March was a game-changer, says state climatologist Nolan Doesken
By Bob Berwyn
SUMMIT COUNTY — Colorado’s water managers and weather experts shouldn’t be surprised by the dry skies and warm temperatures. Historically, the majority of past disaster declarations in the state have been related to drought, which indicates the state’s vulnerability to this particular natural hazard, according to boilerplate language found in hazard mitigation plans in almost every county.
Summit County’s multihazard mitigation plan, for example, says clearly that multi-year droughts can be expected about every 10 years, so this season’s dry conditions are right on schedule after the last major dry spell culminated in 2002. Since 1893, Colorado has experienced six multi-year droughts that are widely considered “severe.” These droughts affected most of the state and involved record-breaking dry spells.
While this year’s early snow melt is not unprecedented, it is unusual, according to state climatologist Nolan Doesken, who said the last time the state saw similar conditions was more than 100 years ago, in the 1910 drought, which was widespread across the West and also led to one of the most destructive wildfire seasons on record. In that year, conditions did turn around a bit in late April and May, he added.
“March was the real game-changer,” Doesken said, explaining that the state’s snowpack usually grows and peaks during the last few weeks of winter; this year, it was the opposite, with a meltdown that saw statewide snowpack dwindle by nearly a third.
In fact, some parts of the central and northern mountains recorded above average precipitation in January and February, but that wasn’t enough to make up for a dry early winter, and when March turned balmy, the snow simply vanished.
“We don’t expect north-facing, high-elevation slopes to melt in March,” said Doesken by phone from Durango, where he was attending an annual water-users conference.
High-wind episodes during some of the dry periods between storms also were a factor, scouring the snow from some of the highest elevations, where the snow usually builds into thick, deep pillows that slowly melt during the spring. Some of the snow that’s blown around by the wind also simply sublimates into the atmosphere, Doesken said.
That means most of the snow ended up at mid-elevations, in many cases in the lodgepole pine belt, where the beetle-kill may also be a factor in the rapid melt-down. Instead of being shaded by thick evergreen branches, the snow on the ground was exposed to the early spring sun.
There’s still a slight chance that a big wet storm — like a 2003 mid-March deluge that ended the 2001-2002 drought — could make up some of this winter’s deficit, but those odds diminish with each passing week, he said. And the outlook for the next few months from NOAA’s Climate Prediction Center continues to trend toward the dry side, with better than average odds for warmer-than-average temperatures and below-average precipitation, he added.
There’s also the question of whether the early meltdown will have an effect on the summer monsoon, when moist subtropical air streams northward into the Rockies to trigger thunderstorms and showers. The average start date for the monsoon is mid-July.
Some weather experts have suggested that a dry winter can affect the monsoon because there’s not as much localized moisture available for evaporation and condensation. But overall, there doesn’t seem to be much statistical data to support that conclusion, Doesken said.
“There’s an ever-so-slight bias toward more precipitation after a dry winter,” he said, explaining that one theory is that early heating in the Southwest helps develop the big desert heat low pressure system that becomes the pump for monsoon moisture.