At a Glance

The last three years have demonstrated that flooding remains the biggest killer in tropical cyclones.Intensity forecasts remain challenging, with rapid intensification just before landfall particularly concerning.The forecast for tropical cyclone tracks, however, continues to improve.The danger from tropical cyclones exists even after the storm.

Several destructive hurricanes have slammed the U.S. the past three years and taught us a few important lessons about the state of tropical cyclones as we head into this Atlantic hurricane season.

Eight Atlantic hurricanes made landfall in the U.S. over the last three years. In 2016, hurricanes Hermine and Matthew made landfall and 2017 saw Harvey, Irma, Maria and Nate. Then, in 2018, Hurricane Florence and Hurricane Michael struck the U.S.

Below we take a closer look at what these recent storms have shown us.

Flooding Is the Biggest Killer

Water remains the greatest threat from tropical cyclones.

The National Hurricane Center (NHC) said that 83 percent of direct deaths from tropical cyclones in the U.S. from 2016 through 2018 were water related.

In addition, most of the water-related deaths over the past three years were from inland flooding, with only 4 percent due to storm surge. Historically, storm surge has been the biggest killer from tropical cyclones. Storm surge accounted for 49 percent of U.S. tropical cyclone fatalities from 1963 through 2012, according to a study by .

Extreme rainfall from slow-moving systems with ample available moisture is the reason for that change.

Percentage of direct deaths due to tropical cyclones that are water related. Data excludes Hurricane Maria due to uncertainty related to causes of direct deaths.

(Ken Graham/National Hurricane Center)

For example, in 2018, Florence set new in both North Carolina and South Carolina. Elizabethtown, North Carolina, reported 35.93 inches of rainfall from Florence and Loris, South Carolina, received 23.63 inches. This excessive rain and inland flooding caused .

Five people also died in 2018 in Virginia due to freshwater flooding during Hurricane Michael, while occurred in Florida.

dumped 60.58 inches of rain near Nederland, Texas, and brought tremendous flooding to the Houston area in 2017. There were to freshwater flooding from Harvey and surprisingly there were no storm surge deaths reported from the hurricane's landfall.

Twenty-seven people lost their lives in freshwater flooding during 2016's . The storm brought more than a foot of rainfall from Savannah, Georgia, into eastern Virginia. Fayetteville, North Carolina, picked up over 8 inches of rain in 6 hours, prompting serious flooding. And widespread flash flooding and record river flooding were observed in eastern North Carolina.

The communication of the threat of expected storm surge has also improved and shouldn't be understated in reducing storm surge deaths.

The NHC began issuing storm surge watches and warnings in 2015 but the product wasn't official and . The potential storm surge flooding map became operational in 2016. These products provide valuable information on where storm surge is the greatest threat and how high the surge could be and may be helping to limit fatalities due to storm surge.

Storms Can Rapidly Intensify Before Landfall on Short Notice

Hurricanes Harvey and Michael were two of the strongest hurricanes to strike the U.S. in the past three years.

Hurricane Harvey was a hurricane when it made landfall near Rockport, Texas, on Aug. 25, 2017. Just two days before landfall it was a tropical storm. Conditions in the Gulf of Mexico were and unfortunately that occured right as it was approaching land.

In 2018, was a tropical storm just three days prior to making landfall. Michael rapidly intensified in the Gulf of Mexico just as Harvey did. It continued to strengthen right up until landfall. Upon reanalysis, Michael was reclassified as a hurricane when it made landfall near Mexico Beach, Florida, on October 10.

Hurricane Harvey (2017) and Hurricane Michael (2018) both rapidly intensified into major hurricanes right before landfall.

Forecasting the intensity of hurricanes remains difficult, especially rapid intensification.

There may not always be 5 or more days notice that a powerful hurricane is going to impact an area, so it is important to be ready. It is a good idea to prepare for a hurricane that is one category stronger than forecast.

Track Forecasts Continue to Improve

While the intensity forecast for hurricanes remains challenging, forecasting the track of tropical cyclones has seen great improvement over the past couple of decades. One of the reasons is enhancements in the computer models -- one of the tools that the NHC uses to make their forecasts.

Errors in the track were lowest in 2017, and the improvement in accuracy the past three years is noticeable.

The errors in the path of tropical storms and hurricanes improved by 10 to 40 nautical miles over the last two years compared to the 2010 through 2016 seasons, depending on how far out in time the track is analyzed.

The errors in the track have decreased by two-thirds since the 1990s. So for example, the same track for a tropical system 48 hours away would be off by about 160 nautical miles in the 1990s compared to just 55 nautical miles in 2017 and 2018.

The NHC was not able to issue track forecasts past 72 hours until the early 2000's when improvements in the science and accuracy of such forecasts made it possible to produce a reliable product.

Errors in the track of tropical cyclones continue to improve.

(National Hurricane Center)

Also thanks to those improvements in the forecast track, the forecast cone has become smaller in recent years.

The the most likely track of the center of a tropical cyclone and the width of that cone represents the actual track errors over the last 5 years. Therefore, as those track errors decrease, the width of the cone does, too.

This improved accuracy has likely saved lives and reduced unnecessary preparations and evacuations.

Deadly Dangers Before and After a Storm

Indirect deaths are often an afterthought but can be the cause of more deaths than the storm itself.

The NHC defines an indirect death as "occurring from such factors as heart attacks, house fires, electrocutions from downed power lines, vehicle accidents on wet roads, etc."

Based on current data, Hurricane Michael was responsible for 43 indirect deaths, which is more than twice the number of direct deaths reported of 16.

Indirect deaths from Atlantic tropical cyclones in the U.S. from 1963 to 2012. In the graph, cardio. refers to cardiovascular failure.

(Data from Rappaport and Blanchard, 2015)

Hurricane Maria killed thousands of people in Puerto Rico in 2017. And while the exact number and cause of deaths there is not fully known, there was a lack of electricity, clean water, supplies and housing and as a result, the number of indirect deaths due to Maria may have been as high as around 3,000.

This underscores the importance of being prepared well before a storm and for an extended period after the storm has passed. This includes having plenty of supplies, a plan and a way to safely evacuate.