A record El Niño is building in the Pacific. Here is what it could mean for a dry Colorado.
The Pacific Ocean is doing something it has done only three times since record keeping began in 1950, and forecasters say this time may top them all. For Colorado, coming off its warmest water year on record and one of its driest, the question is whether that will bring the snow and rain the state needs.
Thursday, the federal Climate Prediction Center said El Niño, the periodic warming of the eastern tropical Pacific, is still strengthening. Sea-surface temperatures in the region forecasters watch most closely ran 1.8 degrees Celsius above normal in August. The center put the chance of a very strong El Niño this fall and winter above 90 percent, and gave a 75 percent chance that it becomes the strongest on record.
"With an event of this magnitude, the chances of experiencing impacts consistent with El Niño are larger, though not guaranteed," the center wrote.
What El Niño is
Most years, trade winds push warm surface water across the Pacific toward Asia, and cold water rises along the coast of South America. During El Niño those winds weaken, warm water spreads east, and the shift changes weather patterns around the world. It usually peaks in the northern winter.
©
For the United States, a strong El Niño tends to strengthen the southern branch of the jet stream, the river of air that steers storms. That raises the odds of cool, wet weather across the southern tier of states and warmer, drier weather across the northern tier. Colorado sits near the boundary, which is why the state's results have been mixed.
What it usually means here
Colorado's own records tilt in the state's favor. Since 1950, the state has seen 26 water years with an El Niño winter. In 17 of them, statewide precipitation for the year came in above the long-term average of 17.8 inches. In the 25 La Niña years, only seven did. The average El Niño year brought 18.3 inches, about an inch more than the average La Niña year.
The three "super" El Niños on record, 1982-83, 1997-98 and 2015-16, all produced above-average precipitation statewide, at 19.7, 18.6 and 19.0 inches.
©
But precipitation and snowpack are not the same thing, and the Colorado Climate Center at Colorado State University warns against expecting a big snow year on the strength of the ocean alone. In a September 3 analysis, Assistant State Climatologist Peter Goble wrote that El Niño years are generally wetter in Colorado, "especially true in fall and spring," and that the Front Range and Eastern Plains "are more prone to large spring snowstorms and experience fewer dry and windy spring days." That matters in southeast Colorado, where spring wind strips moisture from fields.
The link to mountain snowpack, however, "is weak," Goble wrote. Only the southern basins, the San Juan and the Rio Grande, show a loose tendency toward more snow in El Niño years. Of the three super El Niños, "1982/83 delivered huge snowfall to Colorado, the latter two yielded roughly average conditions."
"Drawing firm conclusions from three events is unreliable," he wrote.
There is one more caution. Colorado is warmer than it was in 1983, and Goble noted that even if the storms come, "less of the precipitation may fall as snow than in a typical year, especially at lower elevations."
"It is a bit like playing a game of poker with a few extra aces and kings in the deck," he wrote. "The deck may be stacked in our favor, but we could still draw a crummy hand."
What the forecasters say
The Climate Prediction Center's seasonal outlooks, issued August 20, lean the same way. For September through November, every part of Colorado has better-than-even odds of above-normal precipitation, with the strongest lean over the southwest corner. For December through February, the wet lean holds across the southern half of the state, including the southeastern plains, while northern Colorado has equal chances of a wet, dry or normal winter. Winter temperatures are a toss-up statewide.
©
The center's seasonal drought outlook, issued August 11 and running through November, expects drought to ease across most of Colorado, with outright removal in some areas, on the strength of monsoon moisture and the El Niño pattern. It expects drought to hang on across parts of the Plains to the east, where the seasonal outlooks show no wet signal.
©
The state's Water Conditions Monitoring Committee, which hears monthly from the Climate Center and the Natural Resources Conservation Service, summarized the outlook at its August meeting this way: federal outlooks point to a wetter-than-normal fall, "but substantial uncertainty remains."
What it cannot fix
Even a wet winter will not undo the last one. Snowpack peaked in early March at 51 percent of the median, the lowest on record, and statewide streamflow from April through July was 28 percent of the median, according to the monitoring committee. Statewide reservoir storage stood at 60 percent of the median in late August, and Blue Mesa Reservoir was at a record low. The state climatologist wrote in June that the Arkansas River was on track for its lowest water-year flow on record.
Colorado remains under the statewide drought emergency Governor Jared Polis declared June 4. As of September 8, 75 percent of the state was in severe drought or worse, and Kiowa County was 95 percent in severe drought and 43 percent in extreme drought.
The Climate Prediction Center updates its seasonal outlooks September 17, the monitoring committee meets September 21, and the Drought Monitor's final map of the water year comes out October 1.
