Look for a near-normal Atlantic hurricane season this year.
That's the outlook from several groups who produce seasonal forecasts for the Atlantic hurricane season, which began Friday.
Here's a sampler:
• The National Oceanic and Atmospheric Administration anticipates a 70 percent chance that from nine to 15 storms will reach tropical-storm status and earn names. Between four and eight will become hurricanes. Of those, from one to three are expected to become major hurricanes, where maximum sustained winds top 111 miles an hour.
• At Colorado State University, where atmospheric scientist William Gray pioneered seasonal tropical cyclone forecasts, he and colleague Philip Klotzbach anticipate 13 named storms, five hurricanes, and two major hurricanes.
• Researchers at Florida State University give the season a 70 percent chance of producing 10 to 16 named storms, and from five to nine hurricanes.
• Tropical Storm Risk.com, a consortium of scientists and insurance-industry specialists based in Britain, is anticipating (in round numbers) 13 tropical storms, six hurricanes, and three major hurricanes.
The average between 1981 and 2010, the base period federal forecasters now use for determining above- or below-normal conditions, is 12 named storms a year, of which half reach hurricane status.
Several factors are contributing to the near-normal outlook. Among them, sea-surface temperatures in regions where the Atlantic's tropical cyclones form, and conditions in the tropical Pacific that have an extended influence over weather patterns elsewhere.
Tropical cyclones form over and feed off of warm surface waters. And while still warm by snow-bird standards, waters across the tropical Atlantic from Africa to Central America have posted their third-coolest May readings since 1995, when the current, relatively intense long-term period of tropical-cyclone activity began, notes Jeff Masters, a meteorologist and co-founder of the website Weather Underground. Sea surface temperatures are running only about a half a degree Fahrenheit above normal, suggesting that it may take longer for waters to warm enough to fuel tropical cyclones.
But he also points to a broad patch of unusually warm water running up the East Coast from Virginia through southern New England. There, average surface temperatures are running up to 7 degrees or more Fahrenheit above normal – perhaps setting the stage for sustaining or intensifying the strength of any hurricane that travels up the eastern seaboard.
Another key factor is the state of the tropical Pacific's El Niño-La Niña cycle.
Over the past two hurricane seasons, La Niña has held sway – characterized by cooler than normal sea-surface temperatures in the eastern tropical Pacific and warmer than normal temperatures in the western tropical Pacific. These conditions flip-flop under El Niño. Each phase of this cycle alters wind patterns in ways that inhibit tropical-cyclone formation in the Atlantic during an El Niño and favor cyclone formation during a La Niña.
La Niña fizzled in April, leaving current conditions in a neutral phase some forecasters whimsically refer to as La Nada. Some models are forecasting the development of a weak El Niño between July and August, but for now it's unclear whether it would last much beyond that.
Indeed, the season's two early storms in May, Alberto and Beryl, highlight the influence wind conditions have. Both strengthened to tropical-storm levels over warm Gulf Stream waters off the coasts of Florida and the Carolinas. But they did so because the system of cloudiness they grew from traveled into a gap between two high-altitude rivers of air flowing west to east – the polar and subtropical jet streams, Dr. Masters explains on his Weather Underground blog.
Both can meander over the area where the two storms intensified. If the storms had encountered either, the high winds would have sheared off cloud-tops and prevented intensification.
Alberto formed off the East Coast and became a tropical storm just off South Carolina. Beryl formed from a patch of storms that formed off the Yucatan Peninsula. It, too, became a tropical storm when it encountered the warm waters of the Gulf Stream off the southeast coast. With maximum sustained winds at one point reaching 70 miles an hour, Beryl briefly flirted with hurricane status, where maximum sustained winds must reach at least 74 miles an hour.
Both brought much-needed rain to the southeastern corner of the US. But the area still is in the grip of a long-term drought, which ranges from severe to exceptional throughout much of Georgia, Florida, South Carolina, and in significant portions of Alabama.
Although the Atlantic hurricane season officially starts June 1 and runs through Nov. 30, Alberto and Beryl highlight the climate's tendency to ignore the calendar.
Since 1865, some 23 tropical storms have formed during May, appearing as early as May 6. But only two other years have seen two tropical cyclones form ahead of the official season. Separate storms formed on May 15 and 17 in 1887. In 1908 one storm formed May 24, the other on March 6, the earliest on record.