The project entailed a detailed examination of dynamically downscaled temperature and precipitation projections available from the North American Regional Climate Change Assessment Program (NARCCAP) for southwestern Colorado for two 30-year periods: a historic period (1971-2000) and a future period (2041-2070). The area of investigation was centered on the San Juan Mountains. Overall, the results show at least 4°F warming in all seasons, with summer having the highest warming in maximum (daytime) temperatures (>6°F) and greater increases at higher elevations (>8,000’). By the mid-21st century, the normal daytime summer temperatures are projected to be similar to those observed in 2002, the year with the highest observed summer temperatures in the San Juans. The consensus among six regional climate model (RCM) runs is that southwestern Colorado can expect near-average precipitation or a slight increase during December and January, near-average precipitation or a decrease from April to June, and a decrease in precipitation in July and August. However, none of the RCM simulations captured the monthly precipitation climatology of the region. In particular, all of the models had trouble reproducing various features of the North American Monsoon from July through September. In most cases, no monsoon was simulated, and in the remaining ones, the monsoon was not maintained for a long enough period. Additionally, a majority of the RCM simulations had problems reproducing the observed trend of increasing precipitation with elevation during the fall, winter, and spring months, the period during which the snowpack is accumulated. This analysis indicates that climate model projections of changes in precipitation have much greater uncertainty than temperature and, therefore, should be treated with greater caution.

Examining Regional Climate Model (RCM) projections: What do they add to our picture of future climate in the region? (Intermountain West Climate Summary July 2011)