Five Lessons Learned in the Hyperactive 2020 Hurricane Season
December 29, 2020
As we look back on the extreme weather stories from this past year, the hyperactive 2020 Atlantic Hurricane Season stands out as the highest-impact topic. This hurricane season shattered records for both storm activity and landfalls. The Atlantic observed 30 named storms, which was an all-time record, surpassing the 28 named storms that formed in 2005. Of these 30 storms, 13 intensified into hurricanes, producing winds of at least 74 mph, while six strengthened into major hurricanes, generating winds of at least 111 mph.
Unfortunately, many of these storms tracked westward in the Atlantic Basin, impacting the United States, Caribbean and Central America. These tracks enabled a record-breaking 12 named storms and six hurricanes to make landfall in the United States.
On September 14, five named storms were simultaneously churning over the basin, ranging in strength from Tropical Depression Rene to Hurricane Paulette. At the time, Sally, Teddy and Vicky were tropical storms. All of these systems fit within the broader classification of tropical cyclones, with hurricanes producing maximum sustained winds of at least 74 mph and tropical storms generating maximum sustained winds of at least 39 mph. Like hurricanes and tropical storms, tropical depressions have a closed low pressure system that fuels itself off warm ocean water, but maximum sustained winds are less than 39 mph.
According to Phil Klotzbach, research scientist at Colorado State University, the five simultaneous named storms in the Atlantic Basin had only been surpassed one time, when six storms churned at the same time in 1971.
This blog post reflects on five lessons learned during this hyperactive hurricane season. While enhanced hurricane threats increased stress for coastal populations, lessons learned from such seasons can help better prepare us to become more resilient in the future.
LESSON #1: Seasonal Hurricane Forecasts Are Improving
As the Atlantic Hurricane Season approached, scientific consensus forecast above normal hurricane activity for the basin. In May, NOAA predicted a 60% chance that hurricane season would be more active than normal, and prominent hurricane climate research groups, like Colorado State University, also forecast above normal activity.
Ocean water temperature influences seasonal hurricane activity forecasts, and the presence of warmer than normal water in the tropical Atlantic indicated that enhanced hurricane activity was likely, as warm ocean water provides the fuel for hurricanes to develop. At the same time, the Equatorial Pacific ocean was becoming cooler than normal, a process called La Nina, which typically correlates with increased Atlantic hurricane activity. Cooler water near the Equatorial Pacific ocean reduces thunderstorm activity and wind shear, or cross winds, that can form near Central America and the Caribbean Sea, which inhibit hurricane development in the Atlantic Basin.
The sea surface temperature map on June 1, 2020, clearly depicted warmer than normal water temperatures throughout the Main Development Region of the tropical Atlantic, while the Equatorial Pacific region called Nino 3.4, was generally running cooler than normal.
While hurricane activity forecasts are important, so are hurricane track forecasts, because an inactive season with one or two hurricane strikes near populated areas could have a bigger impact than a more active season when all of the storms stay out to sea.
The North Atlantic Oscillation (NAO) is a climate index that provides insights into forecast positions of the Bermuda High, a dominant area of high pressure in the Atlantic around which hurricanes often track. This year the May-June index values of the NAO were clearly negative, which meant the Bermuda High was likely to be weaker than normal and offset to the southwest. This set up usually favors more hurricanes tracking towards the western part of the Atlantic Basin, with more impacts in the U.S., Mexico, Central America and the Caribbean.
Looking back, both the forecasts for an active hurricane season and for storm tracks to shift farther west were spot-on this year. Although long-term hurricane forecasting still retains considerable uncertainty, forecasts based in credible science that even slightly shift wind and flood risks up or down enable professional sectors from construction to shipping to make optimal decisions.
LESSON #2: Warming Ocean Waters Likely Related to Rapid Intensification
Not only did the 2020 Atlantic Hurricane Season observe a record number of named storms and landfalling U.S. storms, it also broke the record for landfalling rapidly intensifying hurricanes. For the first time on record since 1950, two hurricanes intensified by at least 40 mph in the 24 hours before landfall. When added to hurricanes Michael (2018) and Harvey (2017), we have observed four hurricanes in four years that have rapidly intensified in the 24 hours before landfall along the U.S. Gulf Coast.
The statistics on rapidly intensifying hurricanes are impressive when we zoom out to look at hurricane activity since 1950. In the 71 years from 1950-2020, only nine storms have intensified by 40 mph or greater within the 24 hours before landfall. For the first 50 years of that record, from 1950-1999, only three hurricanes intensified so quickly, or one hurricane approximately every 17 years. Yet in the past four years it has happened four times.
Warming ocean water may explain some of the cause behind the apparent trend in hurricane rapid intensification. In 2017, Kerry Emmanuel, a professor at MIT, published a paper in the Bulletin of the American Meteorological Society that suggested warmer ocean waters will substantially increase the chance of rapidly intensifying hurricanes making landfall by the year 2100, when compared to the climate of the late 20th Century.
We may be seeing that connection already starting to take place. One indication that the Gulf of Mexico may be warming substantially in recent decades can be found in the overnight air temperature trends found at coastal weather stations, like Galveston, Texas. As Galveston observes consistent onshore winds during the summer and early autumn, these increasing temperature trends may indicate the Gulf of Mexico is warming substantially. This season, Galveston observed overnight low temperatures of 87F for the first time since records began in 1871, and it happened four times. Last year was the first time that overnight low temperatures remained at 86F and that happened three times. The graph below shows the remarkable trend in hot nights at Galveston since 1900.
