La Nina weather patterns are likely to continue into the winter according to the National Weather Service. La Nina was partially to blame for lower snowfalls through the month of December last year, and the pattern could return this year, although meteorologists say it’s too early to be certain.

La Nina tends to bring wetter than normal conditions for the northern half of the U.S. and drier than normal conditions for the southern half, with the dividing line falling on Colorado’s equator. As a result, northern Colorado tends to see increased precipitation with southern Colorado receiving less precipitation.

For Summit County, a mountain environment nestled in the heart of the state, predicting precipitation from La Nina can be tricky, Boulder’s National Weather Service Meteorologist Bruno Rodriguez said.

“Realistically, it’s very hard to say what that means in terms of precipitation,” he said. Summit County should expect more accurate winter weather predictions closer to the winter season, he said. The combination of La Nina and the county’s location can create unpredictable outcomes. Last year’s lower December snowfall may not repeat this year.

The National Weather Service expects La Nina to taper between December and February. The probability of La Nina is at 86%, but the percentage is at 60% in December through February, the National Weather Service predicted.

While a majority of North American Multi-Model Ensemble models suggest La Niña will transition to a neutral impact between January and March 2023, forecasters are split on this outcome resulting in equal forecast probabilities for that season, the National Weather Service reported.

La Niña’s greatest influence on weather and climate is during the winter, but National Oceanic and Atmospheric Administration Emily Becker indicated the climate pattern could have a minor role in the monsoon season by possibly creating an earlier start in the southwest U.S.

If La Nina does persist, it will be the third time the climate pattern has repeated for three years in a row in the 73 years the National Weather Service has tracked it. The climate pattern appears when the sea surface temperature in the east-central Pacific Ocean is cooler than the long-term average by at least 0.5 degrees Celsius.

All but three of the last 24 months have shown the same trend, according to climate data.

In July, the sea surface temperature in the Niño-3.4 region of the tropical Pacific, our primary monitoring region, was 0.7 degrees Celsius cooler than average according to the Extended Reconstructed Sea Surface Temperature, Version Four dataset, a set of temperature data taken from ships and buoys.

Stronger atmospheric circulation above the equator must also be present for a La Nina pattern. Rising air over the warm waters of the western Pacific, paired with westerly winds at high altitudes bringing air across the Pacific toward the U.S. In the central and eastern Pacific, that air descends over the relatively cooler eastern side of the ocean and hits east-to-west trade winds closer to the water’s surface. The whole loop is referred to as the Walker Circulation.

July recorded all the expected strengthened Walker circulation features, including substantially stronger-than-average trade winds which can further cool the water’s surface temperature. Weekly measurements from the National Weather Service show July starting at -0.5 degrees Celsius and ending at -1.0 degrees Celsius.

Stronger trade winds can also create an upwelling Kelvin wave: cooler-than-average water traveling east under the surface waters of the equatorial Pacific. The trade winds will drag waters west towards Indonesia and cooler water from below will rise. The cooler waters rising to the surface will keep the surface colder the next few months. Colder waters increase La Nina’s likelihood of sticking around, Becker wrote.