Those who like the drier than normal weather Colorado has experienced in recent weeks will like the forecast for the rest of the winter.
According to University of Colorado at Boulder and NOAA atmospheric scientist Klaus Wolter, a moderate to strong La Ni帽a in the equatorial Pacific Ocean is not likely to fend off the threat of drier than normal weather for the state this winter.
"What I've seen so far is, it's just not too encouraging, especially for December, January and February," Wolter said of the prospects for winter moisture in Colorado.
La Ni帽a is an ocean-atmosphere phenomenon that is roughly opposite to its better-known sibling, El Ni帽o. During a La Ni帽a event, the sea-surface temperatures across the equatorial eastern central Pacific Ocean are colder than normal, while an El Ni帽o event brings warmer than normal ocean temperatures to the region.
Typically an El Ni帽o event means a wetter than normal winter across the southern United States while a La Ni帽a means drier conditions for these areas.
While much of southern Colorado tends to be dry with La Ni帽a, the central and northern mountains typically receive at least average snowfall during a La Ni帽a winter. But that most likely will not happen this year, said Wolter. Indications point to a much drier winter than he would expect from a typical La Ni帽a, he said.
"The high pressure cells that typically develop over the eastern central Pacific Ocean during a La Ni帽a phase appear to be setting up further to the north than in other such winters," Wolter said. "This may shunt the northwesterly jet stream to the north, which would otherwise bring moisture-laden storms into this region, translating into fewer storms throughout the winter."
Wolter said his winter outlook, if it holds up, could be devastating to regions already hard hit by drought, such as southern California and the desert Southwest. And he adds that Colorado, which is one of the few western states without current drought concerns, could lose some of the ground it has slowly regained after the severe drought of 2000-2002.
"I am very concerned that Colorado, which is essentially drought-free on the national drought monitor, might see regions of drought develop by spring," he said. "Most vulnerable, I believe, would be the Four Corners area and southeastern Colorado."
This La Nina also is unusual because it came on quickly and strong so late in the year, said Wolter. La Ni帽a most often develops in the spring, but this one took off in August.
"It was one of the most dramatic transitions from near normal or neutral in the Pacific to a moderately strong La Ni帽a," based on records from the last 60 years," he said.
Wolter is a research scientist for CU and the National Oceanic and Atmospheric Administration through the joint institute CIRES or Cooperative Institute for Research in Environmental Sciences.