The Climate Prediction Center, a branch of NOAA, has declared La Niña conditions. They say there is a 65 to 75 percent chance that La Niña will last through the entire winter.
A La Niña is when there is below-average sea surface temperatures in the equatorial Pacific Ocean. This changes the flow of air in the upper atmosphere and alters the Polar Jet Stream that determines winter storm tracks in the U.S.
La Niña conditions typically bring warmer, and drier air to the State of Colorado, with the northern mountains being the exception. Places like the Park Range usually get above average precipitation.
The Front Range metro areas have received mostly below average snowfall historically from La Niña's. Last winter was the least amount of snow that Denver had received in 128 years.
The last four La Niña's have resulted in below average snowfall for Denver, and 15 out of the 19 La Niña's since 1950 have resulted in below average snowfall as well.
Denver averages about 57 inches of snow every season. Last year Denver only totaled 21.8 inches. Based on historical La Niña data, there is a higher probability of Denver receiving less than 57 inches of snow once again this winter.
That doesn't mean Denver won't get any snow storms this winter, we always do. It just means there will likely be less of them than normal.
The Colorado mountains have a different La Niña story, with mixed results through history. Last years La Niña brought record snowfall to some parts of the mountains. That is because the mountains can benefit from different types of jet stream patterns, as opposed to the urban corridor, and foothills, that rely on up-sloping easterly and northeasterly winds to create heavy snowfall.
Last year, there were varying patterns for the mountains. There were several dry spells, but the season was salvaged by a tremendously wet pattern from late December, through the month of January. This was brought on by a long lasting, straight westerly jet stream pattern that is sometimes referred to as the Pineapple Express by some meteorologists.
The Pineapple Express type of jet stream pattern brings a river of atmospheric moisture off of the Pacific Ocean, which gets intercepted by the mountains, creating heavy snowfall. That air sinks down the east side of the Continental Divide and brings dry conditions to the Interstate 25 corridor. As air sinks, in warms, and dries.
There are other climate oscillations that impact winter weather in the United States. The Pacific Decadal Oscillation, the North American, and the Arctic Oscillation, as well as the timing of the Madden-Jullian Oscillation.
The Madden-Jullian Oscillation (MJO) is an atmospheric disturbance that circles the planet about every 4 to 8 weeks. When this oscillation is near the U.S., it promotes upward motion in the atmosphere. NOAA climate scientist Dr. Emily Becker describes in a recent article, how the MJO can counter the effects of a La Nina circulation.
The impact of these other oscillations in conjunction with the ENSO, which is the La Nina/El Nino, is not well defined, but is being heavily researched right now.
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