Slow-Moving Hurricanes Near Coasts Are Flood Nightmares - Why Sally Is A Threat

Several years ago I was monitoring Hurricane Harvey (2017) as it approached Texas. I knew that it was going to be a major storm with all of the things associated with that categorization. However, phase 2 of Harvey looked equally daunting because the storm was projected to stall. With my meteorology background, I knew that meant a flood disaster was on our hands. Hurricane Florence (2018) evoked similar concerns as it stalled in the Carolinas. Tropical Storm Sally is currently on approach to the U.S. Gulf Coast and is expected to intensify to a Category 2 hurricane before landfall. There is something else about the forecast that worries me, however. It slows down near the Gulf Coast and that could be a recipe for flood problems too.

I want to draw your attention to the 3 circles with an “H” in the graphic above. The time frame between the first one at 8 pm (Monday) and the third one at 8 pm (Tuesday) is 24 hours. It is important to remember that hurricanes are not dots on a map but large rain-producing machines. Hurricane Sally slows down as it approaches the Gulf Coast and lingers for roughly a day before making landfall. Even after it weakens to a tropical storm, it will still cause rainfall in the region. That’s a problem folks. A 2019 study published in one of the top science journals, Nature, found that hurricanes are stalling more often along North American coasts so this is something we better get used to, unfortunately.

Like Hurricane Harvey (2017) and Hurricane Florence (2018), Sally has the potential to be a life-altering rainfall event. The latest projections from NOAA suggests that rainfall totals will be within the 1 to 2 feet range, and isolated higher amounts are never out of the question. I always sound the alarm for this type of storm because the manner in which hurricane impacts are communicated is often biased towards the wind threat or category. The Saffir-Simpson scale, however, conveys nothing about rainfall. In fact, Harvey and Florence dumped the majority of flood-producing rainfall well after being “downgraded” to a lower category hurricane or tropical storm.

I have been fascinated by the rainfall aspects of hurricanes since publishing a study in 2007 on the relative rainfall contributions of different category storms. A more recent study, of which I am a co-author, was published in the Bulletin of the American Meteorological Society. It proposes an alternative way to communicate hurricane rainfall threats to emergency managers, media, and the public. Instead of focusing on wind and category, it talks about the rainfall risk in a statistically-robust but easy-to-understand way based on frequency and magnitude of extreme rainfall events.

I do have other concerns about the slow-moving storm:

1. Sustained “push” of water to the east of the storm from Louisiana to Florida, a region quite vulnerable to storm surge.
2. A persistent tornado threat mainly to the right of the center as outer bands rotate over the coastal regions.
3. The ever-present wind threat associated with a category 1-2 level storm.
4. New Orleans and other places in this region are very low-lying and flood vulnerable.

Hurricane and storm surge warnings/watches have already been issued by the National Hurricane Center for much of the area. Remember, it is the water aspects of a hurricane that are typically deadliest (surge and inland freshwater flooding) not wind. Be prepared and resist the urge to say, “it’s just a Category 1 or 2 storm” because that is a potentially fatal underestimation.