Tag Archive for: rainfall

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.


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.


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.”


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.


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


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.


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