Understanding the Strength of Tropical Storms: Pressure, Winds, and Surge

One week ago today, Hurricane Laura made landfall near Cameron, Louisiana…a community whose residents are no stranger to the devastation of tropical storms. Category 3 Hurricane Audrey caused over 300 deaths in the small town in 1957, and nearly 50 years later, the town was struck again. While everyone fortunately evacuated before Category 5 Hurricane Rita, the storm devastated the town in September 2005. Then, in the midst of recovery from Rita, in came Hurricane Ike, leveling the town with a 12 foot storm surge. Ike destroyed over 90% of the homes within the parish and caused catastrophic flooding in every part of the parish. The damage sustained by both Rita and Ike led to stricter building codes and higher insurance costs, leading to the town’s dramatic reduction in population – from 1,965 people in 2000 to just 406 in 2010.

Credit: NOAA

Laura barreled onto shore as a Category 4 hurricane, with a minimum central pressure of 938 millibars, maximum sustained winds of 150 miles per hour, and storm surge over ten feet at approximately 1:00 AM CDT August 27, 2020. As Laura continued inland, the storm weakened to Category 2 hurricane by 5:00 AM August 27, 2020. By 12:00 PM on August 27, Laura had been downgraded to a Tropical Storm with maximum sustained winds of 70 miles per hour. By 4:00 AM on August 28, Laura was over northeastern Arkansas and the National Hurricane Center reported that the storm was losing tropical characteristics as it continued trekking northeast. Laura became a post-tropical cyclone with maximum sustained winds of less than 39 miles per hour by 5:00 AM August 29, 2020.

While we will likely delve deeper into the aftermath of Laura at a later date and have discussed tropical weather fairly frequently on the blog, we feel that it’s important to first take the time to discuss the three main attributes generally discussed when talking about hurricane strength at landfall: winds, pressure, and storm surge. Each tropical storm can bring a variety of unique, life-threatening hazards to a given location. It’s critical to understand each risk, and to know your individual risk, particularly if you live in a storm surge evacuation zone or in an area where flooding could occur. Even if you’ve previously survived a storm in your area, future hurricanes may bring different hazards.

Winds

The Saffir-Simpson Hurricane Scale utilizes a 1-5 ranking system (listed below) that estimates property damage based on a hurricane’s sustained wind speed and has provided an excellent tool for alerting the public about the possible impacts from hurricanes. However, the scale only tells part of the story, as it does not address the potential for other hurricane-related impacts, such as storm surge, rainfall-induced floods, and tornadoes. Thus, meteorologists are required to convey these risks to the general public independent of the Saffir Simpson scale.

  • Category 1: 74-95 miles per hour
  • Category 2: 96-110 miles per hour
  • Category 3: 111-129 miles per hour
  • Category 4: 130-156 miles per hour
  • Category 5: 157+ miles per hour

Hurricane Allen (1980) attained the strongest winds of any Atlantic hurricane with peak sustained wind speeds of 190 miles per hour, followed by the “Labor Day” storm in 1935, Gilbert in 1988, Dorian in 2019, and Wilma in 2005 (all of which had peak sustained winds of 185 miles per hour).

Pressure

Another frequently reported attribute that is very closely related to wind speeds (but generally less understood) is the pressure (also known as the barometric or atmospheric pressure) of the storm. Changes in air pressure tell meteorologists how a storm is evolving over time. For example, if the central pressure within a tropical storm is increasing, it may be a sign that the storm is losing strength. On the other hand, as the central pressure of a storm drops, it indicates that the storm is gaining strength, leading to increasing wind speeds and a more dangerous storm.

Hurricane Wilma (2005) reached the lowest barometric pressure ever recorded in an Atlantic Basin hurricane: 882 millibars. Sustained winds of 185 miles per hour attest to the ferociousness of the storm. Hurricane Gilbert (1988; 888 millibars), the “Labor Day” hurricane (1935; 892 millibars), Rita (2005; 895 millibars), and Allen (1980; 899 millibars) round out the five lowest pressures in Atlantic hurricane history.

Storm Surge

Although hurricanes are well known for their strong and destructive winds, storm surge is the greatest hurricane-related threat. Storm surge is water that is pushed toward the shore by winds swirling around the storm. This rise in water level can cause severe flooding in coastal areas, with the potential storm surge for a location dependent on a number of different factors, including storm intensity, forward speed, size, angle of approach to the coast, as well as the shape and characteristics of coastal features such as bays and estuaries. With much of the United States’ densely populated Atlantic and Gulf Coast coastlines less than 10 feet above mean sea level, the danger from storm surges is tremendous.

Hurricane Katrina (2005) is a prime example of the damage and devastation that can be caused by surge. At least 1500 people lost their lives during Katrina and many of those deaths occurred directly, or indirectly, as a result of storm surge. Historically, nearly half of direct fatalities from land falling US hurricanes are attributed to storm surge.

Credit: NOAA; Chart adapted from Fatalities in the United States from Atlantic Tropical Cyclones: New Data and Interpretation by Edward N. Rappaport, 2014.

Extreme Rainfall

While landfalling storms are generally characterized by the three metrics listed above, we would be remiss not to mention extreme rainfall. Tropical cyclones have particularly warm air that holds a tremendous amount of moisture. The moisture cools as it rise and condenses into heavy rain, which can occur not only at the coast, but also many miles inland – leading to flooding that can continue for days or weeks after a storm. It is critical to follow evacuation orders and never drive into flooded roadways.

Over a four-day period, Hurricane Harvey (2017) brought more than 40 inches of rain to many areas of southeastern Texas (including peak accumulations of 60.58 inches in Nederland), causing catastrophic flooding and many deaths. Inland flooding, far away from storm center and after storm has weakened in strength, can cause devastating rainfall, particularly in mountainous areas (such as Irene in 2011 and Floyd in 1999).

“Run from the water, hide from the winds”

When you’re at risk from flooding, it’s important to get to high ground away from bodies of water and any flood-prone areas. When there are high winds, but no danger of flooding (rare in hurricanes, but also important during any storm), it’s important to shelter in place in a sturdy structure, away from doors and windows. This is why most evacuation orders are given for water, not wind.

Lingering Dangers

Hurricane-related hazards continue after the storm has passed. Many fatalities are reported after storms end, due to heart attacks, issues related to power outages, and accidents. At the time of this writing, 41 deaths have been confirmed from Hurricane Laura in the U.S. and Caribbean. Of the 18 deaths in the U.S., most have been from carbon monoxide poisoning when using generators indoors. When using generators, remember the following:

  • Use portable generators outside the house and garage.
  • Keep generators at least 20 feet away from windows, doors, and vents.
  • Install a battery-operated or battery-backup carbon monoxide detector in hallways outside bedrooms.
  • NEVER ignore a beeping carbon monoxide alarm. Go outside and call 911 if the alarm sounds.
Courtesy: CDC (https://www.cdc.gov/cpr/infographics/br-hurricanes.htm)

As our climate changes (as discussed in this blog), the impacts from hurricanes are also changing. Coastal communities are particularly vulnerable as the rising sea levels worsen storm surge flooding. Understanding the changes and risks are key in being proactive in hurricane preparation.

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