By Robert Tann – Summit Daily News, (Via AP Storyshare)

The seasonal pattern is transitioning to its opposite heading into the summer, bringing with it a change in the jet stream

After months of being under an El Nino pattern, Colorado and the rest of the U.S. will begin to shift to the inverse, known as La Nina. 

Both terms refer to the change in surface temperature of the Pacific Ocean off the coast of South America — with El Nino signifying seasonally warmer temperatures and La Nina signifying cooler temperatures that impact the jet stream. Generally speaking, El Nino pushes the jet stream south and can bring more precipitation and cooler temperatures to southern regions while La Nina does the opposite. 

This past winter marked the first in three years to experience an El Nino season. But what impact the pattern had on the Rocky Mountains is harder to tell compared to other parts of the state.

In Breckenridge, for example, the majority of winter and early spring netted above-average precipitation, something that would be associated with a La Nina year, said Kenley Bonner, meteorologist for the National Weather Service office in Boulder. November was the only month to see below-normal precipitation, while the rest of the months through March were above normal, Bonner said. Temperature wise, this past winter was warmer than average, according to data collected in Dillon. 

The same can be said for much of the Western Slope. In Grand Junction, monthly average temperatures have hovered around 4 degrees above normal since November, said Lucas Boyer, meteorologist for the National Weather Service office in Grand Junction. 

“Time and again it’s proven hard to say, ‘It’s El Nino, it’s going to predict this or that,’” Boyer said of High Country and Western Slope areas. 

Still, in some areas, the trend in snowpack appears to have somewhat followed expectations based on previous El Nino years. 

Ahead of the 2023-24 winter season, OpenSnow meteorologist Sam Collentine predicted that, based on an analysis of past El Nino seasons at Copper Mountain in Summit County, winter snowfall may lag before increasing near the end of the season. 

“It tends to be that the shoulder seasons are above normal, while the winter months are below normal,” Collentine wrote in an Aug. 30 blog post on OpenSnow.com. 

Snowpack in the Blue River Basin, which encompasses all of Summit County, had a slow start to the season, with levels below the 30-year median for much of November through the first half of January. Snowpack climbed afterwards, trending along the 30-year median line for much of February before rising above normal for all of March and the first two weeks of April. 

The same was true for the entirety of the Colorado River Headwaters Basin, which includes some central and northern mountain areas as well as parts of the Western Slope. 

Looking to the rest of 2024, signs point to the return of a La Nina pattern brought on by cooling ocean temperatures. 

According to an April 11 report from the Climate Prediction Center, a transition from El Nino to a neutral system, where ocean temperatures are seasonally normal, is 85% likely to happen between April and June. There is currently a 60% chance that a La Nina system will then develop between June and August. 

Early reports show the transition could make for a hotter, dryer than normal summer across the U.S. 

“As we see the strong La Nina forecast evolve, there’s indications that we get into a dryer than expected seasonal pattern for what we normally expect for our monsoon season for the summer,” Boyer said. “If we don’t have monsoon showers or thunderstorm activity in the afternoon, we typically warm up and our temperatures stay above that climatological warmer mark.”

The reason has to do with the change in the jet stream, with La Nina patterns typically bringing warmer, dryer air from the south upwards. 

A three-month outlook released by the prediction center on April 11 shows Colorado has between a 33% and 50% chance of experiencing above-normal temperatures for May, June and July in various areas. The southwestern portion of the state also has between a 33% and 40% chance of seeing below-normal precipitation during that period, while the northeastern portion has equal chances of seeing above- or below-normal precipitation. 

Meteorologists noted that these predictions are long-term forecasts and that a lot can change in the coming weeks to shift their trajectory. When it comes to the High Country, especially, the correlation between La Nina and weather will be difficult to know. 

“Typically, if we do experience anything from La Nina it is from May to September, and that would favor warmer, dryer conditions in eastern Colorado,” said Bonner with the weather service’s Boulder office.

The post El Nino is winding down. Here’s what the winter season looked like for Colorado’s mountains — and what comes next. appeared first on Yellow Scene Magazine.