Hurricane season kicks off today, and odds are it will be busier than normal


October 2016 satellite image of Hurricane Matthew, the most intense storm in the Atlantic in nearly a decade. (NOAA)

The six-month-long Atlantic hurricane season begins today, and the growing consensus is that it will end up more active than usual, despite the possibility of El Niño. Nature even provided a preseason tropical storm, Arlene, in April to get things started early.

This season’s name list was first used in 1981 and repeated in 2005 and 2011. In 2005, five names were retired and replaced: Dennis, Katrina, Rita, Stan and Wilma. After 2011, the name Irene was retired because of the storm’s destruction across Puerto Rico, Hispaniola, the Bahamas and the Northeast.

Hurricane season spans June through November but around 85 percent of the action occurs in August, September and October.

Current conditions across the Atlantic basin and beyond portend a favorable environment for tropical cyclone development in the coming months.

Sea surface temperature anomalies indicate that not only are the key formation areas warmer than average but also the Atlantic Meridional Mode (AMM) is in a positive phase, which generally enhances tropical cyclone activity in the Atlantic. This phase is characterized by warm water in the North Atlantic and cool water in the South Atlantic, which you can see is what’s roughly in place now — although there is region of warm anomalies in the northeastern South Atlantic.


Sea surface temperature anomalies analyzed May 31 across the Atlantic basin.  The equator is marked with a thick black line for reference. (tropicaltidbits.com)

In the Pacific, things are neutral. Neither El Niño or La Niña are present at the moment, though things are trending toward El Niño. Sea surface temperatures around the equator are higher than average but not quite enough to meet the criteria.

The latest computer model forecasts indicate a 60 percent chance that El Niño will appear during hurricane season and a 35 chance that things will remain as is. The skillful European model has even higher odds (about 75 percent) of an El Niño developing by the peak of hurricane season.

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All other things being equal, Atlantic tropical cyclone activity tends to be suppressed during El Niño.

This morning, Colorado State University released a new hurricane season outlook. The team, which is led by Capital Weather Gang expert Phil Klotzbach, is also predicting a slightly above-average season. with 14 named storms, 6 hurricanes, 2 major hurricanes and an accumulated cyclone energy of 99. The climatological average is 12 named storms, 6.5 hurricanes, 2 major hurricanes and an ACE of 92.

The group also highlights a few years to which this hurricane appears to be similar: 1957, 1969, 1979 and 2006. Among the 44 tropical storms that occurred during those years, a few were infamous: Audrey (1957), Camille (1969), David (1979) and Frederic (1979).


Tracks of four infamous storms that occurred during the four analog hurricane seasons identified by the Colorado State University forecast team. (Brian McNoldy)

Last week, NOAA released its latest hurricane season forecast, which calls for a 45 percent chance of a slightly above-average season with anywhere from 11 to 17 named storms.

The important footnote on these forecasts is that they say nothing about how many storms will make landfall. The forecast could be for one hurricane in the entire season, but if that hurricane happens to make landfall over your house, it will have been a bad season no matter what. It takes only one.

What’s new at the National Hurricane Center

The National Hurricane Center is launching new tools this season:

  • Fully operational storm surge watches and warnings
  • The ability to issue advisories and warnings for storms that have not yet “formed”
  • A “time of arrival” of dangerous winds graphic
  • An updated look and content for the forecast cone graphic

Storm surge is typically the deadliest risk associated with landfalling tropical cyclones, so it makes sense to have a separate watch and warning for that, since the hurricane watch/warning is meant to convey a wind hazard. The storm surge products were experimental for the past two seasons but finally got put through the wringer during hurricanes Hermine and Matthew and were successful and well-received.

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Before this year, NHC forecasters didn’t have the ability to issue advisories/watches/warnings for “potential tropical cyclones.”  This can be useful for systems that are forecast to develop quickly near land.

The NHC product suite has included a wind speed probability swath since 2006, but new this season is the addition of time of arrival lines for the onset of tropical-storm-force winds.

The left side of the graphic shown below is an example from Hurricane Matthew last October — the same five-day wind speed probability swath that we’ve had now incorporates useful timing information. The right side of the graphic shows an example of the new track forecast map using Hurricane Sandy from October 2012.

The 1-to-3-day cone of uncertainty is still shaded in white, the 4-to-5-day portion of the cone is dotted, the intensity is marked with a letter at each forecast position (D, S, H, M), the watches and warnings are still color coded, but the most recent wind field has been added.  Because the cone does not relate at all to storm size, showing the extent of tropical storm and hurricane force winds is great for perspective.


Example of the earliest reasonable time of arrival of tropical-storm-force winds graphic, left, and example of the updated forecast cone and wind field graphic, right. [NOAA/NHC]

And once again, this season’s cone is slightly smaller than last year’s.

Cone of uncertainty refresher:

The same cone is used all season long, for every storm, and every forecast. At least for now, it does not reflect the situation-specific uncertainty in the track forecast, only a historical average.

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The cone is designed to show where the center of the storm may track with a 2/3 probability — there’s a 1/3 chance that the center will not remain inside the cone at all.

The cone is not meant to indicate the storm size or where the hazards and effects will be experienced.  To illustrate how track forecast improvements have reduced the cone size in the past decade, we can look at a few snapshots of actual cones for a hypothetical storm.


Using a hypothetical hurricane forecast, the actual cone of uncertainty from 2008, 2012 and 2017 is drawn for comparison. [B. McNoldy]

Incredible major hurricane drought persists for U.S.

Last September, Hurricane Hermine broke Florida’s record “hurricane drought,” which lasted about 11 years. But another record that continues is the “major hurricane drought” for the entire United States.

A major hurricane is defined to be a Category 3+ on the Saffir-Simpson scale.  The last major hurricane to hit the United States was Wilma in October 2005.  That’s not to say that other storms since then (Ike, Irene, Sandy, Matthew, etc.) have not been destructive, deadly or otherwise significant, but they (fortunately) were not accompanied by the damaging winds associated with stronger hurricanes.

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