What made the rain in Hurricane Harvey so extreme

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The rainfall from Harvey has now exceeded the amount from the previous record-bearer, Tropical Storm Amelia in 1978.
AP Photo/Charlie Riedel

Russ Schumacher, Colorado State University

Fifty inches of rain. Nine trillion gallons of water. The Gulf Coast of Texas, and especially the Houston metropolitan area, has been inundated by rain produced by Hurricane Harvey. And as of this writing, the rain continues along a broad swath of the Gulf Coast, with a flood threat extending all the way east through New Orleans to the Florida Panhandle.

Even for one of the wettest and most flood-prone parts of the United States, the rainfall totals and flooding are breaking records. So, what has made Harvey such a prodigious rain producer?

Left panel: Rainfall accumulation for four days ending Tuesday, Aug. 29, 2017 at 9 a.m. CDT. Right panel: Rainfall forecast for the 24 hours from 9 a.m. Tuesday, Aug. 29 to 9 a.m. Wednesday, Aug. 30. The National Weather Service said forecasters needed to change their usual color scale to illustrate the extreme rainfall amounts in Texas.
National Weather Service

A ‘train’ of rainstorms

The amount of rain that falls at a given location can be boiled down to a surprisingly simple equation: The total precipitation equals the average rainfall rate, multiplied by the rainfall duration. In other words, the most rain falls where it rains the hardest for the longest.

Tropical cyclones in general are very efficient rain producers, because they draw large quantities of water vapor into the atmosphere from a warm ocean. That moist air rises and the water vapor condenses, and a large fraction of that water falls as rain. Tropical cyclones can also last a long time; if their motion slows, then a particular region can experience that heavy rainfall for multiple days.

Even compared to other tropical cyclones, the rain from Harvey has been very hard, and gone for a very long time. On Saturday evening (August 26) into Sunday morning (August 27), an intense band of storms developed to the east of Harvey’s center, and lined itself up right over Houston. This is a process known as “echo training,” in which it appears that the individual thunderstorm cells are like train cars that repeatedly pass over the same spot and bring with them heavy precipitation.

This precipitation band was producing up to six inches of rain per hour – an extremely high rate – and it remained over the Houston metro area for several hours, with a couple more that followed immediately after. One location just southeast of downtown Houston recorded 13.84 inches in just three hours. These rains from Saturday night into Sunday morning initiated the massive flooding in the Houston metro area.

Image from NOAA’s GOES-16 satellite at 9:42 p.m. CDT on August 26, 2017. Infrared image shows the temperature of cloud tops, with red and black colors indicating very intense thunderstorms.
Colorado State University/Cooperative Institute for Research in the Atmosphere
Animation from the Houston, Texas National Weather Service radar on Saturday evening, August 26, 2017. The left panel shows radar reflectivity, which is related to rainfall intensity. The right panel shows radial velocity (green colors toward the radar, red colors away). The green polygons show flash flood warnings issued by the NWS, including a rare ‘flash flood emergency’ for Houston.

Relentless rainfall

Then, after this initial intense burst, there has been no respite. Usually, when a tropical cyclone turns poleward from the tropics toward the United States, it will interact with one or more midlatitude weather systems that will send the storm on its way after a day or two. But this August, the jet stream has been positioned well to the north of Texas, so none of these disturbances has approached, and Harvey’s center of circulation has barely moved since it made landfall. As a result, across the Texas (and now Louisiana) coast, there have been periods with intense rainfall (in more of the rainbands described above), along with lighter, but still substantial, accumulations.

Hourly rainfall amounts (in inches) between 8 p.m. CDT Saturday, August 26 to 8 p.m. CDT Monday, August 28, at Harris County Gauge C106_310 Berry Bayou @ Nevada Avenue. This station is located just southeast of downtown Houston. By the morning of Wednesday, August 29, this station had received over 44 inches of rainfall over the previous week.
Harris County Flood Warning System

This combination of unusually high rain rates and long duration has resulted in a very large area with 30 to 45 inches of rain in a few days.

Those of us who study extreme rainfall and flooding, and those who live in and around Houston, know this area is vulnerable to both very heavy rainfall and destructive and deadly floods. The previous standard-bearer for extreme rainfall in the region was Tropical Storm Allison in June 2001, which produced just over 40″ of rain around Houston. But the excessive accumulations were fairly localized. Major floods again occurred on Memorial Day in 2015, and on April 18-19, 2016.

