Shockingly quiet in the tropics as study confirms hurricane season is starting sooner | WeatherTiger

·7 min read

With the Tropics remaining quiet for the moment, today’s hurricane column is devoted to explaining a scientific paper released this week on the hot topic of whether hurricane season is getting longer.

As it happens, I wrote this paper along with a great team of scientists from across the field over the last year and a half. So, no pressure, but I hope you like it.

Before we get to that, here’s a quick look at the goings-on, or lack thereof, in the Atlantic.

The key feature to watch for any potential U.S. impacts in the upcoming week will be a tropical wave cutting across the southwestern Gulf of Mexico on Friday and Saturday. There is some chance of short-lived development with this feature, though with or without development the primary influence of the low will be beneficial rains for drought-plagued southern Texas.

Who is WeatherTiger? Hurricane expert Ryan Truchelut provides Florida storm forecasts, analyses

Elsewhere, the Tropical Atlantic is shockingly quiet for mid-August, though there are signs that waves emerging from the African coast this week will eventually find a more favorable environment down the road. Next week’s column will either be forecasting where these waves will track or devoted to explaining why the Atlantic is remaining so eerily quiet.

I’m hoping for the latter, but there are no guarantees in late August.

In the meantime, keep watching the skies and read on.

Earlier season?: Is the Atlantic hurricane season cranking up earlier? Study says yes, thanks to climate change

Why rethinking the start of hurricane season matters

One spring day a few years ago, my one-year-old son and I went for our usual daily walk around our neighborhood to look at clouds, colorful flowers, and lizards.

On this particular afternoon, soon after cresting what passes for a hill in Northern Florida, we were chased back home by a formidable outer band of Tropical Storm Alberto.

While I should have done a more thorough job of checking the radar prior to leaving home, I do have some excuse for not expecting brisk winds and a torrential downpour from a tropical cyclone: it was not yet, in fact, hurricane season.

Alberto, which caused 18 deaths and $125 million in flood damage in May 2018, is not alone in impacting the continental United States outside the June 1-November 30 bounds of hurricane season.

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In fact, Alberto was one of seven tropical or subtropical storms since 2012 to require U.S. tropical storm watches or warnings to be hoisted by the National Hurricane Center before June 1.

In our new paper in Nature Communications, I, along with co-authors Phil Klotzbach, Erica Staehling, Kim Wood, Daniel Halperin, Carl Schreck, and Eric Blake, investigate whether initial tropical cyclone (TC) formation dates and U.S. landfall risks are shifting earlier in the year, and the potential causes of these trends.

As my experience shows, understanding trends in TC activity onset has real-world implications. Not only do pre-season or early-season storms disproportionately impact populated areas, especially through excessive precipitation, but they strike at a time when the general population (or even a sleep-deprived hurricane forecaster) is not primed to prepare for such threats or react to warnings.

The 7 pre-season storms.
The 7 pre-season storms.

With around 30% of tropical cyclone casualties arising from flooding, any divergence between perception and reality of risk is of serious concern.

Day by day: Each decade since 1979 has seen earlier storms

We began our study by using quality-controlled TC historical data from the National Hurricane Center reaching back to 1900 for U.S. landfalls and 1979 for Atlantic storms. The 1979 start represents the earliest time for which observational tools like satellite imagery were sufficiently available to be confident that storms in the open ocean were not systematically going undetected.

For landfalls, 1900 is the year in which the full U.S. coastline was populated with enough weather stations and observers to detect strikes.

The season start trendlines.
The season start trendlines.

Using this data, we show that there is a significant trend toward earlier onset of North Atlantic TC activity.

The date at which the first few percent of overall TC activity within a hurricane season occurs is shifting earlier at a rate exceeding five days per decade since 1979, and the initial continental U.S. named storm landfall is trending earlier at a rate of about two days per decade since 1900.

To address whether these trends might be caused by the increased detection of short-lived storms in recent years due to improved observational tools, we also repeated all of our analyses with and without named storms lasting less than 48 hours. The shift towards earlier TC development and U.S. landfall risks was accentuated when excluding these "shorties."

Warmer oceans mean earlier storms

Satisfied that these trends were real, we then investigated why more tropical storms are developing earlier in the season in the southwestern Atlantic, the Gulf, and the western Caribbean Sea. We found that some parameters that matter for whether storms form, like moisture and wind shear, were shown not to have changed much in these areas since 1979, or to weakly modulate early season activity.

Rather, increased TC activity in the pre-season and earliest weeks of hurricane season is primarily driven by warming ocean temperatures in the western Atlantic. For each 1°C of ocean warming in this area, the initial few percentiles of Atlantic TC activity are expected to shift about one month earlier.

The observed spring warming trend in the Caribbean and western Atlantic of 0.75°C since 1979 roughly explains the observed change in season onset, even when normalizing for quieter and busier years overall.

Plotting out where early storms start.
Plotting out where early storms start.

This suggests that the threshold date at which the first percentiles of TC activity and U.S. landfalls occur might continue to shift earlier, irrespective of whether future Atlantic hurricane seasons overall become busier, quieter, or stay around the same.

By definition, the initial percentiles of the season only comprise a small portion of total Atlantic TC activity, but pre-season and early season TCs can have outsized societal impacts, particularly with respect to flash flooding.

Alberto's impacts in 2018 underscore these rising risks; another example of an extreme, destructive TC precipitation event in the opening days of hurricane season is 2001's Tropical Storm Allison, which deposited up to 35" of rain and caused nearly $10 billion in damage in eastern Texas.

Tropical Storm Allison killed 41 in 2001.
Tropical Storm Allison killed 41 in 2001.

The final analysis

So, what does all this mean for hurricane season itself?

As it turns out, the concept of "hurricane season" currently lacks a precise, scientific definition, and actually originated in the 1930s as the annual operating dates of a telegraph line connecting U.S. Weather Bureau offices.

While official efforts are underway to develop a more rigorous delineation, that hurricane season is a social construct underscores additional considerations like public communication, net risk mitigation, and emergency management perspectives are needed to determine when to draw the line.

The research suggests that hurricane season could be objectively defined as beginning prior to June 1 based on the last 50 years of data.

Overall, the additional ocean temperature increases likely in the coming decades will probably continue to widen the mismatch between the actual onset of TC risks to the U.S. coastline, and the current start date of hurricane season on June 1.

Our study shows initial Atlantic TC activity is shifting earlier by at least half a day per year on average, and reasonably may be expected to continue to do so given the likely cause of the changes. Should the definition of hurricane season adapt to include portions of May?

Our study suggests perhaps it should, at least from a quantitative perspective. After all, it's no fun to get caught in the rain without an umbrella.

Dr. Ryan Truchelut is chief meteorologist at WeatherTiger, a Tallahassee start-up providing forensic meteorology and expert witness consulting services, and agricultural and hurricane forecasting subscriptions. Get in touch at ryan@weathertiger.com, and visit weathertiger.com for an enhanced, real-time version of our seasonal outlook.

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This article originally appeared on Tallahassee Democrat: Florida hurricane season: Still quiet; Study highlights early threats