NHC releases official Tropical Cyclone Report for Hurricane Harvey

Catastrophic flooding and high death tolls detailed

By Rebecca Barry - Meteorologist

The National Hurricane Center released the official Tropical Cyclone Report for Hurricane Harvey detailing the unique movement, catastrophic flooding, and high death rates from the storm. 

The NHC's report begins with an abstract, " Harvey started as a typical weak August tropical storm that affected the Lesser Antilles and dissipated over the central Caribbean Sea. However, after re-forming over the Bay of Campeche, Harvey rapidly intensified into a category 4 hurricane (on the SaffirSimpson Hurricane Wind Scale) before making landfall along the middle Texas coast. The
storm then stalled, with its center over or near the Texas coast for four days, dropping historic amounts of rainfall of more than 60 inches over southeastern Texas. These rains caused catastrophic flooding, and Harvey is the second-most costly hurricane in U.S. history, after accounting for inflation, behind only Katrina (2005). At least 68 people died from the direct effects of the storm in Texas, the largest number of direct deaths from a tropical cyclone in that state since 1919." 

Next the report details the beginning stages of the storm and its development as it initially formed in the Atlantic and made its way Westward.  The report goes on to say, Harvey began to rapidly intensify late on August 23rd in an environment of light shear, very warm water and high mid-level moisture. The storm turned northward, steered around the western edge of the distant subtropical ridge, and the track gradually bent toward the northwest during the next day or two. The cyclone’s rate of intensification increased early on August 24th  as a large mass of deep convection formed over the center, and an eye was noted on reconnaissance observations by 12:00 UTC that day. Harvey became a hurricane later on 24 August, and by that night a well-defined eye appeared in infrared satellite pictures. The hurricane reached category 3 status by midday on 25 August while it
approached the middle Texas coast and intensified into a category 4 hurricane by 0000 UTC August 26th. Harvey’s center made landfall on the northern end of San Jose Island about 5 nautical miles east of Rockport, Texas at 3:00 UTC that day. Sustained winds of 115 kt and a minimum central pressure of 937 mb are estimated for that landfall. The hurricane then made a second landfall on the Texas mainland 3 hours later, slightly weaker due to land interaction, with 105 kt winds and an estimated central pressure of 948 mb southeast of Refugio on the northeast coast of Copano Bay, west of Holiday Beach.  Harvey rapidly weakened over land to a tropical storm within 12 h after landfall and maintained a 35-kt intensity for the next day or so, aided by the sustaining effects of the southeastern portion of its circulation remaining over water. 

The storm made a slow loop late on August 26th into August 27th, and drifted eastward or southeastward for the next few days. Although the center passed well south of the Houston Metro and Golden Triangle (southeastern Texas between Beaumont, Port Arthur and Orange) areas, torrential rains fell in these locations near a stationary front on the north and east side of Harvey. 

Winds Speeds and Pressure

 

  • Harvey’s maximum winds of 115 kt occurred during a several hour period concluding with its first Texas landfall. 
  • The highest observed sustained winds on land were 96 kt near Arkansas Pass, with the highest observed gust being 126 kt near Rockport, Texas
  • The minimum pressure of Harvey is estimated to be 937 mb, based on a dropsonde
    measurement of 938 mb with 10 kt of surface wind at 0215 UTC 26 August. 
  • The lowest observed pressure on land was 940.8 mb reported by a storm chaser in Rockport at 0331 UTC 26 August. 

Storm Surge

 

  • The combined effect of the surge and tide produced maximum inundation levels of 6 to 10 ft above ground level to the north and east of Harvey’s center landfalls in Texas in the back bays between Port Aransas and Matagorda, including Copano Bay, Aransas Bay, San Antonio Bay, and Matagorda Bay.
  • The highest inundations (8 to 10 ft above ground level) likely occurred along the western shores of San Antonio Bay and adjacent Hynes Bay.
  • The highest measured water level by a tide gauge was 6.7 ft above Mean Higher High Water (MHHW) at a Texas Coastal Ocean Observing Network (TCOON) site at Port Lavaca.
  • Copano Bay, where Harvey made its second Texas landfall, also had significant storm surge flooding of 4 to 7 ft above ground level.
  • Similar coastal flooding of 4 to 7 ft above ground level occurred in locations south of Port Aransas to the north entrance of the Padre Island National Seashore.