In the April 2016 event, an intense line of overnight storms produced up to 15″ of rain in a few hours, similar to the “training” rainbands in Harvey. But with Harvey, the area covered by the heavy rainfall has been vastly larger, and the rain has persisted for days. For comparison, in just one day (ending Sunday morning, August 27, 2017) the area covered by rainfall from Harvey exceeding 16″ is several times larger than the entire April 2016 flood event, and at least two more days of similar accumulations have followed.

Tornado risks

To make matters even worse, there were also numerous tornadoes reported as the rainbands came on shore. It’s fairly common to have tornadoes occur in association with landfalling hurricanes, but what struck me in this case was that tornado warnings were being issued in the same places that had just received massive amounts of rain.

My research group has studied the challenges associated with multi-hazard situations, and specifically when the threats of tornadoes and flash flooding occur in the same place at the same time, as the protective responses to those hazards can be at odds with each other. For people to be under both a tornado and a flash-flood warning at the same time is surprisingly common – these overlapping warnings occur around 400 times per year on average.

But this situation was taken to a new extreme during Harvey, when tornado warnings were being issued at the same time that emergency officials were sending messages for people to go to their roofs for safety (rather than risk getting caught in the attic). The heartbreaking (but also heroic) video footage of water rescues speaks to the immense human impact of this multifaceted storm.

Spot-on forecast

One final remarkable aspect of Hurricane Harvey’s rainfall is how accurate numerical weather prediction models – and the human forecasters who use them to make official forecasts – were at highlighting the incredible precipitation accumulations.

Medium-range forecast models at least a week in advance were showing Harvey stalling out along the Texas coast and producing extreme rainfall. As the event neared, essentially every numerical model was showing accumulations over 25 inches. Often, when meteorologists see models making predictions of events that would be unprecedented, we are rather skeptical of that guidance, because there are no points of reference to compare to. But in this case, the models were in close agreement about the potential for a truly major event, and forecasters saw the gravity of the situation.

The NOAA Weather Prediction Center, which makes official rainfall forecasts (and rarely includes extreme amounts), on Friday afternoon (August 25) predicted a broad swath of over 20 inches, with isolated areas up to 40″. Never before had the Weather Prediction Center issued a “high risk” of excessive rainfall three days in advance, as usually the uncertainties with forecasting precipitation don’t allow enough confidence to do so. In fact, their protocol didn’t even allow for such an alert so far in advance! But for Harvey’s rainfall, they did this on consecutive days, and with high accuracy, because of the expected extremity of the event. (The primary errors in the rainfall forecasts in advance of the storm is that they placed the maxima a bit southwest of Houston, instead of centered over Houston.)

Forecast graphic from the NOAA Weather Prediction Center. Tweeted at 5:47 p.m. CDT Friday, August 25, 2017.

The ConversationOne of the fascinating aspects of studying extreme rainfall and flash floods is the wide variety of storm systems that can produce heavy rain, and trying to figure out how the ingredients came together in each of those diverse situations to inform and improve future forecasting. For Hurricane Harvey, researchers and forecasters will be analyzing the ingredients that led to this record-setting flood for many years to come.

Russ Schumacher, Associate Professor of Atmospheric Science, Colorado State University

This article was originally published on The Conversation. Read the original article.

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  • kimyo

    As the event neared, essentially every numerical model was showing accumulations over 25 inches. Often, when meteorologists see models making predictions of events that would be unprecedented, we are rather skeptical of that guidance, because there are no points of reference to compare to.

    harvey is simply NOT ‘unprecedented’. the odds are that a harvey-like storm will drop 40 inches of rain on texas once every decade, if recent history is a guide. (for your daily dose of ‘fake news’, type ‘500 year storm’ into google news and weep as what passes for ‘reporting’ today)

    Texas, No Stranger to Tropical Cyclone Flooding Disasters: Allison, Claudette and Amelia Brought 40-Plus Inches

    Since 1978, three tropical storms or their remnants have produced more than 40 inches of rain in the Lone Star State

    Allison meandered for days, and when slow-moving thunderstorms flared up, it unleashed epic amounts of rainfall that caused massive flooding in the Houston metro area. The top rainfall total from Allison was 40.68 inches at Moore Road Detention Pond in northwestern Jefferson County, Texas, according to the Weather Prediction Center branch of NOAA.
    ~ ~ ~ ~ ~ ~ ~ ~
    The heaviest rain amount was recorded to the south of Houston in the town of Alvin. An observer measured 43 inches of rainfall in just 24 hours – the greatest 24-hour rainfall total in United States history. The storm total beyond the 24-hour record was 45 inches.
    ~ ~ ~ ~ ~ ~ ~ ~
    When the rain subsided, Medina, Texas, had received 4 feet (48 inches), the highest total from an Atlantic tropical cyclone or its remnants dating to 1950.

