ARI SHAPIRO, BYLINE: It's ALL THINGS CONSIDERED from NPR News. I'm Ari Shapiro. The 2013 Atlantic hurricane season ends tomorrow. It'll be remembered as one of the quietest on record. Since June, there have been just two hurricanes, both were relatively weak. As NPR's Jon Hamilton reports, forecasters were expecting something very different.
JON HAMILTON, BYLINE: The National Oceanic and Atmospheric Administration told Americans to expect an unusually active year with between seven and 11 hurricanes. Other forecasters offered variations on that theme.
MATT SAMPSON: And from the looks of things, it's going to be a busy season.
JOE BASTARDI: So we're expecting a very active season.
PHIL KLOTZBACH: About 175 percent of the average hurricane season, so about 75 percent more activity than normal.
DAN LEONARD: Probably a little bit more intense with the storms this year.
HAMILTON: That was Matt Sampson of The Weather Channel, Joe Bastardi of Weatherbel, Phil Klotzbach of Colorado State University and Dan Leonard of Weather Services International. Leonard says six months later, he and his colleagues are looking for ways to describe just how little hurricane activity there was.
LEONARD: This is probably one of the quietest seasons of all time. In fact, we have to go back into the early 1970s to find another year where we had only two minimal category one hurricanes.
HAMILTON: Leonard says he and other forecasters will spend their offseason trying to figure out how they could've been so far off the mark.
LEONARD: This is one of those years where all of us sort of have to take a step back and say, you know, what went wrong? How are we going to learn from this? Because in the past few years, to be honest, we've been pretty good at predicting the number and intensity of storms and this year was sort of a rude awakening.
HAMILTON: Early on, Leonard says, all the signs were pointing toward a busy season. For example, water temperatures in the Atlantic were very warm. That usually means storms moving west off the coast of Africa have the energy they need to develop into hurricanes. Leonard says another factor was the lack of El Nino conditions which tend to produce winds that are less favorable for hurricanes.
LEONARD: This year, without any El Nino, we figured, hey, the door is open for a lot of strong hurricanes in the Atlantic. But that just wasn't the case so it really wasn't El Nino's fault this time around.
HAMILTON: Leonard says it took forecasters quite a while to realize that hurricanes weren't developing the way they usually do.
LEONARD: Roughly halfway through the season, about July, August or so, we started to get these indications that a lot of the waves that move off Africa, that we typically see spin up into storms and move across the Atlantic just weren't developing as they should have.
HAMILTON: Leonard says one reason appears to be that those waves of low pressure air just weren't as powerful as usual. Also, the waves were encountering surprisingly dry air as they traveled across the Atlantic. Phil Klotzbach is at Colorado State University.
KLOTZBACH: Typically, in the Atlantic, during the peak of the hurricane season, you have wind that blows out of the south across the equator and that brings in kind of moisture rich air. This year, we had winds out of the north.
HAMILTON: Which carried drier air into the Atlantic basin. Klotzbach says that discouraged the sort of weather systems that can produce a hurricane.
KLOTZBACH: Basically hurricanes are kind of groups of deep thunderstorms and if you have a lot of dry air, you don't really get the moisture that you need for the clouds to condense and to develop into these thunderstorms.
HAMILTON: Klotzbach says forecasters don't have models yet that can fully account for things like dry air and Saharan dust when making seasonal forecasts and most years, he says, that doesn't matter.
KLOTZBACH: We've being doing these forecasts for 30 years and that's probably the biggest forecast bust that we've had so, obviously, that's obviously not a great feeling.
HAMILTON: But Dan Leonard says there's an upside. Forecasters learn more when they're wrong then when they're right.
LEONARD: This actually, I think, is a good thing going forward. It's going to cause us to lose some of our complacency when it comes to just making very basic seasonal forecasts for hurricanes. I think a lot of research is going to go into this year and we're probably going to come out better off in future years and probably more accurate going forward.
HAMILTON: Starting, he hopes, with the 2014 forecast. Jon Hamilton, NPR News. Transcript provided by NPR, Copyright NPR.