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.