LESSON #3: Another Year, Another Stall
This hurricane season marked the sixth consecutive year in which a tropical weather system stalled out near the coastline, exacerbating the impacts of rain, wind and storm surge. Hurricane Sally crept towards the Alabama coast with a forward speed of only 2 mph, as it approached landfall near Gulf Shores. The slow forward motion put the eyewall over small inlets along the Alabama and Florida coastlines, for more than one hour in some cases, rapidly pushing storm surge into places like Terry Cove, near Orange Beach.
Sally’s slow-motion landfall, follows similar storms over the past six hurricane seasons. This list includes the following storms that caused massive flooding due to prolonged rainfall.
October 2015: Hurricane Joaquin stalled near the Bahamas and shot a plume of moisture that produced prolonged rainfall in South Carolina.
August 2016: A hybrid tropical system stalled in South Louisiana, dumping more than 30 inches of rain in some areas and generating record-high levels on creeks and bayous.
August 2017: Hurricane Harvey stalled over Texas for days, dumping more than 45 inches of rain on much of metro Houston and Beaumont-Port Arthur. Nederland, Texas, recorded 61 inches of rain, breaking the U.S. rainfall record.
September 2018: Hurricane Florence stalled after landfall, dumping more than 30 inches of rain in eastern North Carolina. The runoff from Florence’s rain produced devastating flooding for many days near rivers as far south as South Carolina.
September 2019: Tropical Storm Imelda stalled near the Texas coast, enabling it to generate more than 40 inches of rain in southeast Texas. Imelda’s rains flooded some areas struck by Hurricane Harvey just two years before.
September 2020: Hurricane Sally makes landfall with a 2-mph forward speed in South Alabama, generating extensive flooding from rainfall and storm surge, as well as widespread tree falls.
LESSON #4: Proactive Measures Increase Resiliency
As a Gulf Coast hurricane scientist, I often do field work before, during and after hurricane strikes. This gives me the opportunity to spend time in many disaster-prone communities and learn lessons about how they are making decisions to improve their resiliency.
One of the most encouraging lessons I learned was from Spring Market, a grocery store in Lake Arthur, Louisiana. As I walked into the store on Saturday, October 10, I felt a mixture of confusion and amazement. This area just took a direct hit from Hurricane Delta less than 24 hours before, and the wind impact was so severe that my smart phone could not get a signal. Yet, as I walked into Spring Market, I saw a fully functional, stocked, lit and air conditioned grocery store, serving their community.
When I asked how it was possible that their store was fully functional in the middle of a disaster zone, a worker recounted that the store made the decision to switch to generators before Delta’s destructive winds arrived. This was a different approach than they had taken six weeks before, when they were struck by Hurricane Laura, which knocked out power and spoiled food. The improved response during Hurricane Delta meant they never lost power, saved all their food, and were able to serve the community throughout the disaster. This was one of the greatest examples of improved resiliency that I came across during the 2020 hurricane season.
LESSON #5: Citizen Scientists Get Excited about Deploying Community Flood Monitoring System
As Tropical Storm Beta slowly tracked towards the Texas Coast, CNC Catastrophe and National Claims, in partnership with Flood Information Systems, launched a community flood monitoring program to document flood waters in real time.
The system deployed water level meters that were calibrated on the same scale as local elevation certificates. Citizen scientists reported flood levels throughout the storms, enabling CNC to produce real-time flood inundation maps of impacted areas.
The real-time flood inundation maps enabled people to assess the flood risk for their properties, even if they evacuated from the area. This unique monitoring system, funded on a grant through the Northern Gulf of Mexico Sentinel Site Cooperative, in partnership with Dauphin Island Sea Labs, improves flood risk communication because all of the observations are calibrated to the same system. Typically, flood forecasts are not provided in the same system as elevation certificates that people have for their homes.
In conclusion, the biggest weather story for 2020, at least in terms of collective impact, was the hyperactive hurricane season. We learned that long-term forecasts continue to improve, as this year’s outlook for an active season with storm tracks shifted to the west proved accurate. Warmer than normal water temperatures fueled two hurricanes to rapidly intensify just before landfall along the U.S. Gulf Coast, and we observed a stalled out tropical system make landfall for the sixth consecutive season.
Nonetheless, hardy coastal residents responded in a most resilient fashion. Proactive measures to reduce storm impacts in places like South Louisiana, which observed three landfalling hurricanes, provided lessons for all of us on how to reduce risks. Meanwhile, citizen scientists stepped up to collect flood observations that were fed into real-time flood mapping efforts.
We also learned much about dealing with natural disasters superimposed on a public health crisis, as this most active hurricane season unfolded during the Covid-19 global pandemic. Future blog posts will share stories and lessons learned from these parallel disasters, as well as ongoing challenges and opportunities.
While this topic is too extensive to open up in this blog post, I will close with one observation. While the pandemic and hyperactive hurricane season stretched the resources of disaster-prone communities to deal with joint crises, some unexpected silver linings became visible in the clouds. One of these silver linings was the extent to which online video chat platforms and shared online digital work/ school content enabled people to better connect and reduced potential impacts in disaster zones.
We will discuss these stories in 2021. Have a happy New Year everyone and thank you for your engagement on the topics of extreme weather and disaster science in 2020.