Hurricane season is fast approaching. That is not something we look forward to, but as coastal residents, it is something we must plan for. Two distinct challenges face emergency managers, public officials, and all who live in an area likely to be threatened by a tropical storm or hurricane.
The first challenge has to do with the COVID-19 epidemic, which will be covered elsewhere in this guide. Needless to say, the ongoing epidemic tremendously complicates hurricane planning and preparation.
The second challenge is a longer term, systemic change that will impact hurricane preparedness this year and beyond. This is related to the slower movement and increased stalling observed in hurricanes and tropical storms.
Several research articles have appeared in recent years documenting this trend. Scientists with NOAA and other institutions have found that the forward motion of hurricanes and tropical storms has decreased measurably since 1949 along with a greater tendency for these systems to stall.
We have experienced this trend locally with Tropical Storm Imelda last season, Hurricane Harvey in 2017, Hurricane Isaac in 2012 and Tropical Storm Allison in 2001. Elsewhere, we have seen Hurricane Dorian, which nearly stalled as it slowly crossed the Bahamas in 2019 as a Category 5 and Hurricane Florence in 2017, which slowed to a crawl dumping a state record 33.90 inches of rain in Swansboro, North Carolina.
To explain the significance of this change, a brief description of the movement of tropical systems in the Atlantic Basin is useful.
Tropical systems typically form in the tropics just north of the equator (10-20 degrees north latitude). These disturbances move along the prevailing atmospheric wind flow, which in the summer into fall tends to be east-to-west in that region due to a persistent upper-level high to the north.
The clockwise circulation around this upper high (the Azores or Bermuda High) causes the wind flow and embedded disturbances to track generally in a westerly direction across the Tropical Atlantic Basin.
In a typical season, about 60 tropical disturbances form in the Tropical Atlantic. About two-thirds of the time, a weakness will develop in the upper high at some point as a system moves westward. This influences the storm to turn northwest or northward, and then curve into the mid-latitude westerlies, sending them either into the north Atlantic or northern Gulf of Mexico east of Texas. When weakness fails to occur, then the storms will continue west toward Texas or Mexico.
The Texas coast, however, can find itself situated near the western fringes of the Bermuda High and east of prevailing low and high pressure systems over the Southwestern United States.
This sometimes results in a zone of weak steering currents over the western Gulf of Mexico. The recent research on hurricane movement suggests that these periods are becoming more common due to a general decrease in atmospheric circulation.
The implications of this for hurricane and tropical storm planning and preparedness are staggering.
Slower moving storms are troubling because of the following:
1. Increased risk of excessive rainfall and flooding: We should be prepared for more major flooding events such as Tropical Storm Claudette (1979), which virtually stalled north of Houston (with 42 inches in Alvin); Tropical Storm Allison (1989), which made a slow loop over East Texas (with 21 inches of rain near Beaumont and 30 inches just east of Houston); Tropical Storm Allison (2001) with 36.99 inches of rain and massive flooding in Houston; Hurricane Harvey (2017), which moved inland near Rockport, stalled and then drifted back into the Gulf of Mexico producing an unprecedented 56-inch rainfall total in Friendswood and amounts of 40 to 50 inches over large sections of Galveston County and Southeast Texas; and Tropical Storm Imelda (2019) which made a slow loop north of Houston after ambling ashore southwest of Freeport causing major flooding from the Galveston-Houston area eastward with rainfall totals of 40 inches near Winnie and 17 inches in Galveston, Anahuac and Kingwood.
Understanding your flood zone risk, having adequate flood insurance, building higher raised structures, and having a plan when an excessive rainfall event threatens are good starting points in preparing for potential flooding.
2. Higher tides and water buildup: The longer a storm takes to move inland, the more likely that even for fairly modest storms, winds blowing onshore ahead of the center will result in higher inundation levels. A good example of this factor is the contrast between Category 2 Hurricane Gustav (2008) with peak winds of 115 mph, which produced a high tide of 7.5 feet as it moved ashore over Louisiana, while the very slow-moving Category 1 Hurricane Isaac (2012) with peak winds of 80 mph at landfall produced an 11-foot tide over the same region.
So hurricane planning should include steps for dealing with higher water levels and storm surge potential as well as excessive rainfall.
3. Prolonged exposure to high wind: The slower a storm moves the more likely it is that structures and natural features, such as trees, will be exposed to a prolonged period of high winds. Wind damage is related to both wind intensity and duration. A longer period of strong winds means greater stress to objects in their path. In prolonged wind events, evacuation may be a better option than sheltering in place. In any case, extra supplies will be needed should you stay home due to the prolonged period of disruption from the storm.
4. Longer evacuation periods: Slow moving or stalled storms will dramatically increase the amount of time that evacuees may need to shelter before being able to return home. It also will increase the time required to restore utilities and other essential services, slowing down recovery efforts.
5. Delaying preparations: Finally, another negative consequence of slower moving storms will be a tendency for some people to delay preparations or minimize the potential threat. With water buildup and increasing tides, exit routes could be blocked, trapping those who hesitate as tragically happened with Hurricane Ike.
Rather than being discouraged, knowing what to expect can allow coastal residents to mitigate some of the risks associated slower moving tropical systems.
Awareness is the first step in proper planning and preparation.