Update: As of September 10, 2020 La Nina has formed, with a 75% chance it will last into winter.
The El Nino - Southern Oscillation, AKA ENSO, measures the cycle of tropical Pacific Ocean temperatures.
Simply put, warmer than average temperatures in the eastern Pacific indicate El Nino. Colder than average indicate La Nina. And normal conditions are considered neutral.
This ocean pattern has impacts on the weather in North America on a seasonal scale, which is why it appears in headlines across the country.
Since late 2018, El Nino conditions have been favored and may see the swing towards La Nina conditions late this year. These swings are quite common and happen every few years.
Impacts to Colorado
Colorado is always a tricky zone to pinpoint impacts from ENSO. We are land-locked and aren't as greatly impacted by ocean patterns as say, the U.S. west coast. Weak ENSO swings, either way, can show very little impacts on the seasonal weather. Strong El Nino or La Nina create more prominent
In general, in La Nina, southern Colorado will feel the impacts of fewer storm systems and drier winter weather. Northern Colorado may see more frequent snow thanks to a more active jet stream to the north.
This may prolong the extreme drought conditions well into winter in southern Colorado.
Past La Nina Events
The chart above compares the seasonal snow totals of recent La Nina years to the 30-year average seasonal snow total of 33" in Colorado Springs. In general, the trend shows a lower than normal snow year during La Nina, but that is not always the case. For example, 1999-2000 was a strong La Nina year and the observed snow total was 40.1". But the lowest total in this chart is 12.7" inches from the strong La Nina of 2010-2011.