface temperatures warmer than normal?
If so, how does that affect hurricane formation?
Sea Surface Temperatures Much Higher than Normal
I first researched sea surface temperature anomalies. You can see from the map below that the entire tropical Atlantic, Caribbean and Gulf of Mexico show higher-than-normal temperatures. How much higher?
Anomalies are departures from normal. This map shows anomalies for today. Source: NOAA.
Most of the upper Gulf Coast is 1-2 degrees Celsius above normal. That translates to about 2-4 degrees Fahrenheit.
In order for a hurricane to form, two things must be present: a weather disturbance, such as a thunderstorm, that pulls in warm surface air from all directions and water at the ocean’s surface that is at least 80° Fahrenheit (27° Celsius).
Because warm air and warm seawater spawn these storms, they form over tropical oceans where seawater is hot enough to give the storms strength and the rotation of the Earth makes them spin.
Hurricanes start simply with the evaporation of warm seawater, which pumps water into the lower atmosphere.
Converging winds then collide and turn upwards, where water vapor starts to condense. That releases heat that warms the surrounding air, causing it to rise as well. That causes even more warm, moist air to spiral in to replace it.
As long as the base of this weather system remains over warm water and its top is not sheared apart by high-altitude winds, it will strengthen and grow. More and more heat and water will pump into the air. The pressure at its core will drop further and further, sucking in wind at ever-increasing speeds.
Eventually, hurricanes turn toward mid-latitudes, i.e., Texas. When they move over cold water or land, they lose touch with the hot water that powers them. The hurricane then weakens and breaks apart.
Recent studies have shown a link between ocean surface temperatures and tropical storm intensity – warmer waters fuel more energetic storms.
NOAA
Other Factors Correlate with Higher Rainfall
Energy and intensity, however, do not correlate directly with rainfall. Other factors play larger roles in creating monster rainfall rates.
A slow moving storm that meanders or stalls can dump more rain than fast moving storms that blow through areas quickly. Tropical Storm Allison makes an excellent example.
This week is the anniversary of Tropical Storm Allison (June 5-10, 2001). NOAA has a special web page that tells the story of Allison and its destructive rains. Before Harvey, Allison set records for much, but not all, of the Houston Region. Greens Bayou at Mount Houston Parkway, for instance, received 38.78 inches of rain.
Allison lingered around the Houston area for days, went up to Lufkin, and then backtracked over already saturated ground before moving east.
The absence of strong steering currents allowed Allison to stall and dump huge rainfall amounts on Houston.
“The devastating flooding from Allison is a stark reminder that rainfall from tropical cyclones does not depend upon the strength of the system.”
NOAA
The Hydrometeorological Prediction Center found six factors that impact the rainfall potential of landfalling tropical cyclones:
Storm track (or movement)
Time of day
Storm size
Topography
Wind shear
Nearby weather features
Between June 5th and the 9th, the two major factors leading to heavy rainfall over Southeast Texas turned out to be Allison’s slow movement and the time of day. These were aided by an abundance of available Gulf moisture.
Tropical Storm Allison 5-day rainfall totals in 2001 related primarily to the storms track and slowness, caused by the absence of steering currents and wind sheer.
Time of day deserves more explanation. On Day 4 of Allison, the sun cleared over much of Houston. That increased daytime heating. And the heat caused feeder bands to intensify over areas that previously flooded. No one died during the first three days of the storm. But 22 died during the last two as rainfall from those bands reformed over areas already badly flooded.
Give Your Kids a Science Assignment for the Summer
Weather is one of nature’s biggest puzzles. I find it endlessly fascinating. If your kids are bored already by the summer’s heat, give them a science assignment. Have them research NOAA’s website to learn more about hurricanes and the heat. Hint: ask them how that bright red area in the northern Pacific (in the SST anomalies map). Then ask them how that’s related to drought, trade winds, wind-sheer, and predictions for an above-average hurricane season.
Posted by Bob Rehak on 6/9/22 based on information from NOAA.
Virtual Meeting on Flood Tunnels Thursday Night
The Harris County Flood Control District (HCFCD) will hold a community engagement meeting to share Phase 2 results for the Feasibility Study of Stormwater Conveyance Tunnels. The meeting’s purpose: to inform residents about the status of the Tunnels study and share study information.