    you’d never approve a design for a school or a movie theatre without the ability to evacuate people safely. why do we think it’s fine to design cities without exits? there’s a chemical plant without power and two nuclear plants in the path of the flooding. shouldn’t there be a plan to get children and the elderly out of harm’s way? isn’t that what government is for?

    ps: before you reach into your pocket please remember this: How the Red Cross Raised Half a Billion Dollars for Haiti ­and Built Six Homes
    The Red Cross says it has provided homes to more than 130,000 people. But the actual number of permanent homes the group has built in all of Haiti: six.

    “Millions of Haitians are safer, healthier, more resilient, and better prepared for future disasters thanks to generous donations to the American Red Cross,” McGovern wrote in a recent report marking the fifth anniversary of the earthquake.

    In other promotional materials, the Red Cross said it has helped “more than 4.5 million” individual Haitians “get back on their feet.”

    It has not provided details to back up the claim. And Jean-Max Bellerive, Haiti’s prime minister at the time of the earthquake, doubts the figure, pointing out the country’s entire population is only about 10 million.

    “No, no,” Bellerive said of the Red Cross’ claim, “it’s not possible.”

    • nomadd

      From your link
      “117 hours as a named storm on land” “double the old record”
      Sounds unprecedented to me.

      • kimyo

        amelia dumped rain for 5-6 DAYS (ie: 120 – 144 hours, depending on how you count it).


        Remnants of Tropical Storm Amelia moved inland over the south Texas coast on July 31, 1978. Abundant moisture from the storm, in combination with other meteorological factors, resulted in record rainfalls over portions of the picturesque Hill Country and Big Country areas of Texas (figure X.1). Between August 1 and 3, locations in the Hill Country received over 30 inches of rain. During August 3 and 4, Big Country locations received similar large rainfalls.

        does this sound familiar? (from this writeup on amelia):

        the heaviest rains and most severe flooding did not take place until two or three days after the center of the storm moved inland. for several days there was a continuous flow of moist tropical air from the southeast.

        or this?: A Week-Long Siege of Heavy Rain Triggers Flash Flooding in Texas in May, June 2016

        As Harvey breaks rainfall record, Houston imposes a curfew and death toll climbs to 18

        On Tuesday afternoon, the Mont Belvieu industrial suburb east of Houston recorded 51.12 inches of water since Harvey’s arrival, breaking the highest previous record of 48 inches for a single storm, from Tropical Storm Amelia in Medina, Texas, in 1978.

        51″ is just NOT unprecedented. not when you have 48 inches in 1978. or 45 inches in 1979 (claudette). or 40 inches in 2001 (allison).

        massive rainfall spanning 4-5 days is NORMAL for texas. it would be far more surprising if you went a decade WITHOUT such a storm.


        • nomadd

          im sure youre right. harvey is typical. but like i said

          “double the old record” still Sounds unprecedented to me.

          • kimyo

            ‘double the old record’ is a semantic trick rather than some sort of climatic record (‘named’ storm on land).

            more importantly, why is the media so fixated on bleating ‘extreme’ and ‘unprecedented’ at every opportunity?

            you’d adopt such a position if your goal was to exonerate those who failed to provide for the safety and welfare of the citizens of houston and its environs.

            it’s essentially condeleeza rice saying ‘no one could have imagined them taking a plane, slamming it into the Pentagon … into the World Trade Center, using planes as a missile’.

            of course people imagined this. they drilled for exactly such an occurrence.

            and of course people imagined a houston devastated by flood and subsequent chemical spills.

            if you’re the ceo of arkema, with your plant burning up, it’s necessary that the public believe ‘no one could have imagined’ this ‘unprecedented’ storm.

            if you’re houston’s new mayor, and in 2016 you released a statement saying “Only a small portion of the city of Houston is at risk for major storm surge,”, then you really really want houstonians to believe that ‘no one could have imagined’.

          • nomadd

            i dont know. i havent paid much attention to media reports so the only ‘unprecedented’ claim ive seen is yours. why r u so fixated on whether its unprecedented? it certainly was extreme though, which is the claim of this article. so are you saying that the claim of double the old record is false?

            i dont think its a semantic trick to state how this storm differs from its predecessors. double the record over land seems a distinction that matters.

