No tropical storms over Labor Day weekend: what does that mean for hurricane season?

katrinaMany years ago, Max Mayfield, the former director of the National Hurricane Center (NHC), told me that if you were a hurricane specialist at the NHC, you should expect to be in the office sometime over the Labor Day Weekend.

What he meant, of course, was that NHC very often was issuing advisories on one or more tropical cyclones over that weekend.  Not surprising, as Atlantic tropical cyclone activity normally peaks around the middle of September.

To see just how often the NHC historically issues advisories over this past holiday weekend, I looked back through the past 50 years of Atlantic Tropical Cyclone data, using the Unisys ‘Atlantic Tropical Storm Tracking by Year’ Web site, which in turn uses the historical ‘best track’ information from the NHC.

Unlike the tracks we see in the real-time forecasts, the best-track data uses all available information, coupled with the hurricane specialists’ expert judgment made without the pressure of having to issue an advisory on a tight time deadline.  For the purposes of this study, I defined the Labor Day weekend to include Labor Day, plus the preceding Saturday and Sunday.

Bottom line (1):  Max was right.  From 1963-2012, there were only five years in which the NHC did not issue advisories for at least one system.   On average, the NHC warns on nearly two systems on any given Labor Day weekend, and it’s not unprecedented to warn on up to four different tropical systems.

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It was interesting that the level of activity is so high even though Labor Day itself varies between the first and 7th of September, meaning the weekend can start as early as August 30.   Of course, the atmosphere doesn’t know anything about the Labor Day Holiday, so are we really measuring nothing more than the date of the first Monday in September?

I divided the data into early Labor Day weekends (those with at least one of the weekend dates occurring in August, or about 20 percent of the total sample size) and weekends where all three of the days fell in September.

Quiet weekends with no storms are disproportionately represented in the early weekends; conversely, very active weekends of four separate systems occur most often when Labor Day is later in the month.  However, there is little calendar trend for Labor Day weekends with one, two, or three storms.

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Can we learn anything else from these data?  Perhaps.

I examined the total number of tropical cyclones, or systems (including tropical depressions) for each year, the number forming on or before Labor Day weekend, and the number forming after Labor Day.

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A number of interesting trends become apparent.

First, there appears to be some correlation between activity over the Labor Day weekend and overall activity for the year.  The average number of observed tropical systems for the entire year when there is no activity over Labor Day is 9.2, compared to an average of 16.3 overall and just over 17 when there is at least one advisory issued over the holiday.

However, the activity over the Labor Day weekend seems to tell us more about the relative activity of the season to date, rather than the number of systems yet to form.  In fact, there seems to be a slight negative correlation between very active Labor Day weekends and the remaining number of tropical systems that will form.  Some of that variation can be explained by the higher percentage of early Labor Days with no storms, as there is up to a week more of the active storm season yet to come, relative to most Labor Day weekends that have high tropical activity.

Another way to look at the data is to examine what percent of storms, relative to the year’s total, have formed by Labor Day.

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For seasons with zero or one active tropical systems over the Labor Day weekend, the year is, on average, not even half-way over; that is, there are probably more storms to form than we have seen year to date.

So what does this mean for us?  As usual, when trying to condense a very complex atmosphere into some simple numbers, there are conflicting signals.  A quiet Labor Day weekend in the tropics is unusual, but given how early Labor Day fell this year, it’s certainly not unprecedented.

With no active systems over this past weekend, the past 50 years of data would lead us to believe we should still see another 3-7 tropical systems (again, including tropical depressions), giving us a total of 9-13 storms for the year.  If that projection was to verify, then the season as a whole would be considerably less active than was expected, as indicated by the numerous public, private, and academic forecasts issued starting in May of this year.

However, these numbers do not fully account for the relatively active (if weak) season we have already observed.  And the data also tell us that little activity over Labor Day weekend typically correlates with a season that is not even half done, particularly when Labor Day falls early in September.   Applying these percentages to the observed activity of six tropical storms to date would project a season with up to 12-18 systems, consistent with many of the seasonal projections.  It is worth noting that both the 2001 and 2007 hurricane seasons only had six storms up to Labor Day, yet both seasons finished their respective years with 17 total storms.

The bottom line (2):  Taking an average of both methods would lead to a forecast at this point of 11-15 systems ( including tropical depressions).  It is very important to note that this study says nothing about the strength, size, duration or location of any future system.  And as we know, it only takes one storm to change your life forever.

Whether you were impacted by Hurricane Katrina in a record-breaking year of 31 storms, or by Andrew in an otherwise ‘quiet’ year of only 10 tropical systems, the result is still the same.  So, while there are some indications this may not be a hyper-active season, common-sense and prudence dictate keeping a weather-eye to the tropics for the next three months and keep your preparedness plans up to date.

Guest contributor David Titley is a Senior Scientist in the Department of Meteorology at the Pennsylvania State University, and founding Director of Penn State’s Center for Solutions to Weather and Climate Risk. More information.

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