The next hurricane…
September 10 marks the average day of peak tropical cyclone activity in the Atlantic Ocean basin (Fig. 1). As we approach that day, the atmosphere is delivering. Early this Monday morning (Aug. 22, 2016), there were three tropical systems at play in the Atlantic – Tropical Depression Fiona and two tropical convective areas (Fig. 2). All of these systems started their lives as tropical waves coming off the west coast of Africa. Note that there is another thunderstorm cluster getting ready to exit Africa. So, all eyes across the eastern half of the U.S. are turning to the east.
The media is showing spaghetti plots and folks in Florida are starting to talk about these weather systems. Fortunately, the bread, water and toilet paper are not yet flying off supermarket shelves.
Successive runs of various computer models are showing an ever-evolving set of potential atmospheric solutions. The other day, one of the tropical systems was expected to pass over southwest Florida enroute to Louisiana. The model run six hours later had the storm in the Atlantic completely missing Florida. The solution set continues to grow. Several things are important to note:
(1) Official National Hurricane Center (NHC) center position forecast errors (based on 2011-2015 data for the Atlantic basin) are about 272 miles at day five (120-hour forecast). We are currently at day 7 or 8 before the storm might affect the U.S. If forecasts were made this far ahead, errors would be expected to be even greater.
(2) Longer-term forecast errors in storm strength are about 15 knots (17 miles per hour), roughly equivalent to one storm strength category.
(3) Nearby weather systems (and sometimes systems far removed from the tropical cyclone) can affect the cyclone’s formation, development and movement.
(4) Computer models are based on past computer model outputs and updated observations. Missing or incorrect data, model boundary conditions, and even the thermodynamics and physics built into the model can all generate errors or incorrect outcomes.
So, this far ahead, it just pays to watch the tropics and know what thenext named storm might be called. Keep up-to-date on any storms that have or may form and monitor official forecast tracks. As any storm gets closer to the U.S. (if one does), NHC forecasts and discussions (plus information here at GWCC, and on local and national media) will help guide our readers to risks and any needed safety actions.
© 2016 H. Michael Mogil
Originally posted 8/22/16