At the highest level, the study looks at the potential to reduce flooding risks in Harris County via large-diameter, deep underground tunnels that convey stormwater.
The study includes three phases:
Tunnel Tradeoffs
The primary benefit of tunnels: they add stormwater conveyance without disturbing development on the surface. In highly developed or environmentally sensitive areas, this is important. But tunnels also come with technical and financial challenges. For instance, you must route them around oil wells, water wells, and geologic faults. And the cost can be considerable: up to $150 million per mile for a 40-foot-wide tunnel.
More about Phase 2
In Spring 2022, HCFCD completed Phase 2 of its feasibility investigation. The purpose of Phase 2 was to identify unmet flood mitigation needs in Harris County’s watersheds. Phase 2 also developed distinct tunnel concepts to meet those needs.
This phase of the study focused on identifying:
Phase 2 found that a tunnel SYSTEM, rather than one or more individual tunnel alignments, should be the focus of further study. Thus, we would need additional study before a final decision on whether to move forward with tunnels.
Phase 2 received funding from the U.S. Department of Housing and Urban Development’s Community Development Block Grant Disaster Recovery program.
Meeting Details: How to Register
Community engagement is an important component of this study. So, HCFCD invites your participation.
The Virtual Community Engagement Meeting will be held on:
Thursday, June 16, 2022
6:30 p.m. – 7:30 p.m.
Join online at: PublicInput.com/Tunnels
Or by phone* at 855-925-2801 with Meeting Code: 9622
*If you attend by phone only, maps and other exhibits will not be visible. However, information will be available after the meeting on the project webpage at hcfcd.org/tunnels.
The meeting will begin with a brief presentation to share project updates. A moderated Q&A session will follow. You can submit questions, comments and input before, during and after the meeting. Any comments not addressed during the Q&A session will receive a response after the conclusion of the public comment period.
Even if you can’t attend the live meeting, still register to receive project updates. Video of the meeting will be available on the Flood Control District’s website and YouTube channel after the event.
Accommodations can be made for those with disabilities. If needed, please contact 346-286-4040 at least three business days prior to the meeting. For questions, please contact the Flood Control District at 346-286-4000, or fill out the comment form online at hcfcd.org/tunnels.
Overview of Other Phases
For a brief history of the tunnel investigation, visit this page on the HCFCD website.
Phase 1 took a high-level look into the feasibility of constructing large-diameter deep tunnels to help move stormwater out of Harris County. It considered soil types, geotechnical challenges, hydraulic capacity, impacts, scheduling, and cost projections. Phase 1 was not watershed specific. Nor did it focus on any particular alignment/location.
Phase 1 findings include:
For the complete 1700-page, 300-megabyte final report, click here.
Phase 3 will include preliminary design. The purpose:
This post will give you more background about flood tunnels.
Posted by Bob Rehak on 6/12/2022
1748 Days since Hurricane Harvey
Upstream Addicks-Barker Trial Concludes, But No Ruling Yet on Damages
The damages phase of the Upstream Addicks-Barker class-action lawsuit over Hurricane Harvey flooding concluded Friday, 6/11/2022. Earlier, Judge Charles F. Lettow ruled that the Army Corps was liable for damages. The question being decided now is “How much will they get?” We don’t yet have that answer, but should before the end of the year.
Basis for Claims
After Hurricane Harvey, people and businesses both upstream and downstream of the Addicks and Barker Reservoirs on Houston’s west side sued the Army Corps. Plaintiffs in both cases alleged that the Army Corps’ operation of the dams flooded their homes and constituted a taking of their property without compensation. The Fifth Amendment of the U.S. Constitution prohibits that.
Difference Between Upstream, Downstream Cases
However, the Upstream and Downstream cases also have important differences. Upstream, the Corps did not own all the land inside the U-shaped reservoirs. Worse, the Corps permitted developers to build homes and businesses inside the reservoirs on land that remained in private hands. The Corps did not anticipate it all flooding based on storms they had studied going back to the 1890s. Yet the Corps still built the walls taller and longer than it needed to hold anticipated floods.
When Harvey came along, the water in the reservoir backed up onto that private property and flooded hundreds of homes.
Lawyers for the flooded property owners asserted that the federal government cannot use private property to store federal floodwaters without providing compensation. The judge agreed.
