It looks like we're in for another La Niña winter. What does that mean?

It seems like every year there's some talk of El Niño or La Niña, and every year you intend to figure out what that really means. It's something about the weather getting warmer...right? But only in certain places? Wind is definitely involved, you're just not 100 percent on the where or how or why. You'll get around to it one of these years ...

This year is that year, and today is your lucky day. Buckle up, people, because you are about to finally find out what the heck La Niña is—so you can someday casually drop some knowledge into a conversation as if you've known it for years. Also, you'll understand the weather around you, since 2017-18 is supposed to be a La Niña season. That's probably more important than your cocktail party conversation game.

It begins as life did: in the ocean

The Pacific Ocean seems like one giant body of water. And it is! But within that enormous, sloshing mass is quite a lot of variation in temperature. The top level tends to be warmer, since it's heated by the sun, while the lower bits are increasingly cold.

If a strong enough wind blows across the surface, it pushes that warm layer in the same direction—displacing the cold underlayers. You can watch a nice animation of what's happening here, courtesy of the National Ocean Service.

Those winds are called trade winds (they're consistent and flow in predictable patterns, and so were used by trade ships to saily quickly from one country to another). In the Pacific, close to the equator, the trade winds blow from east to west. Warm waters travel towards the East Indies, and the deep, cold waters near the Americas rise up in their place.

That patch of abnormally cold ocean is what creates La Niña weather, because the surface temperature of a body of water has a big impact on local precipitation. This, in turn, affects global wind patterns. A weak year, which is what scientists are predicting for the 2017-18 season, means the change in temperature is between 0.5 and 1 degree Celsius below average. A moderate year is between 1 and 1.5 degrees colder, and a strong year is anything greater than that.

22 PHOTOS
Severe weather in the US 2017 -- tornadoes, snowstorms, flooding, hail
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Severe weather in the US 2017 -- tornadoes, snowstorms, flooding, hail
Abdel Salah, the owner of Sam's Food and Liquor Store surveys damage after a series of tornados tore through in New Orleans, Louisiana, U.S., February 8, 2017. REUTERS/Ben Depp
Heavy thunderstorm clouds fill the sky over Center City Philadelphia, PA, on Feb. 25, 2017. (Photo by Bastiaan Slabbers/NurPhoto via Getty Images)
Residents clear their cars and street of snow in Weehawken, New Jersey, as the One World Trade Center and lower Manhattan are seen after a snowstorm in New York, U.S. March 14, 2017. REUTERS/Eduardo Munoz TPX IMAGES OF THE DAY TPX IMAGES OF THE DAY
PERRYVILLE, MO - MARCH 01: A utility pole was downed during last night's tornado on March 1, 2017 in Perryville, Missouri. At least one person was killed when the tornado crossed interstate 55. (Photo by Jon Durr/Getty Images)
Birds sit on some branches in Central Park in New York on March 14, 2017. Winter Storm Stella dumped sleet and snow across the northeastern United States on Tuesday but spared New York from the worst after authorities cancelled thousands of flights and shut schools. Blizzard warnings were in effect in parts of Connecticut, Massachusetts and upstate New York, but were lifted for New York City, the US financial capital home to 8.4 million residents, where snow turned to sleet, hail and rain. / AFP PHOTO / Eric BARADAT (Photo credit should read ERIC BARADAT/AFP/Getty Images)
PERRYVILLE, MO - MARCH 01: A house along Pcr 906 is destroyed after last night's tornado on March 1, 2017 in Perryville, Missouri. At least one person was killed when the tornado crossed interstate 55. (Photo by Jon Durr/Getty Images)
Heavy thunderstorm clouds fill the sky over Center City Philadelphia, PA, on Feb. 25, 2017. (Photo by Bastiaan Slabbers/NurPhoto via Getty Images)
PERRYVILLE, MO - MARCH 01: A shed for farm equipment on Pcr 906 collapsed and metal sheeting was tangled in trees after last night's tornado, on March 1, 2017 in Perryville, Missouri. At least one person was killed when the tornado crossed interstate 55. (Photo by Jon Durr/Getty Images)
PERRYVILLE, MO - MARCH 01: A house and garage were destroyed on Hwy V after last night's tornado on March 1, 2017 in Perryville, Missouri. At least one person was killed when the tornado crossed interstate 55. (Photo by Jon Durr/Getty Images)
A resident clears the street of snow in Weehawken, New Jersey, as the Empire State Building and Middle Manhattan are seen after a snowstorm in New York, U.S. March 14, 2017. REUTERS/Eduardo Munoz
Debris covers a street in New Orleans East after a series of tornados tore through New Orleans and other parts of Louisiana, leaving trees, power lines and homes and businesses leveled, in New Orleans, Louisiana, U.S., February 8, 2017. REUTERS/Ben Depp
Tien Nguyen cleans up debris in "Q" Nail studio on Chef Menteur highway which he and his wife own, they survived the tornado by sheltering in the bathroom, a tornados tore through New Orleans and other parts of Louisiana, leaving trees, power lines and homes and businesses leveled, in New Orleans, Louisiana, February 8, 2017. REUTERS/Ben Depp
Two snowmen sit in Central Park in New York on March 14, 2017. Winter Storm Stella dumped sleet and snow across the northeastern United States on Tuesday but spared New York from the worst after authorities cancelled thousands of flights and shut schools. Blizzard warnings were in effect in parts of Connecticut, Massachusetts and upstate New York, but were lifted for New York City, the US financial capital home to 8.4 million residents, where snow turned to sleet, hail and rain. / AFP PHOTO / Jewel SAMAD (Photo credit should read JEWEL SAMAD/AFP/Getty Images)
Trailers lie on their sides behind a Procter and Gamble warehouse after a tornado ripped through the area on Sunday in Albany, Georgia, U.S. January 24, 2017. REUTERS/Tami Chappell
A snowplow clears snow in Times Square during a snowstorm in the Manhattan borough of New York, U.S. March 14, 2017. REUTERS/Carlo Allegri
A gas sign from a gas station sits in a tree nearby after a tornado ripped through the area on Sunday in Albany, Georgia, U.S. January 24, 2017. REUTERS/Tami Chappell
Friends used there snow day to go sledding on a hill at 47th street northwest and Massachusetts Avenue in Washington on March 14, 2017. Winter Storm Stella dumped sleet and snow across the northeastern United States on Tuesday but spared New York from the worst after authorities cancelled thousands of flights and shut schools. Blizzard warnings were in effect in parts of Connecticut, Massachusetts and upstate New York, but were lifted for New York City, the US financial capital home to 8.4 million residents, where snow turned to sleet, hail and rain. / AFP PHOTO / Tasos Katopodis (Photo credit should read TASOS KATOPODIS/AFP/Getty Images)
A police car pushes a cab stuck on a snow and sleet-covered street in New York on March 14, 2017. Winter Storm Stella dumped sleet and snow across the northeastern United States on Tuesday but spared New York from the worst after authorities cancelled thousands of flights and shut schools. Blizzard warnings were in effect in parts of Connecticut, Massachusetts and upstate New York, but were lifted for New York City, the US financial capital home to 8.4 million residents, where snow turned to sleet, hail and rain. / AFP PHOTO / Jewel SAMAD (Photo credit should read JEWEL SAMAD/AFP/Getty Images)
The Rustic Barn, an event hall, which suffered major tornado damage, is seen from an unmanned aerial vehicle in Canton, Texas, April 30, 2017. REUTERS/Brandon Wade TPX IMAGES OF THE DAY
Homeowners clean up debris after a tornado hit the town of Emory, Texas, U.S. April 30, 2017. REUTERS/Brandon Wade
Homes are severely damaged after a tornado hit the town of Emory, Texas, U.S. April 30, 2017. REUTERS/Brandon Wade
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Precipitation (or lack thereof) shifts the jet streams

