Tag Archive for: rainfall

Relationship Between Wildfires and Floods

10/24/24 – It may seem counterintuitive, but there is a well-recognized relationship between wildfires and floods. What is it?

Simple. Wildfires create conditions that increase the risk of future flooding. Specifically, they affect infiltration; runoff volume and speed; erosion; sedimentation; and more.

How Wildfires Increase Future Flood Risk

Here’s an outline of the the relationship between wildfires and floods:

  1. Loss of Vegetation: Forest fires burn vegetation, which normally helps to hold soil in place and absorb rainfall. Fires leave soil exposed and unprotected.
  2. Water-Repellent Soil: Intense fires can cause soil to repel water. This increases the amount of water running off the surface during rainstorms, rather than soaking into the ground.
  3. Increased Runoff: Without vegetation, rainwater flows more quickly and in greater volumes over the landscape. This runoff can lead to flash flooding, particularly after heavy rains in areas recently affected by wildfires.
  4. Erosion and Sediment: Post-fire floods tend to carry a lot of sediment and debris since the soil is no longer stabilized by plant roots. This sediment can settle downstream, reduce the carrying capacity of streams and worsen flooding.

In short, forest fires can create conditions that increase the likelihood of flooding and make flood events more severe in the affected areas. This relationship is particularly common in the period immediately following a wildfire when the landscape is most vulnerable to erosion and rapid runoff.

What the Experts Says

According to FEMA, “While some floods develop over time, flash floods—particularly common after wildfires—can occur within minutes after the onset of a rainstorm. Even areas that are not traditionally flood-prone are at risk, due to changes to the landscape caused by fire. … Flood risk remains significantly higher until vegetation is restored – up to 5 years after a wildfire.”

The National Weather Service says, “Locations that are downhill and downstream from burned areas are highly susceptible to Flash Flooding and Debris Flows.”

The Army Corps of Engineers also warns of the relationship between vegetation loss and flash flooding. They say the risk of debris flows lasts two to five years.

The Corps, FEMA and Weather Service all recommend purchasing flood insurance within their discussions of wildfires.

Current Conditions in Southeast Texas: Growing Fire Risk

So how bad are current conditions?

I already have received an email from one reader who was seriously injured fighting a pasture fire in Liberty County.

No weather monitoring site in southeast Texas has recorded ANY rainfall for the month of October. “That’s impressive because October is usually our second wettest month behind May,” says Jeff Lindner, Harris County’s meteorologist.

October 2024 Rainfall and Departures from Normal

You can see the lack of recent rain in the statistics below:

  • College Station: 0.00, -3.49
  • BUSH IAH: 0.00, -3.82
  • Hobby: 0.00, -4.12
  • Galveston: 0.00, -3.66
  • Conroe: 0.00, -3.67
  • Sugarland: 0.00, -3.29
Drought Worsening

Moderate drought now covers more than half of Texas and severe drought covers more than a quarter of the state. All of Harris and surrounding counties are in moderate or severe drought, except for those along the coast.

Says Lindner, “While the recent dryness has been relatively short thus far compared to other dry spells, it has been intense – accompanied by much warmer than normal late summer and early fall temperatures. This has quickly resulted in drought degradation across the entire region and vegetation health quickly declining.”

Fire Activity Increasing

“The result has also been an increase in wildland fire activity in recent weeks – especially when drier air masses move over the region along with gusty winds,” Lindner continued.

“As the area moves deeper into the fall and winter months, more frequent and stronger cold fronts will bring stronger winds and much drier air masses across the area. Without any significant wetting rainfall, fire-weather conditions will quickly elevate,” he said.

Wildfire Risk Soaring

The Keetch-Byram Drought Index (KDBI) values indicate wildland fire risk. Currently, they show significant risk across the area. The scale is from 0 (completely saturated) to 800 (completely void of moisture to 8 inches deep). Values above 650 indicate an increased risk of wildland fire and values over 700 indicate a significant risk. 

