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What does the [hurricane] season look like?

Last fall, I came home one evening with a raging headache. I’ve never been prone to headaches and any headache I’ve had in the past has been mild and short lived. I didn’t think much of it, took a few Tylenol, and opted for an early bedtime.

The next morning I woke up with chills and a pain in my head that was unbearable. I’ve never had a migraine but I imagined this is what one felt like. I grabbed a thermometer out of the medicine cabinet and took my temperature. 100.5. Low grade but enough to add to my discomfort.

I visited my doctor that afternoon in search of an answer. He did the routine checks but my symptoms were conflicting — severe headache plus fever plus a pain down my arm didn’t add up to a rosy diagnosis, which is what landed me and my fiancée in the emergency room that evening.

They ran about every test you could imagine and, although they weren’t able to pinpoint an exact diagnosis, they were able to rule out the serious stuff. This is much like how we make seasonal hurricane forecasts. We can check the symptoms going into the hurricane season, but you’ll be hard pressed to get a precise diagnosis.

Like a bad headache, there are symptoms of our most active hurricane seasons — tropical waters tend to be warmer and scissoring wind shear is usually reduced over the Gulf of Mexico, eastern Atlantic, and (especially) Caribbean Sea.

Take 2005 for example — the year of Katrina, Rita, and Wilma. It was the most active hurricane season on record going back to 1851. The veritable alley way of light wind shear across the entire tropical Atlantic made for open season, allowing storms to move toward the U.S. virtually untouched:

200–850 mb deep layer vector wind shear for August to October 2005

Now compare this to last hurricane season, with wind shear across the tropical Atlantic clocking out at the strongest on record (back to 1979).

200–850 mb deep layer vector wind shear (kt) for August to October 2015

The subtropical jet stream tore through the Caribbean and into the middle Atlantic, making life a lot harder for fledgling tropical systems. In fact, if not for one storm — Hurricane Joaquin, a near Category 5 hurricane that snuck in like a Trojan Horse from the northwest — 2015 would’ve gone down as one of the top five least active hurricane seasons of the modern era.

Reduced wind shear, though important, is only one symptom of a potentially busy hurricane season. We also consider the warmth of our tropical waters. In fact, the warmth in the tropics actually peaks in late September (around September 27th), after the typical peak of hurricane season activity.

Total tropical storms (1851–2015) versus climatological 200–850 mb vertical wind shear and main development region sea surface temperatures (1981–2010 averages)

But how we examine ocean warmth for seasonal hurricane forecasts is different from how we examine tropical water temperatures in the days leading up to a storm.

Consider first the pattern of warmer and cooler waters in the Atlantic during our most active hurricane seasons. They looked something like this:

Sea surface temperature anomalies (°C) during the most active hurricane seasons since 1982

Now consider the pattern of warmer and cooler waters during our least active hurricane seasons:

Sea surface temperature anomalies (°C) during the least active hurricane seasons since 1982

The pattern of warm and cool waters, known as the Atlantic tripole, is reversed. At first glance it might seem that the presence of warmer waters in tropical belt (between 10 and 20 degrees north latitude) would fuel more hurricanes during the active seasons versus the cooler waters of the inactive seasons. But the cooler waters of the inactive seasons are still plenty warm enough to fuel powerful hurricanes during the late summer and early fall months. So what gives? It’s all about what the relative warmth does to the atmosphere.

When the tropics and far north Atlantic are cold relative to the middle Atlantic, as it is in the lower plot, it messes with our normal atmospheric circulations (the Hadley and Ferrel cells for the meteorologists reading along). Rising air is shifted northward and away from the tropics. Closer to the polar region, the jet stream is stronger. The stronger polar jet stream pumps up the subtropical ridge which in turn increases low level easterly trade winds across the tropical Atlantic, both cooling off tropical Atlantic waters and increasing wind shear. It’s everything you wouldn’t expect if you’re expecting a busy hurricane season.

Which then begs the question: what does the pattern of warmer and cooler waters look like going into this hurricane season?

