As the American heartland continues to be hammered by a late but lethal tornado season, the U.S. East Coast is bracing for what could be another damaging and deadly hurricane season triggered by unusual climate conditions.

The 2013 Atlantic hurricane season, which officially begins Saturday, likely will be “above normal and possibly extremely active,” according to officials at the National Oceanic and Atmospheric Administration.

Meteorologists say the confluence of warm tropical waters and the slim chance of a cyclone-suppressing El Niño event may fuel three to six major hurricanes over the course of the summer, less than a year after Superstorm Sandy ravaged the mid-Atlantic region. Major hurricanes are defined as Category 3 or above.

Sandy was downgraded from hurricane status to tropical storm status just before it battered the northeast U.S. last October.

Gulf rigs stand ready as hurricane season arrives

Although meteorologists cannot say with certainty how many storms will hammer the coast – or where they will strike – there's a 96 percent chance of a hurricane hitting somewhere along the U.S. coast this summer, according to a forecast released in April.

“We really can’t say where the storms are going to go,” said Dr. Philip J. Klotzbach of the Tropical Meteorology Project at Colorado State University, who authored the forecast with his colleague Dr. William Gray. “But we know that more active seasons have more storms that make landfall.”

Above-average sea-surface temperatures create an environment that “will be very conducive for waves to develop and intensify” and potentially generate associated phenomena, such as increased moisture and lower air pressure, that foment giant storms, according to Klotzbach.

Water temperatures are expected to be 0.8 degrees Fahrenheit warmer than usual, according to The Associated Press.

Although an uptick of less than one degree "doesn't seem like a heck of a lot," Klotzbach said, "it makes a big difference in tropical waters."

What’s more, the unlikeliness of a significant El Niño event will make it easier for a cyclone to take shape, according to NOAA. El Niño is a vast stretch of unusually warm water in the Pacific Ocean that typically takes the edge off hurricanes.

“El Niño … is not expected to develop and suppress hurricane formation this hurricane season,” said Kathryn Sullivan, acting NOAA administrator.

NOAA forecasts 13 to 20 tropical storms, seven to 11 of which are projected to become hurricanes and three to six of which are projected to become major hurricanes.

Klotzbach's projection of four major hurricanes is in that range.

The last major hurricane to make landfall in the U.S. was Wilma in 2005, according to Klotzbach. Since then, five Category 1 or 2 storms – defined by winds moving as fast as 100 mph – have struck the U.S.

Atlantic hurricane season typically lasts for six months, usually peaking between late August and mid-October.

In the introduction to their forecast, Klotzbach and Gray warn coastal residents to take precautions in advance of storm season.

“Coastal residents are reminded that it only takes one hurricane making landfall to make it an active season for them, and they need to prepare the same for every season, regardless of how much or how little activity is predicted,” they wrote.

Batten down the hatches.

Forecasters said Wednesday that the 2013 Atlantic hurricane season is likely to be "above normal and possibly extremely active," predicting three to six major hurricanes this season.

The National Oceanic and Atmospheric Administration said in its outlook that it forecast seven to 11 Atlantic hurricanes for the 2013 season, which officially begins on June 1.

"NOAA predicts an above normal and possibly an extremely active hurricane season with a range of 13 to 20 named storms," seven to 11 of which are forecast to turn into hurricanes and three to six of which are forecast to turn into major hurricanes, said Kathryn Sullivan, acting NOAA administrator.

Major hurricanes are defined as Category 3 or above, with winds of more than 110 mph.

The last time a major hurricane made landfall in the U.S. was Wilma, in 2005, according to the Associated Press. The seven-year landfall drought is the longest in the U.S. on record, The AP reports.

Hurricane Sandy was downgraded to tropical storm status just before it made landfall in New Jersey last October. Sandy caused $50 billion in damage.

NASA via Getty Images file

In this handout satellite image provided by NASA, Hurricane Sandy off the East Coast as it moves north on Oct. 28, 2012 in the Atlantic Ocean.

The numbers for 2013 are above the seasonal average of 12 named storms, six hurricanes, and three major hurricanes. Last year was the third-busiest storm season on record.

NOAA's seasonal hurricane outlook does not predict how many storms will hit land or where the storms will strike; it only provides an overview of the season.