Current average KBDI values are: 

  • Austin: 735
  • Brazoria: 630
  • Brazos: 744
  • Chambers: 693
  • Colorado: 705
  • Fort Bend: 706
  • Galveston: 620
  • Grimes: 730
  • Harris: 727
  • Liberty: 739
  • Matagorda: 615
  • Montgomery: 715
  • San Jacinto: 717
  • Walker: 691
  • Washington: 738
  • Wharton: 689

Looking Ahead: Higher than Average Temps/Lower Than Normal Rainfall

We are currently heading into La Niña conditions. “Such global oscillations tend to support southern plains drought,” says Lindner.

The National Weather Service indicates drought development and persistence across all of Texas into early 2025. See below:

Source: National Weather Service Climate Prediction Center
Be Careful with Outdoor Burning

Beryl left a lot of dead vegetative debris lying around last summer. It’s now turning into fuel. So be extremely careful with outdoor fires. It’s probably best not to burn those leaves you rake up this fall.

The fire you prevent now could help prevent flooding next spring.

Posted by Bob Rehak on 10/24/24

2613 Days since Hurricane Harvey

24% Ahead of Rainfall for Year, Houston Slips into Drought

10/18/24 – According to the US Drought Monitor, Houston has officially slipped into a moderate drought.

Talk about wacky weather. According to the National Weather Service, the Houston area has already recorded its average annual rainfall – with two and a half months left in the year.

Rainfall Year-to-Date for Houston Bush Intercontinental Airport

NOAA shows that normally we receive 41.52 inches by October 18 in an average year. But so far this year, at IAH, the official recording station, we have received 51.2 inches.

From NWS NowData. The big jump in the blue line reflects Hurricane Harvey in 2017.

So, we are 24% ahead of the average annual rainfall for this date … and in drought. But it gets even more wacky.

I live in the center of Kingwood just a few miles northeast of the airport. And my rain gage has recorded 64.58 inches so far. That’s 54% ahead of the average YTD rainfall.

Looking Ahead

The NWS Climate Prediction Center says that a weak La Niña has a 60% chance to emerge by November and is expected to persist through January-March 2025.

From NWS Climate Prediction Center

What does that mean for the Houston area?

Effect of La Niña on Texas Weather in Fall and Winter

According to ChatGPT, La Niña has significant impacts on Texas weather, especially during the fall and winter seasons. Here’s how La Niña typically affects Texas:

1. Warmer-than-Average Temperatures

  • Fall and Winter in Texas during a La Niña event tend to be warmer than usual, especially in southern and central Texas. The jet stream tends to shift northward, allowing more warm air to persist over the state. This can lead to milder winters, especially compared to neutral or El Niño conditions.
2. Drier-than-Average Conditions
  • La Niña often brings below-average rainfall to Texas, particularly in the southern and central regions. This is due to the altered jet stream pattern, which pushes storm tracks farther north, leaving Texas and much of the southern U.S. drier than normal. As a result, drought conditions can develop or worsen, especially in the winter months.
3. Increased Risk of Drought
  • Because La Niña tends to cause drier-than-average conditions, it can exacerbate drought conditions. This is particularly concerning for Texas, which is prone to periodic droughts. Reduced winter rainfall means less moisture in the soil and reservoirs, affecting agriculture, water supply, and wildfire risks.
4. Wildfire Risk

With drier conditions and warmer-than-average temperatures, the risk of wildfires tends to increase during La Niña winters. This is especially the case in late fall and early winter when vegetation can become dry and more susceptible to fires.

What a wild ride this year has been!!! In the first half of the year, we thought we would drown. Now we could dry up and blow away.

Of course, all averages include extremes. By definition, they mask variation. And this year, they could mask a lot.

Posted by Bob Rehak on 10/18/24

2607 Days since Hurricane Harvey

Top of SJR Basin Has Received Normal YTD Rainfall Plus a Harvey or Two

6/4/24 – Rainfall in the upper part of the San Jacinto River (SJR) Basin during early May storms rivaled Hurricane Harvey totals there. And if you consider year-to-date rainfall (YTD) totals, the amount above normal equals two Harveys in places!

