A moderate La Nina episode is expected to prevail through the 2007-08 winter months, with some hints that the La Nina could briefly become a strong episode. This expected La Nina pattern strongly influences the Climate Prediction Center’s (CPC) winter weather outlook, with the main storm track for Pacific storms in the western states this winter expected to impact the Pacific Northwest and the Northern Rockies.
For western Colorado (west of the Continental Divide) and eastern Utah, the latest CPC 2007-08 winter weather outlook follows.
The odds strongly favor above normal temperatures to be prevalent, ranging from about 40 percent probability near the Wyoming border to over 50 percent probability throughout southeast Utah and southwest Colorado.
Precipitation: early winter to mid winter
There are no strong trends indicated for western Colorado and eastern Utah. However, for the early-winter to mid-winter period, above normal precipitation is slightly favored (33-35 percent probability) in extreme northeast Utah (Uinta Mountains) and the extreme northwest corner of Colorado (northwest Moffat County). During the same time period, below normal precipitation is slightly favored (33-35 percent probability) across extreme southern Utah and extreme southwest Colorado (including the southwest San Juan Mountains and the eastern San Juan Mountains). The chance of below normal precipitation increases to the south into New Mexico and Arizona.
Precipitation: late winter
For the late-winter period, the trend is for slightly drier conditions than the early-winter to mid-winter period. The area favoring below normal precipitation (33-38 percent probability) spreads northward to cover most of southeast Utah and southwest Colorado south of Interstate 70. Correspondingly, the area having above normal precipitation probabilities shifts northward out of northeast Utah and northwest Colorado.
Moderate La Nina patterns tend to favor below normal precipitation in the Four Corners area, and near normal precipitation in the mountains of northwest Colorado and northeast Utah. However, there is typically less confidence in the long range forecast for precipitation than for temperatures. And a slight shift southward or northward of the jet stream flow coming off the Pacific could increase or decrease, respectively, the potential for near normal precipitation. To illustrate this concept of precipitation variability for our area, let’s examine the two most recent moderate La Nina winter episodes.
The most recent moderate La Nina winter episode occurred during the winter of 1999-2000. That winter resulted in near normal (90-100 percent of normal) snowfall in the northern and central mountains of Utah and Colorado. In contrast, the Four Corners area had below normal precipitation (generally 50-70 percent of normal), including the southwest San Juan Mountains and eastern San Juan Mountains.
The previous moderate La Nina winter occurred during the 1998-1999 season. That winter, southeast Utah and southwest Colorado only received 30-60 percent of normal precipitation, while northeast Utah and the northwest Colorado received only 60-80 percent of normal precipitation.
The two recent examples typify winter precipitation in Colorado and Utah during moderate La Nina episodes; the chance of normal winter precipitation decreases with lower latitudes.