A cold ocean makes for dry, sinking air. Conversely, a warm ocean makes for moist, rising air that turns into precipitation. That means that in La Niña conditions, precipitation occurs in the East Indies, while our region of the Pacific is cooler and drier.

This shift in precipitation causes the polar jet stream to dip down towards the U.S., bringing cold air with it. Northern states will see cooler weather and more rain or snow, depending on how close they are to the edge of the jet stream's path. And since the Pacific jet stream isn't as strong, the south tends to be drier and warmer than average.

La Niña's are predictable, except when they aren't

Here comes that pesky forecast uncertainty we know and love. Weather patterns on a global scale are pretty predictable, but the smaller the area you look at, the harder it is to determine the forecast.

La Niña patterns will mostly hold true year to year, but the exact precipitation distribution can vary quite a bit. Check out this comparison of La Niña years from the National Oceanic and Atmospheric Administration: It's ranked by how strong the effects were, and you can see that even within strong or weak types, the inches of precipitation aren't super consistent.

In 2005-06, the Pacific Northwest saw a ton of rain. Just a few years later in 2008-09, they'd see far less than normal. Both years were weak La Niña patterns.

That's because there are just too many other weather phenomena that affect our winters. The variation in pressure over the Arctic and the Tropics plays a significant role in determining wind flow and temperature, for example, and then there's always some randomness to make meteorologists' jobs even more difficult.

And by the way, we're not actually sure it will be a La Niña year

NOAA scientists are predicting that a La Niña will form, but we're not there yet. Right now, they're saying there's a 55-65 percent chance. For it to officially be a La Niña year, the sea surface temperature needs to hold at least 0.5 degrees below average for three months. September looked to be cooler than normal, but in the last half of the month the ocean temperature went back up a bit. So even though the wind and cloud patterns look La Niña-esque, NOAA hasn't yet declared it so.

There will be a further update coming from NOAA this week, so stay tuned for more detailed information on whether you're likely to get a ton of snow days—or just a lot of rain.

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