VERIFY – YOU’VE GOT QUESTIONS, WE’LL FIND ANSWERS
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La Niña, the reverse cool phase of El Niño, is now underway for the second year in a row.
Federal government forecasters officially announced its formation on Nov. 9. There is now a La Niña advisory, which is issued when conditions are observed and expected to continue.
National Oceanic and Atmospheric Administration said this winter’s La Niña is predicted to be relatively weak with a 65 to 75 percent chance it will continue at least through winter.
The 9NEWS Verify team and meteorologist Cory Reppenhagen dug into the data to find out if this year’s La Niña will result in less snowfall in Denver – a commonly held assumption.
SNOWFALL IN DENVER
Based on historical La Niña data for Denver's official weather station, chances are pretty good Denver will see less snow than average this winter.
The Denver metro typically has below average snowfall during La Niña winters, according to Reppenhagen.
Russell Danielson, a meteorologist for the National Weather Service, said since this is a weak La Niña, it is more difficult to tell exactly what conditions will be in Denver. Basically, he said there is less confidence in it being the major player in our winter weather patterns.
But he confirmed Reppenhagen’s findings.
“La Niña winters do favor slightly drier than normal conditions and less snowfall in Denver,” Danielson said.
There have been 22 winters since 1950 where we spent at least part of the season in a La Nina. 18 of those have resulted in less than the average 57 inches of snowfall for Denver.
Out of the top 5 biggest snowstorms in the mile-high city since 1950, one of those happened during a La Nina - 21.5 inches in November of 1983.
The last 6 La Nina's have brought below average snow to Denver, including the two driest seasons in more than 100 years.
The 2016 winter season is the most recent example in which only 21.8 inches of snow fell.
While La Niña data indicates there is a higher chance Denver will receive less than 57 inches of snow again this winter, that doesn't mean there are no big snow storms in a La Nina year.
People in the Denver area can expect fewer snowstorms than normal, but those snowstorms could still bring large amounts of snow, according to Reppenhagen.
La Niña conditions typically bring warmer and drier air to Colorado, with the exception of the northern mountains. Reppenhagen said areas such as the Park Range usually get above average precipitation.
The mountains - and even the foothills - have far less conclusive data, and La Nina has less of a consistent impact there.
WHAT IS A LA NIÑA?
A La Niña refers to below-average sea surface temperatures in the equatorial Pacific Ocean.
According to Reppenhagen, during a La Niña, the flow of air in the upper atmosphere changes and alters the Polar Jet Stream, which determines winter storm tracks in the U.S.
“The surface winds across the entire tropical Pacific are stronger than usual, and most of the tropical Pacific Ocean is cooler than average,” according to NOAA. “Rainfall increases over Indonesia (where waters remain warm) and decreases over the central tropical Pacific (which is cool).”
NOAA states there is more rising air motion and lower surface pressure over Indonesia, but more sinking air motion over the cooler central and eastern Pacific waters.
According to the Climate Prediction Center, La Niña associated oceanic and atmospheric conditions are usually opposite of El Niño.
“El Niño and La Niña are the warm and cool phases of a recurring climate pattern across the tropical Pacific—the El Niño-Southern Oscillation, or “ENSO” for short,” according to NOAA.
NOAA’s website said the pattern can swing back and forth every two to seven years on average.
While scientists can't prove a La Nina equals less snow, looking back over the years almost all La Niña winters in Denver have been more dry.
If this season follows suit, people in the Denver area can expect to trudge through less than 57 total inches of snow once again this year.
But if Denver gets a big storm this winter, it doesn't mean La Nina is gone.
Even the years with a few big individual storms still ended up being below average for the entire snow season.