The comparisons with Harvey help put into perspective some extraordinary rainfall in a relatively small geographic area that affects a much larger area.

The comparisons help explain why many people in the Lake Houston Area felt the May flood was far out of proportion to the amount of rainfall received. The most extreme rains did not fall on downstream residents’ heads. For downstream residents, the rains fell unseen – 50 miles north.

Let’s look at data for Huntsville first. Even though the City itself technically sits just outside the San Jacinto River Basin, rainfall south and west of the City entered both the East and West Forks of the San Jacinto.

Harvey Week vs Week Around May Storm

During Hurricane Harvey week in 2017, the SJRA gage southwest of Huntsville received 20.46 inches of rain.

Area near Huntsville received 20.46 inches during Harvey week in 2017.

During the late April/early May storm of 2024, the same gage received just a little less – 18.4 inches.

Huntsville total for week starting April 28, 2024, fell about two inches short of Harvey.

This area got almost as much rain in one week this May as it did during Harvey. Now let’s look at year to date numbers.

Year-to-Date Totals Vs. Harvey

The National Weather Service maintains another gage in Huntsville which accounts for a slightly different total. But I’m using it for the YTD comparison because of the powerful graph. It illustrates how much rain the area recently received compared to normal. (We already know the Harvey total from the first bar graph above.)

How wet has 2024 been? YTD accumulation graph.
From National Weather Service Climate Page.

From the brown line above, we can see that area normally gets 20 inches of rain through the end of May. This year it got 58.97 inches, almost triple the annual average at that point in the year.

The amount above normal (38.97 inches) is almost twice what the Huntsville area received during Harvey (20.46 inches)!

National Weather Service Data

So far this year, the upper river basin has received almost three Harveys worth of rain, or two above the normal YTD rainfall…for that latitude.

I should note here, that inland areas usually receive less rain than coastal areas during hurricanes. So if you’re saying, “Wait a minute! We received more than 20 inches of rain during Harvey,” you’re right. You also probably live south of Huntsville.

How to Compare Totals at Other Gages

Want to see what happened at a gage near you? HCFCD’s Flood Warning System lets you enter any date range using the historical feature. Just click on the “More Info” button associated with any gage. Or do it for the whole river basin to see the distribution of totals.

YTD rainfall distribution across upper SJR basin.

To quickly compare the distribution during Harvey, just change the dates to 8/25/17 and 8/29/17. It’s fun to explore. And it makes a fun learning experience for your kids. Teach them how to become “data detectives.” It could turn them into homeroom heroes.

Alternatively, you can compare Harris County gages during Harvey by consulting the tables at the end of HCFCD’s final Harvey report. It contains peak rainfall totals for all the gages above for time periods ranging from five minutes to four days.

The Reports Page of this website contains similar reports on other storms under the Major Storms tab.

Significance and Recommendations

There’s a point to all this data. We can draw several conclusions and recommendations from it:

  • Extreme rainfall in the northern tier of the river basin contributed to the flooding that many experienced 50 miles south.
  • Rainfall coming from the west and east was less intense and likely accounted for less damage downstream.
  • Heavy rains can fall outside of hurricane season. In fact, spring storms can exceed hurricane rainfall totals and rival hurricane intensity.
  • We need regional flood control. The people near Huntsville were not prepared for this event. Neither were people to the south protected.
  • Hurricane Harvey wasn’t a once-in-a-1,000 year rainfall. Pretending it was will jeopardize public safety. We need better building codes and drainage regulations in areas that haven’t already been updated since Harvey.
  • We also need updated flood maps that show current risk to help protect homebuyers. New floodplain surveys were conducted after Hurricane Harvey. But FEMA hasn’t yet released new risk maps based on those surveys. Many fast-growing areas in the region still base development decisions on data from the 1980s. That puts homebuyers at risk.