Second of Two Phases Nearing Completion
In the first phase of the upstream case, the court found the Corps liable. In the second phase, the court considered damages, i.e., how much compensation property owners should receive.
Although the trial portion of the damage phase just concluded, the case is not yet over. McGhee, Chang, Landgraf & Feiler, one of the law firms representing plaintiffs in the class-action suit, said they must still submit post-trial legal briefings. Then they will make final closing arguments in Washington D.C. in a few months. “We expect a decision to be rendered by the Court thereafter – probably sometime in late fall/winter,” said a press release by the firm.
Exponential Growth, Larger Storms, But No Mitigation
After reading the 46-page decision, I gained a better grasp of the history of the dams and the nature of the claims.
The Corps built the dams much higher than they needed to hold a 100-year flood based on what they knew at the time.
But the Corps did not purchase all land inside the reservoirs. They left private property outside the area expected to flood. At the time the dams were constructed, that land was used for ranching and rice farming.
If the land flooded, reasoned the Corps, not much damage would result. But then came Houston’s exponential growth in the 1950s. Those ranchers and rice farmers sold their land to developers. And developers started to build inside the reservoir.
Then the Corps realized that the storms on which it based the reservoirs’ designs (including a storm from the 1890s) were smaller than storms hitting the Houston area in the modern era. But by then, it was too late.
When the Corps realized future floods would likely invade homes, it launched an awareness program and held some public meetings. But the judge felt that information didn’t filter down to most homebuyers.
Also, the Corps took no concrete steps to reduce flood risk when it realized the severity of the problem. Worse, the Corps continued to issue permits and authorizations for more developments.
To sum up 46 pages in a sentence, “The Corps knew it had a problem and did nothing to fix it.” (That’s my takeaway, not the judge’s language.) The Corps remained focused on its primary objective – preventing downstream flooding.
Downstream Focus Looms Large in Upstream Decision
Said Judge Lettow, “Equipped with the knowledge that storms of the design-storm magnitude were probable, the Corps did not stray from its primary objective to prevent downstream flooding (indeed, it probably could not), even when it knew that could well mean impounding water on private property.”
Lettow cites a 2012 Water Control Manual which the Corps followed during Harvey. It instructs the Corps to operate the dams in a manner consistent with their original purpose: to protect downstream property by impounding water in upstream reservoirs. It states “…operate the reservoirs in a manner that will utilize to the maximum extent possible [Emphasis added] the available storage to prevent the occurrence of damaging stages on Buffalo Bayou.”
Knew Larger Floods Probable
According to the judge, the Corps continued to follow that policy even though it understood that rainfall events – larger than ones they designed the dams around – were “probable, rather than merely possible.”
Lettow also found it “undisputed that plaintiffs did not know their properties were located within the reservoirs and subject to attendant government-induced flooding.”
Government Planned for Years to Impound Floodwater on Private Property
Said one hydrologist who reviewed a detailed history of the Corps’ decision making, “The Corps of Engineers did NOT buy the entire area they knew would be inundated if Addicks and Barker reservoirs were at peak storage capacity.”
Judge Lettow said, “The government had made a calculated decision to allow for flooding these lands years before Harvey, when it designed, modified, and maintained the dams in such a way that would flood private properties during severe storms. Defendant cannot now claim that this harm was unavoidable when it planned for years to impound floodwaters onto plaintiffs’ properties.”
The Corps made the best decisions it could with the information AVAILABLE at the time. But as we all know, things change! And that’s what worries me most about this case.
Right now, developers are building projects all around the region based on flood maps that will soon be replaced.
To read the original complaint by one of the law firms (Irvine $ Conner), click here.
For the complete text of the liability ruling, click here.
Posted by Bob Rehak on 6/11/2022
1747 Days since Hurricane Harvey
The thoughts expressed in this post represent opinions on matters of public concern and safety. They are protected by the First Amendment of the US Constitution and the Anti-SLAPP Statute of the Great State of Texas.
Different Factors Affect Hurricane Strength, Rainfall
The factors that create hurricane strength may not be the same factors that create intense tropical rainfall. According to NOAA, warm sea surface temperatures can increase storm intensity. Meanwhile, the absence of steering currents and wind sheer can cause even weak storms to stall over an area and dump huge amounts of rainfall.
Two things happened this week to bring these factors into focus.
