The 1-2-3 punch of Harvey, Irma and Maria made this year's hurricane season seem especially violent. But was it?
For weeks this fall, tropical storms dominated headlines as they caused flooding in Texas, destruction in Florida and devastation in Puerto Rico, among other locations.
Here, we connect with Tom Walsh, head of weather research at Thomson Reuters, to learn more about the three most major storms of the 2017 Atlantic hurricane season:
- Hurricane Harvey: The first Category 3 or higher storm to make landfall in the U.S. since 2005, Harvey broke rainfall records as it dropped as much as 61 inches in parts of southeast Texas, causing catastrophic flooding.
- Hurricane Irma: Hot on the heels of Harvey, Irma killed 43 as it swept through the Caribbean and resulted in 12 deaths in Florida, Georgia and South Carolina.
- Hurricane Maria: This storm wreaked havoc as it swept across the Dominican Republic, Haiti and Puerto Rico. All in all, it is believed to have caused 78 deaths.
Has this been an especially bad hurricane season, or does it just seem that way?
“The way I would describe it is as a ‘very active’ season. When you combine the number of storms and the strength and duration of the storms, there has been a lot to consider. It’s quite pronounced, especially because in recent memory, hurricane seasons have not been as eventful.”
“That being said, has it been especially bad in comparison to other seasons? That depends on how you compare. An extremely strong storm that develops over the Atlantic without making landfall doesn’t seem very severe because it doesn’t affect a whole lot, whereas a weaker storm that hits a major population area can be much more impactful and newsworthy. Storms that don’t hit major population areas don’t get as much attention, but that doesn’t mean they aren’t powerful or that the season is not active.”
What’s been unusual about this hurricane season?
“In this particular season, we had several storms before Harvey, but they weren’t as impactful. Then you had three major hurricanes in a row, Harvey and then Irma and then Maria. We don’t often get hit with a one-two-three punch like that, and I think that created a heightened awareness.”
“These 3 storms in particular also impacted very concentrated population areas and put a lot of property at risk. Houston, for example, is a very large metropolitan area, so the damage and crisis there attracted a lot of attention. It’s not unusual for Houston to get weather that’s affected by tropical storms, or even take a direct hit by a hurricane, but it hasn’t seen a level of destruction like this in a long time.”
The most unusual/historic aspect to this season was the level of activity during September. That month is typically the most active in an average season, but 2017 has been either the most active or nearly the most active on record depending on how it is measured. This was especially noticeable after a quiet August, when hurricane activity tends to ramp up on average.
What has led to this level of storm activity?
“These events tend to be complex, with many factors, but generally speaking, a tropical storm is produced by a cluster of thunderstorms over warm open waters. Tropical systems can develop from thunderstorms associated with a cold front that moves off the Gulf Coast of the U.S., or from thunderstorms moving off the west coast of Africa westward into the Atlantic Ocean, among other key areas of development. When over the ocean, warm sea surface temperatures and low wind shear [the difference in wind speed and direction over a short atmospheric distance] can enable a storm system to develop. How strong it gets depends on the degree of these factors.”
In the particular case of 2017, we entered hurricane season with above average Atlantic SSTs (warm ocean temperatures), and these warm temperatures created the potential for an active season. What was unknown was what the wind shear might be like, which is controlled largely by whether we are in an El Niño-like or La Niña-like. Expectations were uncertain on that front, but by the time September arrived we had trended strongly into La Niña-like conditions, which created favorable low wind shear in the Atlantic. Combined with the warm SSTs, September had high-end potential that was realized and not surprising given the conditions that developed.”
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