Rainfall and Flooding

Harvey was the most significant tropical cyclone rainfall event in United States history,
both in scope and peak rainfall amounts, since reliable rainfall records began around the 1880s.
The highest storm total rainfall report from Harvey was 60.58 inches near Nederland, Texas, with
another report of 60.54 inches from near Groves, Texas. Both of these values (and from five
other stations) exceed the previously accepted United States tropical cyclone storm total rainfall
record of 52.00 inches at Kanalohuluhulu Ranger Station, Hawaii, in August of 1950 from
Hurricane Hiki. 

For the continental United States, the previous tropical cyclone rainfall record was 48.00
inches in Medina, Texas from Tropical Storm Amelia in 1978. It is remarkable that during Harvey,
eighteen values over 48 inches were recorded across southeastern Texas, with 36 to 48
inches recorded in the Houston metro area. These rains caused catastrophic flooding in Harris
and Galveston counties, with 9 out of the 19 official river gauges in Harris County (which includes
the city of Houston) recording all-time high flood stages. 

The meteorological situation that caused Harvey to produce these extreme rains deserves
additional explanation. While Harvey was very slow moving over Texas, not all drifting cyclones
produce such torrential rain totals, and it is notable that the heaviest rainfall fell outside of the core
of the cyclone. Harvey moved into a somewhat baroclinic environment over Texas, with slightly
cooler and drier air over the southern United States behind a weak stationary front. The
weak front was situated across the Houston metro area from August 26-27th , enhancing surface
convergence and lift within the very warm and humid air on the eastern side of Harvey, leading to
several episodes of heavy rain. Upper-level divergence was also occurring near the front, further
contributing to large and intense rain bands. The rain rates observed in these bands were
exceptional, with 6.8 inches of rain in just one hour documented in southeastern Houston from
extremely heavy rain bands training over the same location. The front hardly moved from
August 27-28th, leading to the extreme rainfall totals in the Houston metro area since the
main inflow band originated over the very warm waters of the northwestern Gulf of Mexico, which
provided multiple influxes of warm and humid air. It should be noted that while the magnitude of
this event was unprecedented, the synoptic situation was not, and previously has been associated
with other tropical cyclone flood events near the coast.  

Harvey also produced heavy rain over Louisiana, with a peak amount of 23.71 inches
recorded west of Vinton. Radar data, however, suggests an estimate of about 40 inches for a
maximum value, which is considered more representative of peak rainfall in that state since there
were few observations over extreme southwestern Louisiana.

Tornadoes

Harvey was a prolific tornado producer. There were 57 tornadoes preliminarily reported
during Harvey, about half of which occurred near and south of the Houston metro area.
Over 150 tornado warnings were issued during the event. Tornadoes were also noted in
Louisiana, Mississippi, Alabama and Tennessee as the cyclone moved near or over those states.
Fortunately, almost all of the tornadoes were relatively weak, of EF-0 and EF-1 intensity, with
generally minor damage, few injuries and no deaths attributed to them.

Casualties

Harvey is responsible for at least 68 direct deaths in the United States, all in Texas. Over
half of the deaths (36) were in Harris County in the Houston metro area. All but three of the deaths were from freshwater flooding, and none of the deaths can be linked to the storm surge, which is quite remarkable for a category 4 hurricane landfall. Still, Harvey is the deadliest U.S. hurricane in terms of direct deaths since Sandy (2012) and is the deadliest hurricane to hit Texas since 1919. About 35 additional deaths are ascribed to indirect causes, such as electrocution, motor-vehicle crashes and isolation from necessary medical services. Four people were reported injured by a tornado north of Reform, Alabama.

Damages

  • The latest NOAA damage estimate from Harvey is $125 billion, with the 90% confidence interval ranging from $90 to $160 billion. 
  • The mid-point of the estimate would tie Katrina (2005) as the costliest United States tropical cyclone, which was also $125 billion
  • The damage caused by Harvey’s flooding was catastrophic over a large area of
    southeastern Texas. Over 300,000 structures in that region were flooded, with up to 500,000 cars reported flooded as well
  • The damage caused by Harvey’s flooding was catastrophic over a large area of southeastern Texas. Over 300,000 structures in that region were flooded, with up to 500,000 cars reported flooded as well
  • About 336,000 customers lost power during the hurricane. 

If you are interested in reading the report in its entirety, you can access it by clicking here...

Copyright 2018 by WJXT News4Jax - All rights reserved.