Data Literacy & Hurricane Sally
Correct Observations + Incorrect Assumptions = Trouble
As tropical cyclones approach land, they draw the attention of seasoned meteorologists, emergency management decision-makers, and the general public. Every hurricane season, named storms dominate news cycles- perhaps most aggressively immediately before and after they come ashore. During those times, two pieces of critical information are occasionally misrepresented; the strength of a storm and the timeline of projected landfall. When presented without the appropriate context, these data points can be misunderstood and dangerously skew risk perception, to the detriment of decision-makers and exposed populations.
In the second week of September of 2020, an area of low pressure that originated over the central Bahamas crossed Florida and into the warm waters of the Gulf of Mexico, where it became the earliest named “S” storm on record. Tropical Storm, then Hurricane Sally intensified into a Category 2 hurricane as forecasts showed the storm tracking towards the Louisiana/Alabama border.
As Hurricane Sally closed on the Gulf Coast, it slowed. On the 15th of September, the storm’s speed of advance was reported at 2 mph (1). Hurricane Sally’s reported wind speed dipped below 96 miles per hour as the storm’s center, some 60 miles offshore, proceeded to the north at a snail’s pace. In weather bulletins, operational briefings, and on the news, information about the storm was presented as follows: Hurricane Sally had been downgraded to a Category 1 hurricane, and landfall, previously expected to occur on the evening of the 15th, was not expected until mid-day on September 16th (2).
Nothing about either of those two points are incorrect. It’s true: the storm had been downgraded to a Category 1 hurricane and the storm’s landfall was projected to occur mid-day on the 16th. In a vacuum, these data points might suggest that the overall threat had decreased. However, with the appropriate context, the threat of Hurricane Sally had actually increased considerably.
How could that be?
First, downgrading the storm from a Category 2 to a Category 1 hurricane reflects a change in the storm’s strength per the Saffir-Simpson Hurricane Wind Scale. Developed in the early 1970s, civil engineer Herbert Saffir and meteorologist Robert Simpson wanted to create a simple scale for communicating the potential damage of hurricanes, and their progression of Tropical Depression to Tropical Storm to Category 1–5 hurricane is one of the most widely used systems for communicating storm strength today (3).
The Saffir-Simpson Scale is solely a reflection of wind speed. While the Saffir-Simpson Scale may be an excellent indicator for gaining a general awareness of a storm’s destructive potential, it doesn’t account for the most significant tropical cyclone hazard factors. The National Hurricane Center lists the following tropical cyclone hazard factors by order of severity: storm surge, storm tide, heavy rainfall, inland flooding, and finally, high winds (4). This agrees with the prevailing emergency and disaster management community’s assertion that water-related hazards are typically more severe (and often more lethal) than wind-related hazards for tropical cyclones. The Saffir-Simpson Scale does not take into account anything relating to rainfall, storm surge, the size of a storm or its speed of advance. As a measure of the overall threat associated with a tropical cyclone, the categorization of a storm along the Saffir-Simpson Scale is less comprehensive than it is often presented, particularly for “weaker” storms.
Hurricane Sally was never projected to be a significant wind event, compared to recent storms in the same region. As Sally’s wind speeds decreased, reflected in the downgrade from a Category 2 to a Category 1 Hurricane, the storm grew in overall size. Pre-impact storm surge estimates suggested a surge as high as 12 ft to accompany projections of over two feet of total rainfall in some areas (5). When taken to represent the total potential impact of a storm, the Saffir-Simpson Scale alone failed to accurately represent the actual threat to impacted areas. This doesn’t mean that the Saffir-Simpson Scale is a poor metric, but rather that it does not tell the whole story.
As Hurricane Sally grew, its speed of advance slowed. Although the center of the system became near-stationary offshore, the outermost bands of the storm brought heavy rainfall to the Gulf Coast on the morning of September 13th. Tropical storm force winds arrived to the Alabama and Mississippi coasts late on the 14th and torrential rain continued for days. The center of the storm finally crossed over land in the late morning of September 16th, marking landfall.
