People walk down a flooded street as they evacuate their homes after the area was inundated with flooding from Hurricane Harvey on 28 August 2017 in Houston, Texas. Photo: Joe Raedle / Getty Images

By Dr. Jeff Masters
28 August 2017

(Weather Underground) – The center of Tropical Storm Harvey edged out over the Gulf of Mexico late this morning, and satellite images and radar loops show that Harvey is responding by building new intense thunderstorms near its center. These thunderstorms will bring renewed rounds of torrential rains to Houston through at least Wednesday morning. Houston got a brief reprieve on Monday morning from the heavy rains that have submerged the city, when a slot of dry air to the west of Harvey rotated into the core of the storm and stifled the heavy rains over Southeast Texas. Harvey’s heaviest rains fell instead near the Louisiana/Texas border, where an intense band of heavy thunderstorms fed by Gulf moisture brought 12-hour rainfall amounts in excess of ten inches to three WU personal weather stations near Beaumont, Texas. Rainfall amounts in western and central Louisiana of 4 – 8 inches were common, in the 12-hour period from midnight to noon Monday.

Mind-boggling rainfall amounts

When we first saw the model predictions last week of widespread rain amounts for Houston of 15 – 25 inches, with some amounts as high as 40 inches, we issued the required “Catastrophic rains coming” forecast, but our view of the forecast was tinged with a sense of unreality. Could the models be wrong? What would that kind of rainfall would do to Houston? Surely the heavy rains wouldn’t center directly over the nation’s fourth-largest city, would they? But they did. Here we are, in the midst of a mega-disaster on the scale only surpassed by Hurricane Sandy and Hurricane Katrina in recent decades, from a hurricane hazard we’ve never seen on such a large and destructive scale—torrential rain. The damages from Harvey will undoubtedly run into the tens of billions of dollars, making Harvey’s rains the most destructive ever experienced from a hurricane.

The highest 3-day official rainfall amount from Harvey was over 39” as of 10 am CDT, as compiled by NOAA/NWS/WPC. The official Houston-area record is kept at Bush Intercontinental Airport, well north of downtown. Even there, it appears a new 24-hour record has been set, according to WU weather historian Christopher Burt, who calculated a preliminary total of 18.10” between 9:53 p.m. CDT Saturday, August 26, and 9:53 p.m. CDT Sunday. If confirmed, this would top the previous record for any 24-hour period of 15.65” on Aug. 27-28, 1945, at Hobby Airport, which was Houston’s officially designated NWS reporting site at the time. Saturday was the wettest calendar day on record for NWS/Houston, which recorded 16.07" from midnight to midnight. And August 2017 is already the wettest month in Houston’s century-plus record by far, with 32.68" through Sunday beating out 22.31” in October 1949 at Hobby Airport.

Hydrograph for the Brazos River at Richmond, where floodwaters from Hurricane Harvey are not expected to peak until at least Wednesday, 30 August 2017. Graphic: NOAA / NWS / AHPS

A number of rainfall amounts topping 30” were reported from 8:00 pm CDT Thursday through 10 am CDT Monday, mainly across the Houston metro area:

39.72”   Dayton (0.2 mi E)
34.90”   Waller (3 mi WSW)
31.56”   Berry B Forest Oaks
30.90”  Brookshire (6.9 mi NNW)
30.32”  South Houston (4 mi SSW)
30.25”  Santa Fe (0.7 mi S)

As of 12 pm CDT Monday, at least eight personal weather stations (PWSs) in the Houston area had received 60-hour rainfall totals over 30”, and two had rainfall amounts in excess of 40”. The PWS numbers given below are the 60-hour storm total, followed by Monday’s rain, Sunday’s rain, and then Saturday’s rain. Given that this event still has 2 – 3 more days to go, it appears likely that some location in Texas will break the all-time record for U.S. rainfall from a tropical cyclone or its remnants: 48.00” in the Texas hill country from Tropical Storm Amelia of 1978.