Flood Risk: A Shifting Target

Even as FEMA finalizes new flood-risk maps, understand that Mother Nature, sand mines, and insufficiently mitigated upstream developments are constantly changing the landscape through erosion and deposition.

The resulting blockages and reduction in conveyance may now contribute to increased flooding on smaller rains. Consider River Grove Park and the Kingwood Diversion Ditch. Both are now blocked by sand from upstream.

Kingwood Diversion Ditch at River Grove silted in again.
Kingwood Diversion Ditch where it passes through River Grove Park. Photo from 5/26.

It’s not as bad as it was after Harvey…yet. But up to five feet of sand was deposited in this area during the recent flood. So give it a few more floods.

As the City of Houston readies a $34 million dollar dredging program (that doesn’t even include River Grove), look what’s coming downstream to us.

Confluence of West Fork and Spring Creek near US59 bridge. West Fork, on right.

The West Fork snakes through 20 square miles of sand mines between US59 and I-45. And due to pit capture, the West Fork currently runs through a mile-long sand pit.

That raises one final recommendation: I wish the City could use its influence in Washington and Austin to have the EPA and TCEQ eliminate these blatant abuses.

In the meantime, the Kingwood Service Association will debate its own dredging program and the future of River Grove this Thursday night. To attend the online meeting, contact the KSA office at 281-358-5192 for a Zoom link.

Posted by Bob Rehak on June 4, 2024

2471 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.

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.

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.

Hurricanes start simply with the evaporation of warm seawater, which pumps water into the lower atmosphere.

NOAA

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.

Graphic showing rainfall totals for Harris County, Texas for June 5 - 9 2001 during Tropical Storm Allison. The highest recorded rainfall was 38.8 inches. Image courtesy of Tropical Storm Allison Recovery Project.
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.

1745 Days since Hurricane Harvey

How Much Rain Would It Take To Flood Elm Grove Again?

As Tropical Storm Beta bears down on the Houston Area, many people in the Elm Grove/North Kingwood Forest area worry that they might flood again. How likely is that, given the current predictions of 6-10 inches? After all, on May 7 last year, Elm Grove flooded on what was officially a six inch rain according to the nearest gage at West Lake Houston Parkway and the West Fork.

Additional Detention in Woodridge Village Now…

First of all, understand that upstream conditions have changed. On May 7th, only about 11% of the planned detention pond capacity had been constructed. And only 23% was constructed by Imelda. Today, 100% is in place.

…But Detention Based on Pre-Atlas 14 Rainfall Rates

Even though that’s far more than Woodridge Village had during the May or September floods, the detention calculations by LJA Engineering were based on pre-Atlas 14 rainfall rates. A 100-year rainfall then was about 40% less than the official 100-year rainfall now.

So, the questions is, “How much rain would Beta have to dump on Woodridge Village before it overwhelmed the detention ponds that exist today?”

Figures Used by LJA

The chart below shows the rainfalls that the ponds were designed to hold without flooding. The bench mark its the 24-hour, hundred year rain.

These figures come from the hydrology report submitted by LJA to Montgomery County. A table buried on page 32 of the PDF shows that they based their analysis on a pre-Atlas 14, 100-year storm that dropped 12.17 inches of rain in 24 hours.

From Page 2.1 of LJA Hydrology Report Addendum, 8/28/2018 (page 32 of pdf.)

The ponds should also hold any of the shorter-duration rainfalls in the last column above.

Assumptions Underlying the Answer

To answer the question – How much would it take to flood Elm Grove again? – we need to make several assumptions:

With those caveats in mind, it would take 12+ inches of rain in 24 hours to exceed the capacity of the detention ponds currently on Woodridge Village. After that, water would start to overflow.

Short, High-Intensity Downpours Can Cause Different Type of Flooding

However, consider the other durations in the chart above. Seven inches in three hours or nine inches in six hours could also exceed the capacity.

Actually, as you get into these short-duration, high-intensity rainfalls, you introduce the risk of flooding from a second source: overwhelming the capacity of storm drains.