"With the devastation of Sandy fresh in our minds, and another active season predicted, everyone at NOAA is committed to providing life-saving forecasts in the face of these storms and ensuring that Americans are prepared and ready ahead of time," Sullivan said.

Several climate factors are contributing to the upcoming season being busier, forecasters said.

"These factors include a continuation of the climate pattern that has been responsible for the ongoing era of high activity in the Atlantic that began in 1995; warmer than average sea-surface temperatures across the tropical Atlantic Ocean and Caribbean Sea; and near-normal, year-average seasonal temperatures in the tropical Pacific Ocean, which means El Nino ... is not expected to develop and suppress hurricane formation this hurricane season," Sullivan said.

Atlantic hurricane season lasts for six months, typically peaking between late August and mid-October.

"This year, oceanic and atmospheric conditions in the Atlantic basin are expected to produce more and stronger hurricanes," said Gerry Bell, Ph.D., lead seasonal hurricane forecaster with NOAA's Climate Prediction Center. "These conditions include weaker wind shear, warmer Atlantic waters and conducive winds patterns coming from Africa."

Related content:

• Satellite's failure before hurricane season ruffles meteorologist
  

If you found yourself bundling up in scarves, hats, and long underwear less than usual last year, you weren't alone: 2012 was the warmest year on record in the contiguous United States, according to scientists with The National Oceanic and Atmospheric Administration.

The average temperature for 2012 was 55.3 degrees Fahrenheit, 3.2 degrees above normal and a full degree higher than the previous warmest year recorded -- 1998 -- NOAA said in its report Tuesday. All 48 states in the contiguous U.S. had above-average annual temperatures last year, including 19 that broke annual records, from Connecticut through Utah.

“We’re taking quite a large step,” said Jake Crouch, a climate scientist from the NOAA National Climatic Data Center, which has recorded temperatures in the contiguous U.S. for the past 118 years.

It was also a historic year for "extreme" weather, scientists with the federal agency said. With 11 disasters that surpassed $1 billion in losses, including Superstorm Sandy, Hurricane Isaac, and tornadoes across the Great Plains, Texas, and the Southeast and Ohio Valley, NOAA said 2012 was second only to 1998 in the agency's "extreme" weather index.

A long-term warming trend for the U.S., combined with drought and a northerly jet stream, led to the record heat, explained Crouch. 

"During the winter season, the jet stream tended to stay further north of the U.S.-Canadian border, so that limited colder outbreaks in the country. It also limited precipitation. So that led to a warm and dry winter season, and that persisted through the spring," he said. 

Matt Rourke / AP file

People play in water from an open fire hydrant during the afternoon heat on July 18, 2012, in Philadelphia. July was the hottest month ever on record in the contiguous U.S.

"That warm and dry spring and winter laid the groundwork for the drought we had this summer... . When we have drought, it tends to drive daytime temperatures upward."

The unprecedented warm weather wasn't contained to the United States.

A corresponding rise in global temperatures prompted the World Meteorological Organization to call the rate at which the Arctic sea ice was melting "alarming" in its Nov. 28, 2012, report.

“The extent of Arctic sea ice reached a new record low. The alarming rate of its melt this year highlighted the far-reaching changes taking place on Earth’s oceans and biosphere. Climate change is taking place before our eyes and will continue to do so as a result of the concentrations of greenhouse gases in the atmosphere, which have risen constantly and again reached new records,” World Meteorological Organization Secretary-General Michel Jarraud said.

Each year since 2001 has been among the warmest on record worldwide, with 2012 likely to "be no exception despite the cooling influence of La Niña early in the year," the report added.

'Horrible' sea level rise of more than 3 feet plausible by 2100, experts say 

Watch NBC's special coverage of the 2012 drought 

'Wake-up call': Chicago set to break 73-year-old snowless record

NOAA expects to have global data for 2012 sometime in the coming weeks, but Crouch said scientists already know with certainty "it's going to be in the top ten" warmest years ever.

Adding to the extremes: 2012 was the driest year on record for the U.S., with 26.57 inches of average precipitation -- 2.57 inches below average. Those dry conditions created an ideal environment for wildfires in the West, which charred 9.2 million acres -- the third highest amount ever recorded, NOAA said Tuesday.