          • kimyo

            my link doesn’t mention your ‘117 hours over land’ quote. you must be reading that from a sidebar or something.

            and, it’s only ‘double the old record’ if you use semantic tricks to ignore storms like amelia, which lasted LONGER over texas than harvey did.

            what the link i provided says is: ‘Since 1978, three tropical storms or their remnants have produced more than 40 inches of rain in the Lone Star State.’ generally, most folks would not think to use the word ‘extreme’ when describing something which happens once every 10 years.

            Houston is experiencing its third ‘500-year’ flood in 3 years. How is that possible?

            So is Houston just on a historically unlucky run of flooding, to be followed by a return to normal soon?

            these people are demented. harvey is ‘normal’. what changed is we decided to cram a bunch of people into a flood plain, pave everything over, and store chemicals willy-nilly all about the place.


          • nomadd

            “my link doesn’t mention your ‘117 hours over land’ quote. you must be reading that from a sidebar or something.”

            yes it did. the video at that link which apparently has changed. at any rate, it matters not one whit.

  • Lyle

    I wish this article would have mentioned climate science and the increased amplitude of the jet-stream due to global warming.

    • nomadd

      Is the “increased amplitude of the jet-stream due to global warming.” Or is global warming due to the jets (i.e. chemtrails)?

      • Gary Dalton

        No global warming it only comes from the UN who uses it to give Texas a good smack for not doing as they were told. From there its sent to CIA who direct it and aircraft spraying microscopc coal fly ash also known as chemtrails major additional enhancement from NOAA in the from of microwave (heaters) wich rapidly heat cloud formation into high pressure areas to block movememt of the storm and increase rainfall and winds . The jetsream was moved to block Harvey from moving north it was boxed in. Once there is the basic elements then geoengineering takes over this was developed after the atomic bomb program (1950 s). Its know as Weather Warfare. Far less damage, think you could fight in Harvey. Two sites but lot more coming as the cats out of the bag. 1 (www.geoengineeringwatch.org) 2 (1pacificredwood) on you tube I just watched ( 2 ) good place to learn system. (droughts, fires, floods, tornadoes, hurricanes, chemtrails, Harvey there was hand made all can be seen on weather downloads, hard to hide this. Give prayer to what been put on the people of Texas

  • Pete

    Why no apparent use of weather control technology to slow down or control the direction of this storm? The US has numerous patents on weather control technology.

    • nomadd

      because the psychopaths in charge did not want to slow it down. they probably intensified it to advance their malicious agenda. just like they did with katrina.

  • Open your eyes and mind

    The tiny animated radar images they provided above are just pathetic. They hide the truth.

    Go to http://www.weatherwar101.com and see longer time lapse AND wider geographical area radar animations. Not just for Harvey but also for many other severer weather events, they show the true source of such heavy rain: WSAC (wet surface air coolers) from electricity generation plants DELIBERATELY brought online to inject massive amounts of water vapour into the sky.

    That is a fantastic site and it is definitely worth the time to investigate in depth. You will learn a lot and see with your own eyes how storm events are artificially created.

    • AnaJDavis

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    • nomadd

      havent investigated yet but suspect geoengineering because they have done it before and keep on doing it and doing it and doing it and doing it. its the mo of the geoengineers/weather warriors. will check out that site.

      • Wandawferrell

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  • DebL.

    If this was geo-engineered it was for this main reason–shit down oil refineries to increase the price of oil (which would save Saudi Arabia from bankruptcy thus allowing them to continue genocide in Yemen, treat women like crap and destroy Shiites there, among other things) as well as gasoline, to end the oil glut. Folks it’s all about the money. Who gives a crap that many people will lose everything in these floods? Certainly not the criminal psycho elites!

    • nomadd

      i doubt that was the reason, at least not the main one, though it may be one ‘benefit’ these controllers welcome. here is another more plausible scenario.

  • JerseyCynic

    Defense contractor “Raytheon” does all the “forecast” modeling for the National Weather Service and NOAA. Lockheed Martin does the modeling for the FAA. Since both of these private defense contractors are up to there necks in weather modification patents and programs, the “forecasts” are more or less the “scheduled” weather. Now, with the Rothschild’s acquisition of the dominant private modeling organization, the control of weather “modeling” seems complete.

    • nomadd

      ‘“scheduled” weather’
      good description