The waning days of El Ni帽o 鈥� an atmospheric pattern that can affect the jet stream and, with it, weather conditions 鈥� means the impending arrival of its opposite pattern: La Ni帽a.
Both are influenced by surface temperatures along the Pacific Ocean off the coast of South America, with El Ni帽o signifying above-normal temperatures and La Ni帽a signifying below-normal temperatures. The result can be wetter, cooler conditions in southern regions and dryer, warmer conditions in the north during an El Ni帽o season, with La Ni帽a bringing the opposite.
So what does the transition from an
Early predictions show the state could be in for a hot and dry summer. issued on May 16 shows Colorado has a chance of seeing above-normal temperature and below-normal precipitation from June to August.
Specifically, northeast Colorado has a 33% to 50% of higher-than-normal temperatures while southwestern Colorado has as much as a 60% to 70% chance. Northwest and southeast areas, as well as central Colorado, have between a 50% and 60% chance for higher temperatures.
Practically all of the state has a 40% to 50% chance of seeing below-normal precipitation, according to the projections.
These forecasts are always subject to change, especially when looking at longer-term predictions. And drawing a correlation between a pattern like La Ni帽a and seasonal weather can be tricky, particularly in Colorado, experts say.
鈥淚n the summertime, the connection between La Ni帽a and the U.S. climate is not that strong,鈥� said Nat Johnson, a meteorologist for the National Oceanic and Atmospheric Administration. 鈥淪o generally other factors tend to dominate the forecast, including long-term warming trends, soil moisture anomalies.鈥�
La Ni帽a tends to have the strongest influence on summer weather in areas east of the Rocky Mountains, particularly the midwest. Any impact in Colorado tends to be concentrated around the eastern plains, with La Ni帽a potentially bringing warmer, dryer weather, Johnson added.
OpenSnow meteorologist Alan Smith stated in that while summer weather patterns are 鈥渘otoriously difficult to predict in advance, even more so than winter,鈥� there are some weather signals 鈥渢hat arise across the Western U.S. when we examine past summers featuring a transition from El Ni帽o to La Ni帽a.鈥�
In his post, Smith analyzed five years since 1991 during which El Ni帽o conditions dissipated in the late winter or spring and gave way to La Ni帽a in the summer. The years, which were 1995, 1998, 2007, 2010 and 2016, were then compared to 30-year climatological averages from 1991-2020.
In general, between June and September, temperatures for much of Colorado were average, with slightly hotter temperatures on the east side of the Continental Divide, according to the analysis.
For precipitation, Smith鈥檚 analysis found summers tended to be more wet than normal across a large portion of the western U.S., with the strongest signals across the northern Rocky Mountains. Colorado鈥檚 Front Range, however, was an exception with rainfall that tended to be below average.
June proved to be the biggest 鈥渨ild card鈥� month in the analysis, 鈥渨ith some Junes featuring hot and dry conditions, and others featuring much cooler and wetter than normal conditions,鈥� Smith wrote.
Last summer, Colorado was under an El Ni帽o pattern and in June saw below-average temperatures in virtually all parts of the state, according to data from the National Weather Service office in Boulder. Precipitation that month was right around normal for most areas west of the divide and above normal for parts of the Front Range and eastern plains for the month of June.
Last July, temperatures were near to below normal across the state鈥檚 northeastern plains while above normal in mountain areas west of the Continental Divide and mountain valleys. Precipitation was near to above normal for the eastern plains while the mountains and valley areas west of the divide were significantly dryer.
In August, temperatures were slightly above normal for most of the state. In terms of precipitation, southern and eastern parts of Colorado were near- or slightly-below average while western and northern areas were near or slightly above average.
But forecasts for a hotter, dryer summer this year have more to do with long-term warming trends, said Johnson, the National Oceanic and Atmospheric Administration meteorologist.
鈥淗ere in the summer, it鈥檚 hard to find a good correlation,鈥� said National Weather Service Boulder office meteorologist Jim Kalina. 鈥淭here鈥檚 really no guarantees with these predictions. We had a fairly wet March and April. It鈥檚 really turned dryer in most places in May, but it is pretty uncertain.鈥