Hurricane Florence made landfall Friday morning in North Carolina. While people along large swathes of the Eastern Seaboard have been dreading the storm for days, you can say one thing: it arrived right on time.
We are smack-dab in the middle of Atlantic hurricane season, which runs from June 1 to November 30. Nearly all tropical storm activity in the Atlantic basin occurs between those dates.
The season peaks between August and October, with September 10 as the day you're statistically most likely to find a tropical cyclone somewhere in the Atlantic basin.
The answer has to do with both wind and water.
Wind shear is the variation of the wind's speed or direction over a short distance within the atmosphere. Stronger wind shear in the spring fades through June and July, and by late August, wind shear reaches a minimum. That's too bad, because wind shear can prevent weather systems from organizing into a tropical cyclone.
Then there's the temperature of the ocean. The water temperature in the deep tropics rises as summer goes on – the result of sunny days, warmer air temperatures, and more moisture in the atmosphere. Warmer ocean temperatures drive greater storm activity.
Lack of wind shear in the atmosphere? Check.
Tropical waters as warm as a bathtub? Check.
Put those together, and waves off of Africa can strengthen into a hurricane like Florence.
"Hurricanes start simply with the evaporation of warm seawater, which pumps water into the lower atmosphere," NOAA explains. "This humid air is then dragged aloft when converging winds collide and turn upwards. At higher altitudes, water vapor starts to condense into clouds and rain, releasing heat that warms the surrounding air, causing it to rise as well. As the air far above the sea rushes upward, even more warm moist air spirals in from along the surface to replace it."
In its most recent forecast released last month, forecasters at NOAA's Climate Prediction Center said that odds were good for a below-normal Atlantic hurricane season. They predicted nine to 13 named storms, four to seven hurricanes, and two or fewer major hurricanes during the entirety of the season.
To date, this hurricane season has produced ten named storms, of which four were hurricanes. So far, Florence is the only one classified as major.
Despite Florence's outsize presence at the moment, NOAA says its forecast from last month still seems on target.
"This year, despite the recent uptick in activity, the overall activity remains typical of a less active season," Gerry Bell, NOAA's lead hurricane season forecaster, wrote in an email to NPR. "The recent flare-up in activity is associated with an enhanced west African monsoon system, which has been helping to fuel the development of storms over the far eastern Atlantic."
But he said several environmental factors continue to suppress the overall activity, "which is why we have been seeing overall weaker and shorter-lived storms. These include cooler sea-surface temperatures in the tropical Atlantic, coupled with stronger wind shear, drier air, and increased atmospheric stability" in the Atlantic tropics and the Caribbean.
Elsewhere on the planet, the patterns are different. In the Northwest Pacific basin, for example, tropical cyclones occur regularly, throughout the year. As a result, there is no "typhoon season" there, though there is a lull in February and March and a peak in August and September. In the North Indian basin, typhoons peak in both May and November.
Climate change is also having an effect on Atlantic hurricanes, causing storms that are bigger, longer-lasting, and more intense.
One particular concern is that tropical cyclones are slowing down. Last year's Hurricane Harvey sat over Houston for days, dumping more than 60 inches in some areas. So far Florence is moving very slowly – just 3-6 miles per hour. That Harvey-like behavior could mean major flooding in the Carolinas.
The number of hurricanes drops precipitously in late October and November. As fall temperatures arrive, wind shear increases in the Atlantic basin. The air and the water both cool down, resulting in fewer areas where the storms can form.
And we can breathe a sigh of relief — at least until next hurricane season.