Hurricane Dorian: A study in hurricane impacts

At the time of this writing on Tuesday, September 2, Hurricane Dorian is currently bringing heavy rain, winds and storm surge to the Bahamas as it makes its slow trek towards the eastern U.S. coastline. This storm has proven to be very difficult to forecast, in part because of its rapid intensification and slow speed. Let’s look back and see how this dangerous situation evolved and where the forecast impacts are expected in the next 3-5 days.

Satellite imagery of Hurricane Dorian over the Bahamas, September 1, 2019.

Hurricane Dorian, now a major hurricane, began its life as Tropical Depression number 5, about 650 miles east-southeast of Barbados on the morning of August 24. By evening, the winds inside the storm had become strong enough that it became a named Tropical Storm, Dorian. A tropical depression becomes a named tropical storm when its wind speeds exceed 38 miles per hour. The storm then keeps its name for the remainder of its life cycle. Once the winds exceed 74 miles per hour, the tropical storm becomes a hurricane. Hurricanes are rated on the Saffir-Simpson scale from Category 1 to Category 5.

The storm tracked slowly to the north-northwest at between 10 and 15 miles per hour. By the early morning hours of August 28, Tropical Storm Dorian had entered the Caribbean Sea and was forecast to impact Puerto Rico. At this time, the official track forecast took Dorian to the northwest, and remained east of the Bahamas.

Forecast track and intensity for Dorian, August 28, 2019. Courtesy NOAA/NHC.

This is a good time to go over what exactly what information theforecast graphics from the National Hurricane Center, such as the one above, contain. The location of the storm at the time of forecast issuance is given by the orange circle. The black circled letters represent the center location of the storm; S is for Tropical Storm, H is for Hurricane, and M is for Major (Category 3 or higher) Hurricane. The shaded area around the track is what is known as the ‘cone of uncertainty’. Essentially what this area represents is the range of possible locations of the storm center. Notice that as the forecast time increases, the possible range of solutions increases, and the cone widens. It is very important to note that this cone does NOT represent the area of influence of the storm, only the range of possible locations of the center. Storm effects, such as heavy rain, winds, and storm surge can impact areas very far removed from the center, as we will see in a few minutes.

Forecast track and intensity for Dorian, August 29, 2019. Courtesy NOAA/NHC.

By the next morning, August 29, a marked change had occurred in the forecast. Hurricane Dorian was now forecast to make a sharp turn westrward, and potentially strike the Florida panhandle as a major hurricane. The storm was located in an area of very favorable conditions for intensification: warm water and weak vertical wind shear, or changing of wind speed and direction with altitude. A strengthening upper level ridge, or high pressure area, over the Atlantic, was forcing the storm further westward than the initial track forecasts had indicated. Floridians all up and down the east coast of the peninsula were encouraged to begin making preparations for a landfalling hurricane within 3-5 days.

Forecast track and intensity for Dorian, August 31, 2019. Courtesy NOAA/NHC.

Two days later on August 31, the storm remained a very strong Category 4 storm and continued to move very slowly to the west or west-northwest. By this time, the forecast models had begun to suggest that the storm may begin to be steered northward prior to making landfall over Florida. The forecast issued on the morning of August 31 indicates that the initial turn to the north was expected to begin by 2:00 AM on Tuesday September 3. Slow-moving hurricanes exacerbate many of the hazards which accompany them, prolonging strong winds, storm surge, and heavy rainfall. Notice that, in the graphic below, tropical storm force winds were expected to extend from the Keys all the way into the Carolinas, with the highest confidence still across the eastern Florida coastline.

Probability of occurrence and arrival times of tropical storm force winds for Dorian, August 31, 2019. Courtesy NOAA/NHC.

By the evening of Sunday, September 1, Dorian had become a very dangerous Category 5 hurricane. The storm center was located over the Bahamas. The strongest wind speeds and storm surge effects are found in the region immediately outside the center, known as the eye wall. The satellite image below shows the scale of the storm very well, and in fact lightning and thunderstorms in the spiral rainbands at the outer edges of the storm were already impacting parts of Florida at this time. The highly symmetrical nature of the eye, as well as the rapid change from cloud cover to clear skies in the center, are all indicative of a very strong, well-developed tropical system.

Satellite imagery of Hurricane Dorian over the Bahamas, September 1, 2019.

The storm made landfall on Great Abaco Island with maximum sustained winds of 185 miles per hour, tying the record for the strongest recorded landfall (the 1935 Labor Day hurricane).

This photo, taken from a NOAA reconaissance aircraft, shows impressively how there can be clear skies in the center, or eye, with tall thunderstorms surrounding the center in the eye wall.

At 1:00 AM on Tuesday morning (September 2), the National Hurricane Center tweeted this message for the Bahamas:

Notice that sustained wind speeds in the storm were 180 miles per hour, and wind gusts were expected to exceed 200 miles per hour. Storm surge, or the rise in sea level due to the strong winds pushing water onshore, was expected to be between 18 and 23 feet above normal tides. The threat from the storm could not be underestimated; a life threatening situation was unfolding in the Bahamas, and continues to persist at the time of writing of this blog. The Nassau Guardian reports that many people are stranded and emergency rescuers are not yet able to reach them.

Hurricane Dorian, as of the latest advisory from the National Hurricane Center, is expected to continue to slowly make its turn northward and hug the coast of the Eastern Seaboard over the next 5 days. This means that the storm’s impacts, including tropical storm and hurricane force winds, potentially life threatening storm surge, and heavy rainfall, may impact areas of the eastern United States through Saturday. The most likely landfall at this point looks to be on North Carolina’s Outer Banks, although a only a slight westward shift of the track could result in a landfall anywhere from Florida to South Carolina. Regardless of where the center of the storm makes landfall, the effects will be felt all along the East Coast. Further, inland flooding due to the rising of warm, tropical air over the Appalachian mountains could pose a threat for areas which are very far removed from the storm center itself. The National Hurricane Center is a great source of information on how to ensure you and your loved ones are prepared before a storm, as well as what to do when a threat is imminent. As always, stay tuned to your local National Weather Service for up-to-date forecasts, watches, and warnings.

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