41.77”, 3.77”, 29.96”, 8.04”: Baytown, TX (Country Club Oaks
40.25”, 4.26”, 29.87”, 6.12”: Dayton, TX (Winter Valley)
39.12”, 2.93”, 25.97”, 10.22”: La Porte, TX (Westend LaPorte/SJJC)
37.57”, 2.52”, 29.88”, 5.17”: Baytown, TX (Eastpoint)
37.25”, 3.91”, 27.44”, 5.90: Dayton, TX (Brookstone)       
37.04”, 2.04”, 25.04”, 10.46”: League City, TX (South League City)
33.52”, 1.52”, 23.73”, 8.27”: Seabrook, TX (Baybrook)
32.05”, 1.85”, 25.47”, 4.73”: Baytown, TX (Chaparral Village) [more]

Harvey Moves Back Over Water; Historic Rainfall Will Continue

A Photoshopped image shows a shark swimming through floodwaters on a freeway in Houston, Texas, 27 August 2017. Photo: Jason Michael / Twitter

By Bob Henson
28 August 2017

(Weather Underground) –The nation’s worst flood disaster since Hurricane/Superstorm Sandy continued to unfold late Sunday across Houston and nearby parts of southeast Texas, as Tropical Storm Harvey punished the area with record-shredding rainfall. As of 6:00 pm CDT Sunday, Harvey was centered about 10 miles northeast of Victoria, TX, moving southeast toward the coast at a mere 3 mph with top sustained winds of 40 mph. Despite its minimal tropical-storm-strength winds, Harvey is a superpowered rain producer. Parts of the Houston area have racked up 20” or more in torrential rainbands since Saturday; another 10” – 20” or more is expected, with flooding expanding into parts of southwest Louisiana. The National Weather Service has kept the Houston area in a Flash Flood Emergency since Saturday night—a truly unparalleled stretch—and a Civil Emergency has been declared. And as if we needed any more trouble, a Tropical Storm Watch has been issued for the North Carolina coast as newly designated Potential Tropical Cyclone 10 threatens to strike as Tropical Storm Irma on Tuesday (see details below).                                      

A disaster that won’t stop long enough for us to take stock

The situation on the ground in Houston is gaining uncomfortable resonance with 2005’s catastrophic Hurricane Katrina. Instead of compromised levees, we have an extraordinary prolonged and intense rain event atop one of the nation’s largest and most flood-vulnerable cities. As with Katrina, there have been numerous reports of people trapped in attics, and countless roads across the city are impassable. Given the unrelenting rains and flooding, there is real concern that some people will be unable to leave their homes or places of refuge for multiple days—a potentially life-threatening situation for those with special medical needs or without enough food and water. It is too soon to tell exactly how many people are in such dire need, but the number of 911 calls and other cries for help suggest the number is far too large for comfort.

Harvey is sure to inflict a massive economic toll as well. Katina (2005, around $100 billion) and Sandy (2012, around $75 billion) were the nation’s two costliest hurricanes on record. It’s easy to imagine Harvey rising into that range, although experts say it’s simply too soon to know for sure. Steven Bowen (Aon Benfield) told me: “We've got multiple days to go, and since we're already in uncharted territory for this type of flood event in Houston/Galveston, it's hard to fathom what might come next. There is no question that this is a catastrophic event that is going to have severe financial implications. The economic cost - from both direct damage and direct interruption impacts - is going to be well into the billions. We just don't quite know how many billions as of now. It's going to take weeks to fully take stock of the totality of the damage."  Bowen points out that only 1/6 of Houston residents are insured through the National Flood Insurance Program, so thousands of people are likely to face devastating financial hits. […]

Observed precipitation from Harvey, as of 7 am CDT Monday, 28 August 2017. The Houston area has received 20 – 30” of rain, and the northern suburbs and adjoining areas have received over 30”. Graphic: NWS

A rain-making machine for the ages

Although the phrase is now a cliché, the rain-producing setup around Houston truly is a near-perfect storm. Harvey’s extremely slow motion is allowing bands of rich tropical moisture to pour relentlessly northward across southeast Texas. Individual cells within these bands are racing northward, while the bands translate east very slowly. As the rainbands reach the Texas coast, they are encountering a weak surface front lying parallel to the coast and just inland. There is little temperature contrast, but the front separates east-northeast winds inland from southeasterly winds flowing toward the front from offshore. The result is converging air along the front, and a ramp-up in rainfall intensity as the bands slide north across the front.

Decaying tropical systems often produce more rainfall at night, similar to tropical cyclones over the ocean. Sunday night may bring another pulse of intensified rainfall—likely focused over or just east of the Houston area—with another relative lull possible at some point on Monday. Rainfall production in southeast Texas may become a bit less efficient and slightly more patchy if dry air from western Texas works its way into the south side of Harvey’s circulation. Still, the atmospheric processes locked in place will continue to produce extreme totals wherever training rainbands set up. Moreover, if Harvey moves just offshore and then arcs inland west of Houston on Wednesday, as predicted by NHC and our top global models, then Houston may end up in a more concentrated zone of torrential rain just northeast of Harvey’s center from Tuesday into early Wednesday, before the heaviest rains finally abate. [more]

Flood Calamity Continues In Houston and Beyond; Harvey Edges Toward Coast



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