But with Harvey, the area covered by the heavy rainfall has been vastly larger, and the rain has persisted for days. 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. Major floods again occurred on Memorial Day in 2015, and on April 18-19, 2016. But the excessive accumulations were fairly localized. 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. 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. 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. By the morning of Wednesday, August 29, this station had received over 44 inches of rainfall over the previous week. This station is located just southeast of downtown Houston. CDT Monday, August 28, at Harris County Gauge C106_310 Berry Bayou Nevada Avenue. Hourly rainfall amounts (in inches) between 8 p.m. 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. 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. 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. Then, after this initial intense burst, there has been no respite. The green polygons show flash flood warnings issued by the NWS, including a rare ‘flash flood emergency’ for Houston. The right panel shows radial velocity (green colors toward the radar, red colors away). The left panel shows radar reflectivity, which is related to rainfall intensity. Tropical cyclones can also last a long time if their motion slows, then a particular region can experience that heavy rainfall for multiple days.Īnimation from the Houston, Texas National Weather Service radar on Saturday evening, August 26, 2017. That moist air rises and the water vapor condenses, and a large fraction of that water falls as rain. Tropical cyclones in general are very efficient rain producers, because they draw large quantities of water vapor into the atmosphere from a warm ocean. In other words, the most rain falls where it rains the hardest for the longest. 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. National Weather Service A ‘train’ of rainstorms The National Weather Service said forecasters needed to change their usual color scale to illustrate the extreme rainfall amounts in Texas. Right panel: Rainfall forecast for the 24 hours from 9 a.m. Left panel: Rainfall accumulation for four days ending Tuesday, Aug.
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