Warm sea-surface temperatures and the long-reaching effect of conditions in the tropical Pacific are among the factors that could mean above-normal activity during the 2013 Atlantic hurricane season.
James Franklin, Branch Chief of Hurricane Forecast Operations, reported on April 4th, the National Weather Service is changing how it issues hurricane and tropical storm warnings. Starting June 1, watches and warning will be issued for storms that threaten life and property even after they are no longer hurricanes or tropical storms.
The 2013 Atlantic hurricane season, which begins June 1, is shaping up to deliver above-normal activity again this year, according to several seasonal forecasts.
According to Kathryn Sullivan, acting administrator of the National Oceanic and Atmospheric Administration (NOAA), this season could be extremely active.
Forecasters at the agency's Climate Prediction Center anticipate from 13 to 20 tropical storms over the six-month season. Of those, between seven and eleven are expected to become hurricanes, with three to six of the hurricanes expected to reach "major" status, meaning they host maximum sustained winds topping 111 miles an hour.
The federal forecast, released Thursday afternoon, brackets other seasonal outlooks that also point to a busy season.
In April, for instance, researchers at Colorado State University in Fort Collins released their initial forecast for 2013, which included 18 tropical storms, of which nine are expected to become hurricanes. Four could become major hurricanes.
The spread in NOAA's prediction for tropical storms is a bit wider than normal, Dr. Sullivan notes. It reflects "some uncertainty about whether the activity that will occur this coming season will be a smaller number of longer-lived storms or a larger number of shorter-lived storms," she said.
Broader atmospheric conditions present during the lifetimes of these storms will determine the difference – conditions that forecasters can't predict this far in advance, she notes.
While different approaches account for the differences among seasonal forecasts, they all are starting with some common basic ingredients.
Sea-surface temperatures in the tropical Atlantic are 0.8 degrees F. warmer than normal. "It might not sound like a lot, but that's quite a bit" above normal, said Gerry Bell, who heads NOAA's seasonal hurricane-forecast effort. Tropical cyclones draw their energy from warm surface waters.
Beyond ocean temperatures, "there's an entire set of wind and air-pressure patterns that come together to produce an active season," Dr. Bell said. "We've been seeing that entire set of conditions since 1995." Such conditions are said to go through 25- to 40-year cycles.
In addition, the Atlantic is feeling the long reach of conditions in the tropical Pacific. These currently reflect a state that some call "La Nada" – a climatological no man's land between east-west swings in sea-surface temperatures and atmospheric pressure known as El Niño and La Niña.
The swings take place in two- to seven-year cycles. When El Niño reigns, warm water migrates from the western tropical Pacific to the west coast of Central and South America. The change in air pressure that this warm water brings with it alters atmospheric circulation patterns in ways that stifle hurricane formation in the Atlantic.
With La Niña, El Niño's opposite, and La Nada, those stifling conditions vanish. This favors the formation of more tropical cyclones in the Atlantic.
Meanwhile, forecasters at the National Hurricane Center have created improvements in predicting a storm's path and intensity – in particular sudden changes in storm intensity. This is an issue forecasters have struggled with for nearly 20 years.