Storm Drains Designed for 2″ Per Hour

The storm drains in Kingwood are designed to convey about two inches of rain per hour. When you exceed that, water begins to back up in the streets. Exceed it enough, and water could actually enter homes – without sheet flow from Woodridge Village.

NHC Rainfall Prediction Spans 5 Days

The six-to-ten inch prediction issued by the National Hurricane Center for Beta spans five days. That’s good news. If ten inches were evenly spread out over five days, the streets, drains and ditches could easily handle two inches per day.

But those short, high intensity rainfalls – when you get two inches in five or ten minutes – represent a real danger. There’s just nowhere for the water to when it comes down that quickly.

Perhaps the Biggest Danger

Even if we got the predicted 6-10 inches all in one day, that’s still, at most, about 80% of the old 100-year rain which the detention was designed for.

I suspect the biggest danger from Beta may be those short, high-intensity cloud bursts or training feeder bands that dump a couple inches in five or ten minutes.

So keep your eye on the rain gage. Sign up for alerts at the Harris County Flood Warning System. Also, keep your eye on the forecasts; uncertainty still exists with Beta, its track and rainfall potential.

Posted by Bob Rehak on 9/19/2020

1117 Days after Hurricane Harvey and 1 year after Imelda

SJRA Peak Flow Map from Imelda Shows 1500X Difference Between East/West Sides of Watershed

Here’s a science lesson for the entire family. The SJRA’s peak streamflow and rainfall map for Imelda demonstrated how rain can fall heavily over one part of a watershed and barely touch another. There are huge implications for flooding.

For a high resolution PDF suitable for printing, click here.

Peak Streamflows West to East Vary by 1500X

Note how the gage at Spring Creek in Tomball recorded a peak flow of 22.7 cubic feet per second. The East Fork gage in New Caney registered 34,600 cubic feet per second. That’s a difference of more than 1500X in the peak flow rates!

Rainfall Totals Range from 0 to 30 Inches in 24 miles

The blue figures represent precipitation. That same gage in Tomball recorded none. But a little further east, they picked up more than 5 inches; almost 10 at I-45; more than 15 at I-69, and almost 30 in New Caney.

This is why you need to look at gages upstream on YOUR tributary when flooding is possible! Someday, textbooks will use this map to dramatize that lesson.

Posted by Bob Rehak on 11/5/2019

798 days since Hurricane Harvey and 47 since Imelda

Study Suggests Large Cities Like Houston Can Intensify Rainfall and Runoff From Hurricanes

A November 2018 article appearing in the peer-reviewed scientific journal Nature found that urban growth can intensify both rainfall and runoff from hurricanes. Further, urban growth can increase the risk of flooding and shift the location of flooding. The article specifically studied the effects of Hurricane Harvey on Houston and found that urban growth increased the probability of such an extreme flood across the basin by 21X.

A sister publication, Scientific American, reviewed the article the same month and helped explain the findings in Nature.

The Nature study looks at two distinct effects of urbanization. The first is the impact of impervious surface on RUNOFF. The second is the impact of the urban landscape’s surface roughness on RAINFALL.

The Runoff Component

Numerous studies have looked at the relationship between percentage of impervious cover, runoff, and flooding – a well documented phenomenon. Impervious cover accelerates transport of rainfall from neighborhoods to rivers. That raises peak flows rather than spreading them out over time. Dr. William Dupre, professor emeritus from the University of Houston visualized the relationship this way.

Effect of Urbanization on Peak Stream Flows” by Dr. William Dupre, professor emeritus from the University of Houston.

Rainfall Component Much Less Studied

However, the effect of urban growth and a city’s surface topography on RAINFALL from hurricanes is much less studied. The authors say in Nature that, “Urbanization led to an amplification of the total rainfall along with a shift in the location of the maximum rainfall.” (Page 386).

“Much less is known regarding the urban effects on the organized tropical rainfall of a hurricane, in particular during one like hurricane Harvey, which stalled for several days.” They continue, “…experiments (with computer models) clearly show a large increase in rainfall arising from urbanization over the eastern part of the Houston area.”

