The 2017 North Atlantic Hurricane season was a devastating one in terms of loss of life as well as property damage for the United States and the Caribbean. The National Hurricane Center released its post-season report on Harvey which caused great destruction to parts of Southeast Texas and Southwest Louisiana. What follows is a brief summary and discussion of Harvey based on info from that report as well as other sources related to Harvey’s impacts. The full report is linked at the end of this post in the references.
What became Harvey was originally a tropical disturbance which came off the West Coast of Africa on August 12th. It is common during August and September for land-based thunderstorm complexes known as mesoscale convective systems to move westward off the African coast near or south of the Cape Verde (also known as the Cabo Verde) Islands and later develop into long-lived tropical cyclones. Harvey was a classic “cape-verde” type storm as it would later develop into a tropical depression with a well-defined center on August 16th.
The depression intensified into a storm and given its name 12 hrs after initial development. It peaked over the open Atlantic at 40 knots (~45 mph), moving over the islands of Barbados and St. Vincent on August 18th. However, increasing vertical wind shear (increasing winds with height tilting and blowing the thunderstorms away from the low pressure center) over the central Caribbean Sea lead to Harvey’s dissipation to a remnant low later that day.
The remnant circulation moved over the Yucatan Peninsula on Aug 22nd and redeveloped into a tropical depression over Bay of Campeche on August 23rd, 150 n mi west of Progreso, Yucatan, Mexico.
The initially poor organization of the reformed Harvey transitioned to a period of rapid intensification late on the 23rd as deep convection began to concentrate near the center. This was aided by an environment of light shear, very warm sea surface temperatures and high mid-level moisture. Intensification would continue until landfall on the 26th. Harvey reached Category 3 midday on the 25th and intensified into a Category 4 as it made its landfalls on the Texas coast early August 26th (the evening of the 25th local time). The initial landfall was on San Jose Island, TX as a Category 4 with maximum sustained winds of 130 mph (115 knots) with a second landfall on mainland Texas in northeast Copano Bay as a Category 3 with maximum sustained winds of 120 mph (105 knots). Wind damage was extreme and devastating in Aransas, Nueces, Refugio and the eastern part of San Patricio Counties. 15,000 homes were destroyed and 25,000 homes damaged. The City of Rockport was hit the hardest as the Category 3+ wind field moved into that area causing both extensive wind and surge impacts. The highest surge observed in Harvey was generally in the range of 9-11 ft.
Harvey meandered in light steering currents, “stuck” between a mid-tropospheric high pressure system over the Four Corners states and another mid-troposphere high over the Gulf of Mexico. Torrential rains fell over Houston Metro and the Golden Triangle near a stationary front which formed on the north and east side of Harvey.
The rainfall of Harvey was truly incredible. A storm total of 60.58 inches was confirmed Nederland, TX; 60.54 inches in Groves, TX. Much of the heaviest precipitation fell in the first 72 hrs of the event. Previous continental US record for a tropical cyclone is 48 inches in Medina, TX (1978). The extreme nature of Harvey was displayed in that 18 values over that continental record of 48 inches reported across southeastern TX, with 36-48 inches recorded across the Houston metro area. However, Multi-Sensor Precipitation Estimates (MPE), which includes radar-derived rainfall intensity estimates suggests 65-70 inches where few observations were available or observations failed early in the event. Maximum rainfall measured in Louisiana was 23.71 inches in Vinton, LA, with MPE suggesting a more representative 40 inches as
Southeast Southwest LA obs were sparse.
The large-scale or synoptic set up for the Harvey exceptional rainfall event is not particularly unique. Heavy rain bands formed along a modest frontal boundary situated initially near Houston, then the Golden Triangle region in Southeast TX (Beaumont, Port Arthur, Orange, TX area). Enhanced convergence and convective lift with warm cloud droplet precipitation processes allowed for enhanced rainfall rates in abundant thunderstorms. The combination of extremely high rainfall rates of up to 5-7 inches per hour and the stationary nature of the near coastal frontal boundary and Harvey itself contributed to the extreme total accumulation and massive flooding.
NOAA analysis determined that areas of Southeast TX experience a flood with an annual probability of <0.1% (equivalent to a >1000 year flood event). I believe this is one of the most important parts of the National Hurricane Center report, so I’ll quote it:
While established records of this nature are not kept, given the exceptional exceedance probabilities, it is unlikely the United States has ever seen such a sizable area of excessive tropical cyclone rainfall totals as it did from Harvey.
In addition to storm surge, wind and flooding rains, Harvey produced 57 tornadoes (many in the Houston Metro area) and killed 68 people directly with an additional 35 indirect deaths. All direct deaths were in Texas and it was the deadliest tropical cyclone for Texas since 1919. All but three direct deaths were caused by freshwater flooding.
According to NOAA, preliminary damage analysis suggests estimated damages of $125 billion, making Harvey the second-costliest hurricane on record in the North Atlantic basin, only behind Hurricane Katrina, when adjusted for inflation.
Connection to Anthropogenic (human-caused) Climate Change
During and immediately following the events of Hurricane Harvey, there was intense controversy over even discussing climate change as it related to the extreme events related to Hurricane Harvey. Even mentioning climate change in reference to an individual extreme weather event. A lot of opinions were thrown about, but the science of climate change has evolved dramatically in the past 10 years and climate researchers have a much better understanding of many of the connections between climate variables and the statistics of weather which make up the recent past and current climate. From this, attribution studies can be conducted to determine a likelihood of connection to the changing climate regime. A attribution study was done by World Weather Attribution (#2 below) and the probabilistic statistical analysis determined that the record rainfall from Harvey was approximately a) 3 times more likely and b) 15% more intense in terms of rainfall rate because of climate change. One location witnessed a return period for extreme rainfall of 9000 years with a high degree of statistical confidence. The impacts were consistent with what would be expected with 1 degree C+ of global warming since the late 19th century (the world has thought to have begun warming because of humanity since the mid 18th century). I did an extensive post previously during this most recent hurricane season on the climate change connection with includes references to numerous recent peer reviewed papers HERE.
See my previous posts in this blog on Hurricane Harvey from last August HERE.
–Meteorologist Nick Humphrey