An approaching winter storm is expected to bring heavy snow to the San Juan Mountains, and light rain or snow to Cortez and Durango.
The National Weather Service has issued a Winter Storm Watch from Tuesday afternoon to Thursday morning for the mountains of southwest Colorado and southeast Utah.
Heavy snow is possible mainly above 8,000 feet, with total accumulations of 10-18 inches with upward of 2 feet possible.
Travel could be very difficult, the weather service said. Patchy blowing snow could significantly reduce visibility, especially on ridge tops and mountain passes.
The storm system is being generated from the tropics and has a southwesterly flow into Colorado and Utah, said National Weather Service meteorologist Megan Stackhouse.
So-called Pineapple Express storms are known for bringing good moisture and also warmer temperatures.
“It is a favorable pattern for getting some moisture into the region that really needs it,” she said. “The amount of snowfall will depend on the storm overcoming warmer temperatures.”
High temperatures in the 40s and upper 30s for Cortez means rain may fall, changing to snow overnight.
Preliminary forecasts show accumulation in Cortez may be one inch, with Durango seeing 1-3 inches, Stackhouse said.
More details of the forecast will emerge today, and how much snow falls also depends on storm banding, areas of more intensive snowfall.
At the close of the year, Cortez is below normal for annual precipitation, but this storm will help some, said Jim Andrus, weather observer for the National Weather Service.
Year-to-date precipitation through Dec. 25 is 7.75 inches, or 65% of the average annual precipitation of 11.79 inches.
“It is still shaping up to be a drier year,” Andrus said. “That has been the long-term trend thanks to the triple-dip La Nina.”
The weather phenomenon the past three years generally means drier winters in the Southwest as a result of below average ocean temperatures in the equatorial Pacific.
Andrus said this winter’s snowfall is also below normal for Cortez. So far, 5 inches have fallen since October, or 68% of the average 7.4 inches through December.
Episodic breakthroughs like the Pineapple Express could buck overall drier the trend, and will help build up snowpack desperately needed to fill up McPhee Reservoir. The lake on the Dolores River has been below 50% capacity the past two irrigation seasons.
The Dolores Basin, which feeds McPhee, is 64% of average snow-water equivalent as of Dec. 25, according to SNOTEL snowfall data from the National Resource Conservation Service.
The Animas Basin is at 69% average snow-water equivalent, and the San Juan River is at 75%.
The storm moving into the area will increase avalanche danger in the San Juan Mountains, reports the Colorado Avalanche Information Center.
The Southern San Juans are currently rated moderate, a Level 2 out of five on the danger scale.
Dangerous conditions are expected to increase Wednesday.
This major storm system on the way is both good and bad news for the region’s weak snowpack, the CAIC forecast reported Dec. 25.
Faceted grains, like the ones which have plagued the snowpack structure this season, are notoriously quick to form and slow to go away, the report states. Warmer weather and, more importantly, deeper snow depths are what are necessary to heal this kind of weak grain.
“The good news is that the midweek storm could be a game-changer in the long term for snow depths and a turning point for the snowpack,” CAIC states. “The bad news is that, in the short term, it will fall on such a weak surface and foundation, that a widespread large, natural, and dangerous avalanche cycle is all but guaranteed.”