The authors compared present and past urban landscapes and also modeled a scenario in which the entire region was cropland.

Mechanisms Responsible for Increase Rainfall

To understand the physical mechanisms responsible for the heavier rainfall, they analyzed the vertical convergence of winds and wind fields.

Kingwood Greens Evacuation During Harvey by Jay Muscat
Evacuation During Harvey. Photo courtesy of Jay Muscat.

“The enhanced rainfall … and the shift of rainfall … are tied to the storm system’s drag induced by large surface roughness,” say the authors.

Scientific American explains in more detail. Kerry Emanuel, an atmospheric scientist at Massachusetts Institute of Technology, who did not work on the study said, “We know cyclones are sensitive to characteristics of the surface—mountains, streams, marshland. This new twist is that cities have become big enough to tangibly alter the storm.” Said Gabriele Villarini, an environmental engineer at The University of Iowa and an author on the study, “We removed the urban areas from Houston and replaced them with cropland.”

“The presence of urban areas enhanced all the things you need to get heavy precipitation,” Villarini, one of the study’s authors says. “A stronger drag on the storm winds, associated with a larger surface roughness length” contributed to the increased rainfall.

Emanuel explained, “First, the artificial ruggedness of an urban area slows air down. Whenever air slows in a hurricane, he says, it gets shunted toward the center of the storm and up into the sky. That increases rainfall everywhere [in a metropolitan area].” He added, “A storm moves particularly slowly over downtown areas where buildings are tallest, but the winds bearing down from outside the city are still moving quickly. So, [the storm] is piling up on the city.”

Impact on and Implications for Houston

This increase in urban growth in flat terrain creates problems from a flood perspective, despite mitigation measures already in place.

Urbanization has increased the probability of an event like the flooding associated with Hurricane Harvey by about 21 times, say the authors in Nature on page 388.

The authors make several high-level recommendations.

  • Urban planning must take into account the compounded nature of the risk now recognized.
  • Flood mitigation strategies must recognize the effect of urbanization on hurricanes.
  • Weather and climate models must incorporate the effects of urbanization to increase forecast accuracy on local and regional levels.

“It is critical for the next generations of global climate models to be able to resolve the urban areas and their associated processes,” conclude the authors.

About the Authors and Models

The authors are:

  • Wei Zhang and Gabriele Villarini from the Department of Hydroscience & Engineering, The University of Iowa, Iowa City, IA
  • Gabriel A. Vecchi from the Department of Geosciences, Princeton University and the Princeton Environmental Institute, of Princeton, NJ
  • James A. Smith from the Department of Civil and Environmental Engineering, Princeton University, Princeton, NJ.

This presentation explains the Noah Model that the authors used to calculate air/ground interactions.

Local Questions Raised by Study

To date, the role of a city in altering rainfall during tropical cyclones has received very little attention. Houston has had the largest urban growth and the fifth-largest population growth in the United States in the period from 2001–2011. Much of that growth is now on the periphery of the city. The two fastest growing parts of the region are Fort Bend and Montgomery Counties.

As the city grows, we need mitigation measures that can offset the impact of that growth. That’s why the meeting of the Montgomery County Commissions on August 27th is so important. They will vote on whether to close a loophole that allows developers to avoid building onsite detention ponds. Closing that loophole is important. It will help protect hundreds of thousands of downstream residents as well as those in Montgomery County.

Also, the new NOAA Atlas-14 (rainfall measurements updated after Harvey) does not consider forward-looking urban growth effects. The precipitation frequency data in NOAA Atlas 14 was determined by a statistical analysis of historical rainfall, a key input for FEMA Flood Insurance Rate Map (FIRM) modeling. With all that uncertainty, we need to err on the side of caution in flood planning.

For more about Atlas 14, see this link.