First, sea surface temperatures in June have already reached those not usually observed until late July or August in Galveston.
Second, this week marks the anniversary of Tropical Storm Allison, which set rainfall records for its era and caused all the flood maps to be redrawn (until Harvey). That prompted more research into meteorological factors that affect hurricanes, their formation, and their destructiveness.
Record Heat Tied to Higher than Usual Sea Surface Temperatures
Jeff Lindner, Harris County’s meteorologist, released a report this morning that said, recently all along the Texas coast, the nighttime lows have reached near record highs. Galveston, for instance, has failed to fall below 83 degrees for the last 72 hours and the low yesterday was only 84 degrees which is 1 degree shy of the all-time high “record low” of 85 from last summer.
These extremely high “low temps,” says Lindner, are more typical of August than June and directly tied to the nearshore water temperature which is already 83-86 degrees along the Texas coast.
That raised two questions for me:
Sea Surface Temperatures Much Higher than Normal
I first researched sea surface temperature anomalies. You can see from the map below that the entire tropical Atlantic, Caribbean and Gulf of Mexico show higher-than-normal temperatures. How much higher?
Most of the upper Gulf Coast is 1-2 degrees Celsius above normal. That translates to about 2-4 degrees Fahrenheit.
Eighty-six degrees Fahrenheit is the normal average for August in Galveston. And we’re already experiencing that in June!
Relationship Between Sea Surface Temps and Hurricanes
So how will that affect hurricanes? The short answer: it will likely make them more intense, according to NOAA. Here’s how.
In order for a hurricane to form, two things must be present: a weather disturbance, such as a thunderstorm, that pulls in warm surface air from all directions and water at the ocean’s surface that is at least 80° Fahrenheit (27° Celsius).
Because warm air and warm seawater spawn these storms, they form over tropical oceans where seawater is hot enough to give the storms strength and the rotation of the Earth makes them spin.
Converging winds then collide and turn upwards, where water vapor starts to condense. That releases heat that warms the surrounding air, causing it to rise as well. That causes even more warm, moist air to spiral in to replace it.
As long as the base of this weather system remains over warm water and its top is not sheared apart by high-altitude winds, it will strengthen and grow. More and more heat and water will pump into the air. The pressure at its core will drop further and further, sucking in wind at ever-increasing speeds.
Eventually, hurricanes turn toward mid-latitudes, i.e., Texas. When they move over cold water or land, they lose touch with the hot water that powers them. The hurricane then weakens and breaks apart.
Other Factors Correlate with Higher Rainfall
Energy and intensity, however, do not correlate directly with rainfall. Other factors play larger roles in creating monster rainfall rates.
A slow moving storm that meanders or stalls can dump more rain than fast moving storms that blow through areas quickly. Tropical Storm Allison makes an excellent example.
This week is the anniversary of Tropical Storm Allison (June 5-10, 2001). NOAA has a special web page that tells the story of Allison and its destructive rains. Before Harvey, Allison set records for much, but not all, of the Houston Region. Greens Bayou at Mount Houston Parkway, for instance, received 38.78 inches of rain.
The absence of strong steering currents allowed Allison to stall and dump huge rainfall amounts on Houston.
The Hydrometeorological Prediction Center found six factors that impact the rainfall potential of landfalling tropical cyclones:
Between June 5th and the 9th, the two major factors leading to heavy rainfall over Southeast Texas turned out to be Allison’s slow movement and the time of day. These were aided by an abundance of available Gulf moisture.
Time of day deserves more explanation. On Day 4 of Allison, the sun cleared over much of Houston. That increased daytime heating. And the heat caused feeder bands to intensify over areas that previously flooded. No one died during the first three days of the storm. But 22 died during the last two as rainfall from those bands reformed over areas already badly flooded.
Give Your Kids a Science Assignment for the Summer
Weather is one of nature’s biggest puzzles. I find it endlessly fascinating. If your kids are bored already by the summer’s heat, give them a science assignment. Have them research NOAA’s website to learn more about hurricanes and the heat. Hint: ask them how that bright red area in the northern Pacific (in the SST anomalies map). Then ask them how that’s related to drought, trade winds, wind-sheer, and predictions for an above-average hurricane season.
Posted by Bob Rehak on 6/9/22 based on information from NOAA.
1745 Days since Hurricane Harvey