Hurricane Laura

August 2020

Late August saw two tropical cyclones impact the US Gulf Coast in quick succession. Tropical depression Marco made landfall on Tuesday 25 August and was closely followed by Hurricane Laura on Thursday 27 August.

Classified as a Category 4 hurricane when it hit Louisiana, Laura was the strongest storm to have made landfall in the state in more than a century, surpassing record wind speeds previously observed during Hurricane Katrina in August 2005 (WSJ, 2020). Additionally, forecasts showed Hurricane Laura impacting many of the same locations that were struck by Tropical Storm Marco’s heavy rainfall, with existing wet conditions causing concern about the risks of further storm surge and inland flooding.

Storm surge

Leading up to landfall, Hurricane Laura was forecasted to have an “unsurvivable” storm surge by the National Hurricane Centre, with sustained windspeeds of 150 mph and a peak surge forecast of 4.5-6m (15-20 feet) in places (BBC, 2020a; NHC, 2020).

Fortunately, observed storm surge levels were not as great as forecasted, with a surge ranging between 2.7-4.5m (9-15 feet) (NASA Earth Observatory, 2020). One US Army Corps of Engineers gauge along the Mermentau river measured a peak inundation of 3.9-4.5m (13-15 feet) above dry ground during the height of Hurricane Laura’s passing (Weather Channel, 2020). Calcasieu Pass in Cameron recorded at 2.7m (8.9 feet) at the height of the hurricane (Yale Climate Connections, 2020).

Early explanations about the reduced surge suggest that the predominant wind direction from the storm (east to west) failed to push water up the Calcasieu River basin and therefore reduced the level of inland flooding experienced in urban areas like Lake Charles (NASA Earth Observatory, 2020). Additionally, initial forecasts were based on Hurricane Laura making landfall at the mouth of the Sabine River. As Laura changed direction and made landfall 40 miles further east than originally forecasted, reports suggest that this helped reduce the storm surge seen in more populated areas like Cameron and Lake Charles (NOLA.com, 2020).

Rainfall and River flows

During Hurricane Laura, rainfall concentrated near the border between Louisiana and Texas state. The highest total rainfall recorded on 27 August 2020 ranged from 169.9 – 263.7mm (6.6-10.3 inches) (Table 1 and Figure 1).

Figure 1: (top) Total rainfall (mm) over continental US for 27 August 2020, (middle) 28 August 2020 and (bottom) 29 August 2020. Data source: CPC US Unified Precipitation data provided by the NOAA/OAR/ESRL PS; processed by JBA Risk Management

JBA extracted a historical daily total rainfall from CPC US Precipitation Data from January 1948 until August 2020 and carried out an extreme value analysis to investigate return periods at individual sites, based on the maximum rainfall recorded at the peak of Hurricane Laura (Table 1). As Hurricane Laura moved further inland towards Arkansas and Tennessee, rainfall decreased (Figure 2).

The highest one-day total recorded as a result of Hurricane Laura was at Lacassine station near Lake Charles, with a daily total of 263mm (10.3 inches), which JBA estimates to have a return period of 150 years.

Table 1: Highest rainfall value recorded on 27 August 2020 and associated return period in years (Source: US Climate Data, 2020)

 
Figure 2: Track of Hurricane Laura showing the date and time of its path (USGS data).

Lake Charles was particularly hit during Hurricane Rita in 2005 and wind damage from Hurricane Laura was stated to be equivalent to, or even more severe than, windspeeds experienced during Rita. However, damages are reported to be lower this time due to improved resilience of houses built after Hurricane Rita (New York Times, 2020)

Additionally, strong winds from Hurricane Laura were reported to have forced the water of Neches River, Texas, to move upriver in the opposite direction. At its peak, water was flowing at about 7600 cubic feet per second upstream during Hurricane Laura. Comparatively, two days prior to the arrival of Hurricane Laura, the river flow was approximately 2500 cubic feet per second (USGS, 2020). At Calcasieu River near Lake Charles, the river peaked at about 1.8m (6 feet), around twice the usual river height.

Impacts

The impacts of flood from Hurricane Laura were not as significant as first feared, although the damage caused was still devastating to coastal areas between the Louisiana and Texas border.

At the peak of the storm, 1.5 million people were under some form of evacuation order (The New York Times, 2020), which helped reduce the number of fatalities. Tragically, though, at time of writing, 18 people are known to have lost their lives in the US (Accuweather, 2020), four of whom died as a result of falling trees (BBC, 2020b). Additionally, nearly two dozen people in Haiti and the Dominican Republic were killed by the storm according to various media reports (Insurance Journal, 2020).

The main areas to have flooded were wetland areas, such as Rockefeller Wildlife Refuge, and mainly to the east of the urban area of Cameron, Louisiana. These wetland areas are thought to have received the brunt of the storm surge. Impacted areas with greater population include Crystal Beach, Texas and Cameron, Louisiana. There are additionally some reports of a small number of homes being flooded in Lake Charles, Lafayette, Port Arthur.

Due to a change in direction from the hurricane, the higher populated gulf coast areas were therefore spared from the large, predicted storm surge. Heavy rainfall in the wake of Hurricane Laura also did not cause any significant reports of pluvial (surface water) flooding.

The main source of damage from Hurricane Laura was caused by the extreme wind speeds. Despite only having a central pressure of 938 millibars, Hurricane Laura was able to sustain windspeeds of up to 150 mph, with even higher gusts at landfall. 30 miles inland at Lake Charles, windspeeds were also clocked at 137 mph (Washington Post, 2020). Such windspeeds at the coast and so far inland help highlight the strength of Hurricane Laura.

Estimates suggest insured property losses from Hurricane Laura in the United States are around $8.7 billion, with $200 million in damages in the Caribbean (Reinsurance News, 2020).

Outlook

The National Hurricane Center is currently monitoring a disturbance in the Caribbean Sea with a 90% likelihood, at time of writing, of developing into another tropical cyclone. (NHC, 2020).

JBA Risk Management has nationwide return period flood maps for the USA at 5m resolution and a Global Flood Model to help you understand and manage flood risk more effectively. Please get in touch using the form above for more information, or email the event response team.