Posted by Bob Rehak on 8/18/2019

618 Days after Hurricane Harvey

New 100-year 24-Hour Rainfall Map and Data Released by NOAA Today

New data shows the 100-year rainfall for this area has increased 4-5 inches since the NOAA study in 1961 or 2-3 inches since the USGS study in 2004. This is why flood mitigation and reducing sedimentation are so important. Basically, what we used to think of as a 100-year storm is now almost a 25-year storm.

NOAA Atlas 14, Volume 11: The New Go-By for Everything Related to Rainfall

Today, the Hydrometeorological Design Studies Center of the NOAA’s Office of Water Prediction released updated precipitation frequency estimates for Texas.

They are published as  NOAA Atlas 14 Volume 11: Precipitation-Frequency Atlas of the United States, Texas.

The new precipitation frequency estimates supersede the NOAA estimates published in 1961, 1964, and 1977, and the USGS estimates published in 2004. The new NOAA estimates include data from Harvey and all of the huge storms we have had since 1994 including Tropical Storm Allison, the Tax Day Flood and the Memorial Day Flood. Here’s what the 100-year/24-hour rainfall map looks like. Note that the Houston to Beaumont area is in the bulls-eye.

The new 100-year 24-hour Rainfall Intensity Map. Accompanying documentation, describing the data used in this project and project methodology, will be published in December 2018.

For a full scale map like the one above, download this PDF: tx100y24h rainfall intensity pdf.

Zooming in on the Houston area shows that the new 100-year 24-hour rainfall for the Lake Houston area is between 17 and 18 inches depending on your exact location.

To find precise figures for your location, go to the Precipitation Frequency Data Server – PFDS.

The data varies by location, so…

First, select your location in the map, then click on the gage nearest you.

Next, review the rainfall table associated with that gage. Clicking on the other tabs or “print” brings up additional information.

Then review the new data for different time periods and recurrence intervals. This may be the information you want to keep handy for ready reference.

Comparison to Previous Studies

From this data, we can see that – for the gage at the San Jacinto and US59 – the new, official 100-year rainfall is 17.3 inches in a 24-hour period.

Compare a previous dataset published. Look on page 58 for the 100-year/24 hour data from 1961. Twelve inches in 24-hours represented the old 100-year rainfall for our area for decades.

USGS also published a precipitation frequency study in 2004.  See the USGS Rainfall Maxima Guide for Texas (Warning: 40 meg PDF). I believe it became the basis for the current flood-plain maps redrawn after Tropical Storm Allison that were released in 2007. It shows the 24-hour, 100-year rainfall to be about 13 inches.

How New Data Will Be Used

What does it mean that the 100-year rainfall has increased 4-5 inches?

First and foremost, it means that all of the floodplain maps will be revised. One expert I talked to suspected that the new 100-year floodplain could be close to where the 500-year flood plain is now. However, that is far from certain and not official.

Floodplain Maps

The flood plain maps have not yet been redrawn, as Matt Zeve, Harris County Flood Control Director of Operations, discussed at the September meet of the Lake Houston Area Grass Roots Flood Prevention Initiative. The next step is for the County to process the new rainfall data in a new 2-D model that the Flood Control District has developed with new high-resolution LIDAR data. Contour internals in the new models will shrink from feet to inches. The LIDAR data also reflects new conditions in the watershed (developments, road expansions, siltation in ditches, etc.), so predictions should become much more accurate.

Insurance

Based on the new rainfall data, flood insurance rates could also change.

Construction

Finally, the new data will become crucial in city planning, construction and permitting. The City is already demanding that new construction be raised to two feet above the 500-year flood plain. Perhaps Mayor Turner had a hint of what the new numbers would show when he suggested the new construction standards.

Infrastructure

The larger rainfall totals also mean that cities must use larger storm drains and sewers in new developments. Everything will change.

For more information about the new data, review this quarterly newsletter from NOAA.

