Hurricane Harvey as seen by the GOES-16 satellite at 8:30 am CDT Friday, 25 August 2017. NOAA’s GOES-16 satellite has not been declared operational and its data are preliminary and undergoing testing. Photo: NOAA / CIRA / RAMMB

By Dr. Jeff Masters 
25 August 2017

(Weather Underground) – Hurricane Harvey is poised to deliver a catastrophic flooding blow to Texas after putting on an impressive round of rapid deepening Friday morning that brought the storm to the verge of Category 3 strength. Harvey passed over a warm ocean eddy with high heat content for over 6 hours Friday morning, and the extra energy the eddy provided allowed Harvey’s central pressure to fall a spectacular 15 mb in just two hours, from 967 mb at 4 am CDT to 952 mb at 6 am CDT. It takes several hours for a hurricane’s winds to respond to a rapid pressure fall, so we can expect that Harvey will be a Category 3 hurricane with 120 mph winds by Friday evening. An Air Force hurricane hunter aircraft in the storm this morning was continuing to see falling pressures, with the 8 am CDT eye penetration recording a 949 mb pressure. The aircraft also observed evidence that an eyewall replacement cycle was beginning. In this situation, the inner eyewall would collapse and be replaced by a new eyewall with a larger diameter, which would likely slow down or end Harvey’s intensification phase.

Harvey is a very dangerous hurricane with extreme winds, storm surge, and rainfall. If you live in Texas, please heed the advice of local emergency management officials, and get out immediately if you live in an evacuation zone. Heavy rain squalls and strong wind gusts are already affecting the Texas coast, and tropical storm-force winds will begin affecting portions of the coast late Friday morning or early Friday afternoon, making evacuation difficult. […]

Harvey’s combination of strength and rainfall duration has few if any parallels

The historical record of U.S. hurricanes gives us few, if any, analogs for a major hurricane landfall that transitions into a multi-day rainfall event as prolonged, extensive, and intense as the scenario painted by multiple forecast models for Harvey. All four of the high-probability 0Z Friday European model ensemble members, and all but one of the 20 GFS members, maintain Harvey at Cat 1 strength (or better) for the next five days. The official NHC forecast on Friday morning called for Harvey to maintain tropical storm strength through Wednesday. Even after Harvey weakens below hurricane strength, gale-force winds will continue to pump vast amounts of moisture onshore, fueling several days of heavy rain. The latest 5-day precipitation outlook from the NOAA/NWS Weather Prediction Center (see Figure 3) projects that an area larger than the state of Massachusetts, including Houston and Galveston, can expect more than 20” of rain between now and Wednesday. Amounts of more than 10” cover an even larger area, extending into parts of the Austin-San Antonio urban corridor and including Corpus Christi and Beaumont-Port Arthur, TX, and Lake Charles, LA. Very serious flooding over the next several days can be expected well inland from the areas immediately at risk from Harvey’s initial landfall and storm surge. For example, Austin/San Antonio NWS office notes the potential for life-threatening flash flooding, especially from San Antonio south and east.

Rainfall forecast for the period from Friday morning, 25 August 2017, through Wednesday morning, 30 August 2017. Localized amounts may be even higher, and some shifts to the contours of this area can be expected depending on Harvey’s eventual track. According to NOAA's David Roth, this is the most rainfall ever predicted by NOAA's Weather Prediction Center (WPC), going back several decades. Graphic: NOAA / NWS WPC and NHC

Tropical cyclones that produce inland flooding this widespread and intense tend to be weak, slow-moving tropical storms or tropical depressions, such as Tropical Storm Allison (2001), or even unclassified systems such as the “no-name” flood of 2016 that devastated southeast Louisiana. Both events produced more than $10 billion in damage (2016 dollars), and comparable amounts of flooding and damage are certainly possible with Harvey. Moreover, because Harvey is arriving as a much stronger hurricane, prolonged gale-force winds, perhaps extending well inland, have the potential to knock down many trees and power lines in areas where the soil is saturated, so power outages may be unusually extensive. Residents across south central and southeast Texas and southwest Louisiana should be prepared for the possibility of going multiple days without power. […]

Dangerous compound storm surge and freshwater flooding for Galveston/Houston next week

Official NHC storm surge guidance, which extends out to 72 hours (currently through Monday morning, August 28), is not intended to depict the longer-term threat—perhaps extending well beyond Monday—posed just inland by huge amounts of freshwater blocked by the surge from flowing to the sea. Current storm surge warnings include Brazoria and Galveston, with a 2 - 4 foot surge expected from Harvey’s initial approach; however, more widespread flooding may occur in the Houston/Galveston area early next week. Storm surge expert Dr. Hal Needham, who is riding out the hurricane on Galveston Island, highlights the potential for catastrophic flooding in the Galveston/Houston area if Harvey moves back out over the ocean, intensifies, then moves toward northeast, just offshore of Galveston, as the European model is predicting. In a Friday morning blog post, Dr. Needham points out that Harvey is likely to produce a storm surge for Galveston and Bolivar Islands that is less severe than during Hurricane Ike (2008)—which produced a surge of up to 19 - 22 feet—but far more prolonged. The multi-day onshore flow and storm surge coupled with extreme inland rainfall would push enormous amounts of water from several directions into Galveston Bay and the Houston Ship Channel. The result could be widespread, possibly unprecedented flooding early next week across southeastern parts of the Houston metropolitan area along and near the bay and the ship channel.

This graphic from Dr. Hal Needham illustrates the areas where freshwater outflow from extreme inland flooding could be impeded by storm surge in Galveston Bay early next week, if Hurricane Harvey takes the type of track indicated by the European model, moving slowly northeast along the Texas coast southwest of Galveston. This schematic is not an official or literal depiction of areas expected to flood; for such guidance, please refer to statements from NHC, the Houston/Galveston NWS office, and local authorities. Graphic: Hal Needham

“From personal correspondence with people in the western communities of Galveston Bay, most people are staying put and getting supplies ready, but not anticipating flood levels could come anywhere near Ike. However, compound flooding has the potential to flood locations that did not flood during Ike and inflict a widespread and long-term catastrophe that could last into the better part of next week,”says Needham. Similar levels of compound flooding just inland could affect other parts of coastal Texas and perhaps southeast Louisiana, he adds. A major 2016 investigative report from Pro Publica and the Texas Tribute, “Hell and High Water,” examines the huge economic and societal risks posed by hurricane-related flooding in western Galveston Bay and the Houston Ship Channel, which is one of the busiest shipping lanes in the world. [more]

Hurricane Harvey Strengthens to 110 mph Winds; Catastrophic Flooding Likely in Texas

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