Sea Science: by Jordanna Sheermohamed
With the 2019 hurricane season officially underway, many are actively keeping a wary eye on the Atlantic basin. Portions of the U.S. were lucky to dodge direct impacts during the 2018 season, but luck is never a repetitive guarantee with Mother Nature. All it takes is one system during that six-month window of June 1 to Nov. 30 to change lives forever.
Hurricane preparations often focus on comforts in the aftermath: purchasing enough food and water to last days or weeks (the general rule is 1 gallon per person per day); filling vehicles with gas; or purchasing generators to maintain the power increasingly indispensable in modern life.
There is no doubt that the direct impacts of a hurricane can be terrifying, whether from the 74-95 mph wind speeds of a Category 1 storm or the devastating 157+ mph winds of a Category 5. Substantial rains can produce flooding, such as in 2017, when Hurricane Harvey situated itself over southeast Texas for several days as a result of a blocking high pressure to the north.
Unfortunately, its position also allowed Harvey to access an infinite moisture feed from the nearby Gulf of Mexico, which in turn allowed the system to produce nearly 52 inches of rain over parts of the great Houston area. This is nearly equal to the average amount of precipitation – about 49 inches – that Houston normally receives over the span of an entire year.
Nevertheless, despite legitimate worries about the high winds, the endless rain or the lack of general provisions, there’s another hurricane impact considered the worst of all: storm surge. Many are unaware of how storm surge can exacerbate the destruction, especially if coupled with normal high tides.
The National Hurricane Center defines storm surge as “an abnormal rise of water generated by a storm, over and above the predicted astronomical tides.” If astronomical tides and storm surge simultaneously occur, this produces a storm tide that can produce extreme flooding along coastal areas, “up to 20 feet or more in some cases,” according to the NHC.
Surges are a combination of several factors: the storm’s intensity and size; its forward speed; its direction and angle of impact or proximity to the coastline; the topography and geography of the surrounding regions, such as the presence of rivers, inlets or bays; how far the winds have traveled across the ocean (called the “fetch”); and changes in the ocean floor depth as the system approaches the coast.
“A Cat 4 storm hitting the Louisiana coastline, which has a very wide and shallow continental shelf, may produce a 20-foot storm surge, while the same hurricane in a place like Miami Beach, Florida, where the continental shelf drops off very quickly, might see an 8- or 9-foot surge,” according to the NHC.
Those who live inland sometimes fall victim to the false pretense that they are immune to the effects of the sea, but surges can reach tens of miles inland, especially in areas close to or below sea level. Hollywood often portrays storm surges as the infamous “wall of water” that swallows up a coastal city, but in reality, the water may rise as quickly as several feet in just a few minutes. The storm surge itself travels with the same forward speed as the hurricane, which on average is 10-15 mph.
The National Weather Service has produced a model to aid meteorologists in better predicting storm surge effects. It’s called the SLOSH model – Sea, Lake and Overland Surges from Hurricanes. This model considers a hurricane’s pressure, size, forward speed and track for historical events, hypothetical scenarios, and predicted hurricanes in order to produce a modeled wind field that would indicate the storm surge potential.
There’s an old adage that highlights a general protocol when it comes to Mother Nature: Run from water, hide from wind. This certainly underlines the state of mind necessary when it comes to dealing with hurricane-produced storm surge.
Jordanna Sheermohamed is president and lead meteorologist of Weather Forecast Solutions, a weather-forecasting firm. Comments are welcome below.