If you're hoping for a snowy winter, Southern Colorado may not be your best bet this season.
A La Nina advisory has been active since October 9th, which means cooler than usual ocean temperatures are present in the eastern equatorial Pacific Ocean. This pattern changes the jet stream over the United States leading to several changes that vary between fall and winter in southern Colorado.
This La Nina - which is weak, barely cold enough to meet the thresholds of the definition, is likely to continue through the first half of winter. Neutral conditions become increasingly likely through the second half of winter into spring.
On average, in a weak La Nina, drier and warmer than normal weather is expected in Southern Colorado with below-average snowfall. Above-average snowfall occurs in parts of the northern and western mountains. The areas that are likeliest to be well below average are the eastern plains, with more local-scale variance near our prominent terrain features (such as Pikes Peak, our passes, and the wet mountains). These impacts are less dramatic than with strong La Nina events.
Big Picture Weather Pattern
Of the storms we do get in a La Nina - most are driven by three other patterns — the Arctic Oscillation, the Pacific-North American Oscillation, or PNA, and the Pacific-Decadal Oscillation, or PDO. These are called teleconnections. El Nino and La Nina are also teleconnections. In essence: large scale weather patterns that changes in one part of the world that lead to weather changes far away from where they are.
The Arctic Oscillation pushes cold air into Colorado when high pressure in the Arctic pushes that air south into Colorado. It varies frequently - and can only be predicted a couple of weeks in advance. The Arctic Oscillation has been favorable for cold air in the last few weeks, which is why our cold fronts have been so powerful (and tending to over perform).
The PNA brings storms to Southern Colorado when high pressure near Alaska leads to a wavy jet stream and low pressure in Colorado. It also varies quite often, and has a predictability range of about 2 weeks.
The PNA is why you haven't seen snow yet. The PNA is currently in the opposite end of its pattern (high pressure over the western U.S.)- and sending most storms to north of Colorado.
Other Considerations
In both ENSO neutral and weak La Ninas, the plains tend to be dry - which increases spring fire danger. But there are differences near the mountains. The Pikes Peak region and northwestern Arkansas River Valley tend to see near normal or even above-average precipitation in ENSO neutral cases, and below-average precipitation in weak La Ninas. In other words, precipitation is variable in ENSO neutral set ups, mostly in places where the impact of upslope flow is most pronounced.
Another teleconnection to consider is the Pacific Decadal Oscillation, or PDO, which is currently in a strong negative phase. When the PDO is negative, it generally correlates with below-average precipitation in Southern Colorado.
Other than La Nina, the PNA and AO combination will largely determine snow potential in the early winter, with the PNA being a bigger difference maker in mid-winter when temperatures are more consistently cold.
Colorado Springs typically picks up about 32.5 inches of snow in an average winter with 11.2 inches from September to January. In Pueblo, those numbers are 28.3 and 10.8 respectively.
The Climate Prediction Center's latest outlooks tilt toward above-average temperatures and normal precipitation in Southern Colorado. This is true both in the short term for November and throughout calendar winter as well. While temperature patterns aren't explicitly shown to cover all of southern Colorado, part of the reading audience will experience above average temperatures.
Outlook Summary
Given the setup, we expect below-average snowfall through the end of the year in Southern Colorado and continued above-average temperatures.
It is likely that southern Colorado will still see a few higher impact snowstorms in the late winter to early spring as La Nina conditions reduce and ENSO neutral conditions return.
Temperatures will be closer to average in the second half of winter - particularly from February to May.
Large snow events are more likely if influences from a sometimes active teleconnection called the Madden-Julien Oscillation is active.
This would likely lead to about average snowfall for the season - favoring a back-half of the season catch-up.
We'll check back in later this winter to provide an updated outlook for the second half of the winter season.
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