To start, here is the latest outlook from the Climate Prediction Center for the winter season of December-January-February. For that little black dot that represents Grand Mesa, the outlook shows a shift of odds to a warmer-than-normal winter but with no shift of odds in the precipitation graph seen on the right. But there is more that we can learn from our climate record.
ENSO background information. Modern day numerical weather prediction does an arguably excellent job producing a 7-day forecast. Under ideal conditions, forecast models can track individual storms out to 14 days. But beyond 10-14 days, individual storms are lost in chaos. So, climatologists look out to the state of the oceans for a clue to storm track and intensity. For us here in the American West, we look out to the state of the Pacific Ocean from where our cold season storms come.
The most telling area in the Pacific is along the equator off the coast of Ecuador. Climate scientists have found a correlation between abnormal temperatures in these equatorial waters and cold-season weather patterns in North America. That is actually a really fun concept: water temperatures west of the Galapagos Islands influence our winter weather! This climate pattern is called El Niño Southern Oscillation or ENSO.
For this winter, we are headed into the cold phase of ENSO called La Niña. What does the climate record show for La Niña winters? Let’s look at La Niña effects scaled from the Northern Hemisphere down to Grand Mesa.
Hemisphere effects. ENSO tends to change the jet stream in predictable ways. This is important because winter storms tend to follow the jet stream. La Niña enhances the Polar Front Jet typically located along the USA-Canadian border.
North American effects. Since winter storms tend to follow the jet stream, La Niña tends to bring above-normal precipitation to the Pacific NW and northern Rockies, often leaving the SW USA dry. Now look again at the CPC’s winter outlook charts. Can you see the effects of La Niña? Generally cold and wet in the northern states, warm and dry south. Colorado is sandwiched in between.
Colorado effects. La Niña tends to produce storms coming in from the NW, so it often brings above normal precipitation to NW Colorado including Steamboat Springs and Winter Park. This storm track typically leaves SW Colorado dry. The dividing line of effects is roughly along the Highway 50 corridor. La Niña tends to produce a drier-than-normal fall and spring, but then a wetter-than-normal January. For this season so far, the fall season has been generally dry, just as La Niña predicts.
Grand Mesa effects of La Niña since 1990 (the beginning of the GMNC).
There have been 12 La Niña seasons since 1990. Seven of those La Niñas had normal or above normal seasonal snowfall. Two recent big-snow La Niña winters were 2007-08 and 2010-11 which ended the season with 130% of normal snowpack.
Five of the 12 La Niña were drier than normal. One of those was a record dry season, 2011-12 which ended with 50% of normal snowpack. Last season was the most recent La Niña at about 90% of normal snowpack. (However, most Colorado mountains were at or above normal snowpack at the end of last season.)
This season’s La Niña is forecast to become a strong event. Of four strong La Ninas since 1990, three produced near to above normal snowfall, one season was below normal.
Another impact of La Niña on Grand Mesa: with storms generally approaching from the northwest, the north-facing slopes will be favored for heavier snowfall, from Powderhorn to Mesa Lakes to Skyway-County Line. Ward Lake trails, on the south slopes of the Mesa, are expected to get less snow.
Conclusion. ENSO is the most powerful winter-season outlook tool, but it only explains 15-25% of a winter’s weather. Still, our climate record shows La Niña slightly shifts the odds for a snowy winter that favors the month of January. Other impacts include a drier-than-normal fall and spring.
Also, one final important climate point: This winter outlook is based on past winters’ behavior. As we continue to add heat to the atmosphere, our past climate will be less and less of a predictor for our future weather. We can expect increasing weather variations from the climate normal, and extreme weather patterns we have never seen before.
This is part two of a three part series about weather on the Grand Mesa. Part one was about different types of snow and how they are formed. The third article will offer rules of thumb on how to add quality to the forecast for the Mesa top.