Posted on September 27, 2018 by Bob Rehak

395 Days since Hurricane Harvey

Rainfall Rates, Durations and Frequencies for This Area

The upper Texas Coast is famous for intense, frequent rainfall. Sometimes, like during Harvey, rainfall can last for days. So how do you know when you’re experiencing something truly out of the ordinary? Consult the table below. This table relates three factors: rainfall total, rainfall duration, and rainfall classification. From this chart, you can see that all it will take for us to have our fourth five-hundred year storm in four years is about an inch an hour for 18 hours, or about two inches per hour for six hours.

Rainfall Rates, Intensities and Frequencies for The Woodlands Area on the West Fork, near Humble and Kingwood, Texas

What are the odds of getting hit with three 500-year storms in three years (which we did in 2015, 2016 and 2017)? One might think they are 1 in 125 million which was computed by multiplying 1/500 * 1/500 * 1/500.

The odds of getting four 500-year storms in four years would then SEEM astronomical. Using a similar formula, you would arrive at 1 in 62.5 billion!

But that is not necessarily correct because with that calculation you are inferring that the rainfall events are connected. But they actually are not connected. Just because we had a 500-year rainfall event last year, does not mean we may not see another 500- year rainfall event this year.

EVERY year we have a 0.2% chance or 1 in 500 chances of seeing a 500-year flood for a specific location.

This assumes that the odds are no greater in one year than any other year, and that each event is independent of the others.

How do mathematicians compute the probabilities of these rare events? Obviously, it isn’t through observation. The earth is only about 4.5 billion years old. Humans have only walked the earth for about 200,000 years. And reliable rainfall records in this part of the world only go back a little more than a 100 years.

Probabilities for rare events, such as hundred- and five-hundred year storms are based on a branch of statistics called EVA, extreme value analysis. EVA tries to calculate the probabilities of unobserved events by looking at the distribution of observed events.

But all this technical brilliance is based on one particularly flawed assumption that never gets communicated to the public. The assumption is that for the period under examination, nothing changes. Mathematicians even have a word for it: stationarity. It means underlying factors can neither increase, nor decrease.

Duh! Nothing changes in 500 years? In Houston?

Obviously, those folks never rode around for a day in a Ford F350 with a Houston developer.

In 1900, Houston had a population of 44,000 and was the 85th largest city in the U.S.

Today, the Houston region has a population of more than 6.9 million. That’s growth of 157X in a little more than a century. And that’s a lot more concrete than even Bubba and Jim Bob together  could spit on in a lifetime.

Diane Cooper, a Kingwood resident with more than 20 years of forecasting experience for the National Weather Service points out a couple other problems with these projections. First, the data is very, very, very thin and rarely updated.

Second, the probabilities are computed for a specific point, not a city, county, region or country. Storms know no geographic boundaries.

In fact, she says, it’s a little bit misleading to say that Houston got hit by three 500-year storms in three years. That’s because any given storm may not have equal intensity over all parts of the city. A storm may have had 500-year intensity on the north side. but only 100-year intensity on the south. Following the same line of logic, but in a different direction, if you expanded the boundaries out to the entire U.S., we might have multiple 500-year storms in one year (each in different places).

Cooper also points out that 500-year storms do not necessarily produce 500-year floods. They are two different beasts.

If the ground is dry, say from a drought, a large percentage of a heavy rain might be absorbed, yielding less than a 500-year flood. Conversely, if the ground is saturated and we get a 100-year rain, get out the oars and inner tubes.

Even though charts like the one above have more uncertainty than a dart player who just downed a fifth of Jack Daniels, they do put big storms in perspective.

By the way, the term “500-year flood” originated in the 1960s when the National Flood Insurance Program was being developed. At the time, people intended it to mean “a storm with a .002% chance of happening in any given year.” However, over the years, the meaning became distorted. Because it had a 1 in 500 chance of occurring each year, insurers started calling it a 500-year storm. People mistook that to mean “the interval between intense storms.”

More on that in a future post and how to calculate the chances of getting hit by a monster storm during the life of your 30-year mortgage. Hint: call your insurance agent now!

Posted May 23, 2018 by Bob Rehak

267 days since Hurricane Harvey