Though not a perfect match (the tropical Atlantic isn’t very cool) the springtime Atlantic ocean temperature pattern looks more like the inactive years than the active years. So why is @NOAA, the parent organization of the National Weather Service, going with a near normal outlook for this hurricane season?

Two words: La Niña. The blockbuster El Niño of 2015 is quickly morphing into a La Niña for the fall months. The latest forecasts have a nearly 60% chance of La Niña by the peak months of the Atlantic hurricane season (August, September, and October).

La Niña, marked by cooler than average waters across the eastern equatorial Pacific Ocean, typically means a sharp reduction in hostile wind shear across the tropical Atlantic, especially over the Caribbean, which, as discussed earlier, is an important predictor of tropical activity. In fact, on a year-to-year basis, El Niño and La Niña are the two biggest predictors of hurricane season activity. So despite the seemingly unfavorable (for tropical development) water temperature pattern across the Atlantic, seasonal prognosticators are presented with the wildcard of a budding La Niña, which brings us back to our diagnosis.

It’s my favorite loaded question of the season. What does the [hurricane] season look like? Atlantic ocean temperatures suggest a down year in terms of hurricane numbers, but the potential of a La Niña by late summer say those down numbers could be up. So most seasonal forecasters are straddling the fence with an average year in terms of total named storms and hurricanes.

The overall numbers don’t really answer the question, though, because what most people are asking is “what does the [hurricane] season look like for me?”No one (and by that I mean NO ONE) can tell you with any skill whether a hurricane will hit your hometown weeks or months in advance. Though we might be able to predict overall numbers with some skill going into a hurricane season, we have no skill in forecasting the exact landfall location of those storms this far out. The science just isn’t that good.

So here’s your diagnosis. 2016 is shaping up to be neither a super active nor super inactive season in terms of overall numbers. Your odds of seeing a hurricane landfall this year are the same as they were last year, regardless of the overall numbers. Which means if you live in Florida, you have a 50% chance of seeing a hurricane landfall in 2016. Now compare this to your odds of getting the flu in any given year, which is 13%. You get your flu shot every year — well, at least most of you do — so get your hurricane flu shot today. It’s your protection against hurricane season.

What’s a hurricane flu shot?

Check with your insurance company now to make sure you’re covered for both wind and water damage from a hurricane (water is generally covered under a separate flood insurance policy). There’s a 30-day waiting period before any new flood insurance policy goes into effect, so don’t think you can wait until a storm is approaching.

Know whether you’re at risk for storm surge flooding, since that’s the main reason you’d be asked to evacuate. And if you are asked to evacuate, have a plan on where you will head and where you will stay. You only need to go inland as far as the seawater flood threat.

Everyone that lives in the hurricane zone should have a 3–5 day supply of food and water. You’ll need a gallon of water per person per day, so for a family of four, that’s between 12 and 20 gallons of water.

When buying non-perishable food for your hurricane supply kit, buy food that you’ll actually eat. There’s enough tasty non-perishable food nowadays that you don’t need to buy a week’s worth of SPAM, unless of course you love SPAM. If you don’t use the food by the end of the season, then have a December 1st post-hurricane season feast.

Take this diagnosis and treatment as you would from any doctor. It’s the best the science has to offer and I promise it’ll prevent a lot of headache down the road.


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  11. Mike keeps saying thunderstorms in the context of tropical meteorology. Thunderstorms rare in the tropics because the freezing level is very high.

  12. Thank Michael. I was surprised to learn that cold water and low wind shear contribute to a higher probability of hurricanes. I would have thought just the opposite. So, if we have a strong La Nina, could that significantly impact Sept/Oct? Could the pattern change drastically? All interesting. Thanks.

  13. I predict a busy to serious hurricane season this 2016. From New Orleans – moved to PA. Sort of glad, but will miss it – weather & all. JimboCajun

  14. Loved reading this! Well written and easy to understand! Loved the analogies! Thanks!

  15. We moved to S.E. La. 3 months before Katrina hit. Being from Indiana, hurricanes were something I’d never worried about before… Wow, what a wake-up call. Buy supplies EARLY!

  16. Really good explanation of the factors involved in hurricane formation. Thx!

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