The National Weather Service defines landfall as the intersection of the surface center of a tropical cyclone with a coastline (6). The National Hurricane Center uses the same definition, adding the following qualifier: “Because the strongest winds in a tropical cyclone are not located precisely at the center, it is possible for a cyclone’s strongest winds to be experienced over land even if landfall does not occur. Similarly, it is possible for a tropical cyclone to make landfall and have its strongest winds remain over the water.” In other words, the projected timeline of landfall may not accurately reflect the actual timeline of tropical cyclone-related impacts.
By the time Hurricane Sally officially made landfall per the National Weather Service definition, coastal areas on the Gulf Coast between New Orleans, Louisiana and Panama City, Florida had been subjected to the system’s impacts for three straight days. While many plans that drive evacuation orders and other last-minute preparation efforts follow timelines associated with the arrival of tropical-storm-force winds, some risk communication and public awareness efforts follow the timeline of landfall instead (7). When operational decision-points and personal evacuation plans are based on the timeline of landfall, the probability of missing the window for appropriate action understandably increases.
On their own, the Saffir-Simpson Scale and the National Weather Service definition of landfall are important to understand. It is equally important to understand what those data points do not represent. The importance of clear communication cannot be understated here. Indeed, one of the driving principles of the National Incident Management System, the United States’ overarching framework for emergency and disaster management, is to promote a commonly established terminology across all federal, state, local, and private emergency management entities; the use of commonly understood language is a matter of public safety (8). To that end, accurately representing the threat that tropical cyclones pose is the ultimate responsibility of those who inform and alert the public. Correct observations, such as a change in a storm’s classification on the Saffir-Simpson Scale or a delay in expected landfall, can become dangerous when incorrect assumptions are made about them.
When presented with the opportunity to engage in risk communication, words matter. Definitions matter. Context matters. Decision-making is driven by a decision-maker’s understanding of the information available when a decision is made, whether it occurs on the watch-floor of an operations center or in a kitchen among family members. When describing the potential threat of a tropical cyclone hazard like Hurricane Sally, a purposeful effort to provide the appropriate context to a storm’s strength, anticipated impacts, and timeline of projected landfall can eliminate the opportunity for information to be misinterpreted.
Sources:
- National Hurricane Center. (2020). Hurricane SALLY Advisory Archive. Retrieved September 16, 2020, from https://www.nhc.noaa.gov/archive/2020/SALLY.shtml
- CBS New York. (2020). Hurricane Sally Downgraded to Category 1 Hurricane. Retrieved September 17, 2020, from https://news.yahoo.com/hurricane-sally-downgraded-category-1-162300148.html.
- National Park Service. (2010). Coastal Geomorphology — Tropical Storms. Retrieved September 16, 2020, from https://www.nps.gov/articles/saffir-simpson-hurricane-scale.htm.
- National Hurricane Center. (2020). Hurricane Preparedness — Hazards. Retrieved September 16, 2020, from https://www.nhc.noaa.gov/prepare/hazards.php.
- National Hurricane Center. (2020). Hurricane SALLY Public Advisory 15 Corrected. Retrieved September 15, 2020, from https://www.nhc.noaa.gov/archive/2020/al19/al192020.public.015.shtml.
- National Weather Service. (2020). Glossary of NHC Terms. Retrieved September 16, 2020, from https://www.nhc.noaa.gov/aboutgloss.shtml.
- Andrew Freedman, J. (2020, September 16). Hurricane Sally live updates: Sally strikes Gulf Coast as Category 2 storm, causing ‘catastrophic’ flooding. Retrieved September 16, 2020, from https://www.washingtonpost.com/weather/2020/09/16/hurricane-sally-flooding-florida-alabama/
- Federal Emergency Management Agency. (2006). NIMS and Use of Plain Language. Retrieved on September 16, 2020 from https://www.fema.gov/pdf/emergency/nims/NIMS_ALERT_06-09.pdf.
