Tropical Storm Arlene formed in the middle of the Atlantic Ocean on April 20, 2017, briefly coming to life far away from land—where it was little more than an oddity to gawk at on satellite imagery. Even though the short-lived system wasn’t much of a threat (beyond aggravating some fish), the early start to the 2017 Atlantic hurricane season grabbed headlines.
But if you're a coastal resident anxious about the summer to come, fear not! It doesn't necessarily bode ill for the season. Now is as good a time as any to talk about what you can expect in this upcoming year, and to take a look at the innovative ways forecasters are improving how you can prepare for an approaching storm.
1. DON’T GET TRIPPED UP BY THE TERM SUBTROPICAL.
Tropical Storm Arlene first began its life as a subtropical cyclone. The word subtropical sounds intimidating, but it just describes the meteorological structure of the storm itself. Tropical cyclones are low-pressure systems that form over warm ocean waters and maintain their strength through thunderstorms raging near the center of the storm. They are tight, compact systems that are warm and muggy all the way from the surface to the top of the clouds.
The atmosphere is fluid, though, so not all storms perfectly fit that definition. That’s where subtropical cyclones enter the picture. A subtropical cyclone is one that resembles a tropical cyclone, but it’s not completely warm throughout the storm. It’s also not compact. Unlike a tropical cyclone, where the strongest winds are concentrated right near the center of the storm, the wind field in a subtropical cyclone can be far removed from the center and stretch hundreds of miles across. Sometimes these cyclones progress into tropical versions, sometimes they don't.
2. A STORM IN APRIL ISN’T AN OMEN FOR THE SEASON TO COME.
It’s not too unusual for a tropical or subtropical system to develop before the start of hurricane season. Hurricane season in the Atlantic runs from June 1 to November 30, but that’s just when they’re most likely to develop. The 2016 hurricane season started with Hurricane Alex in January—which was highly unusual—with the season’s second system, Tropical Storm Bonnie, forming in May. The last time we saw a storm in April was Tropical Storm Ana near Bermuda in April 2003.
Since 2007, we’ve seen eight tropical or subtropical cyclones develop before the official start of hurricane season. These early-season storms formed in years that were both quiet and active. In other words, storms that form before the start of hurricane season are usually case studies in their own right rather than a sign of things to come. Plus, no matter how many storms develop, it only takes one storm hitting land to cause major problems.
3. IT’S HARD TO TELL EXACTLY WHAT WILL HAPPEN THIS HURRICANE SEASON.
So much of what happens in the Atlantic Ocean’s hurricane season depends on what’s going on out in the eastern Pacific Ocean. El Niño and La Niña can have a major impact on how many storms are able to form. Years with El Niño conditions tend to see fewer storms in the Atlantic due to increased wind shear, which shreds potential storms apart before they can develop. Years featuring a La Niña can have the opposite effect, as cool waters in the Pacific help reduce destructive wind shear flowing out over the Atlantic—creating more opportunities for tropical systems to develop.
We’re in a “neutral” phase of the El Niño-La Niña cycle right now, which means that water temperatures in the eastern Pacific are right around where they should be. NOAA’s Climate Prediction Center is also calling for the chance for an El Niño toward the peak of hurricane season, though nothing is set in stone. If that happens as forecast, there’s a chance this season might come in a little quieter than average.
4. FORECASTS ARE A LITTLE BETTER THAN THEY WERE A FEW YEARS AGO.
When tropical storms and hurricanes fire up this summer, the most noticeable part of the coverage you’ll see online and on television is the cone of uncertainty, a shaded bubble that stretches along the length of the storm’s forecast track. This cone of uncertainty is the historical margin of error in hurricane track forecasts. Forecasts today are good enough that you can expect that the eye of a tropical cyclone will stay somewhere within that cone of uncertainty about two-thirds of the time.
At the end of each hurricane season, meteorologists at the National Hurricane Center (NHC) calculate the error in their previous forecasts and determine how far off their track forecasts were, on average. The NHC takes this average error at each time step and uses the resulting distance to draw a circle around their forecast points, connecting each circle to make the cone we’re all familiar with. The cone of uncertainty has steadily shrunk over the years—and the cone will grow a little narrower once again this year.
5. GET READY FOR STORM SURGE WARNINGS.
The deadliest part of a landfalling tropical storm or hurricane is flooding from storm surge, or the sea water that’s pushed inland by strong, persistent winds. Most storm surges are small; however, the surge in a large or intense storm can completely submerge a one-story home and push water several miles inland.
Since the threat for storm surge flooding can get lost in the focus on how strong the wind is blowing, the NHC will start officially issuing storm surge watches and warnings this year. Communities placed under one of these new storm surge warnings can expect life-threatening coastal flooding within 36 hours. This new focus on flooding might help convince people who would otherwise attempt to ride out the storm that it’s a better idea to leave for a few days than risk their lives.
6. YOU’LL HAVE A BETTER IDEA OF WHEN THINGS WILL GET UGLY.
Another new product being introduced this year by the National Hurricane Center is an arrival time map [PDF]. This forecast will show you when you can reasonably expect the damaging winds of a tropical storm or a hurricane to reach a certain point based on the storm’s current forecast track. This will help people and agencies gauge just how long they have to prepare for a storm before conditions deteriorate and venturing outside is too dangerous. However, these times are estimates—if the storm changes direction, speeds up, or slows down, the arrival times will change accordingly. Generally with storm systems, you can never be too prepared.