Other notable climate activity from 2012:

• Snowpack totals across the Central and Southern Rockies were less than half normal.
• July was the hottest month ever on record in the contiguous U.S.
• Tornado activity was concentrated toward the beginning of the season, with large outbreaks in March and April in the Ohio Valley and Central Plains, but the final 2012 tornado count will likely be less than 1,000 -- the least since 2002. "The factors behind that are kind of related to what was going on with the drought. We didn't have these large storm systems moving through the country, so that limited precipitation, and that also limited severe weather outbreaks," Crouch said. What made this year so high on the extreme weather index were cyclones, hurricanes, and the heat, he said.
• Alaska was cooler and slightly wetter than average, and had a record-cold January. "Their January temperatures were 14 degrees below average. Many locations in Alaska had temperatures 30 degrees below zero," Crouch said, adding that Anchorage, Alaska, set a new snow record.
• Hawaii experienced growing drought conditions, with 47.4 percent of the state experiencing moderate-to-exceptional drought at the beginning of 2012 and 63.3 percent at the end of the year. Alaska and Hawaii were not included in the bulk of NOAA's 2012 report because of terrain issues, and because scientists don't have records dating back as far as states in the contiguous U.S.

While NOAA made no meteorological forecasts for 2013, Crouch said the drought was going to continue to be an issue.

"The drought got a lot of attention this summer when it was having impacts on agriculture. More than 60 percent of the country is still in drought," he said. "And if things don't change, the drought is going to continue to be a big story in 2013."

More content from NBCNews.com:

• Conn. politician apologizes after saying Giffords should 'stay out of my towns'
• 'Please save us': Teens feared to have fallen through lake ice
• Cops: Fingernail DNA helps catch woman's killer 28 years later
• Teen in crude video about alleged Ohio rape not involved, lawyer says
• New survey helps US companies prove their 'vet friendly' claims
• Oil tanker hits San Francisco's Bay Bridge
• Lottery winner fatally poisoned day after collecting winnings, death ruled homicide
• California man says he can drive in carpool lane with corporation papers
• Supreme Court lets embryonic stem cell research go forward

Follow US news from NBCNews.com on Twitter and Facebook

Folks living along the Gulf and East Coasts can expect a slightly below-average Atlantic hurricane season, the Weather Channel said Wednesday in a forecast that falls in line with an earlier one by university researchers. The federal government, for its part, comes out with its prediction next month.

"After very active tropical seasons in 2010 and 2011, we expect fewer storms to develop this hurricane season," meteorologist Todd Crawford said on weather.com.

The big variable, he added, will be El Nino, the cyclical event that impacts Pacific Ocean temperatures and weather worldwide. An El Nino tends to increase vertical wind shear, the phenomenon of changing wind speed that can tear apart storms before they form.

"There is still uncertainty regarding the development of El Nino, which will impact future forecast updates," Crawford noted. "If the chances of El Nino development increase, our forecast numbers will likely go down even further in future updates."

Cooler North Atlantic sea temperatures are another factor behind the forecast, Crawford said.

The Weather Channel forecast calls for 11 named tropical storms, six of which become hurricanes. Two of those should be major hurricanes of Category 3 or higher, it added.

The 1950-2011 average is 12 named storms, of which seven become hurricanes and three growing to major storms.

Earlier this month, Colorado State University forecasters predicted 10 tropical storms, four of them becoming hurricanes and two "major" in size.

The National Oceanic and Atmospheric Administration will issue the official U.S. forecast in late May.

NOAA this year retired Irene from its list of names for storms because of the 48 deaths and widespread damage Hurricane Irene caused in 2011.

It also slightly modified its hurricane wind rating system. As a result, Category 3 hurricanes are now 111-129 mph (from 111-130 mph), Category 4 hurricanes are 130-156 mph (from 131-155 mph), and Category 5 hurricanes are 157 mph or higher (up from 156 mph). 

More content from msnbc.com and NBC News:

• Dad wires up autistic son, 10, to expose 'bullying' by teaching staff
• Marine who criticized President Obama on Facebook discharged
• Rodney King anniversary: 20 years after LA riots, have race relations improved?
• Supreme Court hears arguments over Arizona immigration law
• Video: Confederate flag dress gets teen banned from prom

Follow US News on msnbc.com on Twitter and Facebook