Southern Rocky Mountains

The vegetation in the Southern Rocky Mountain region consists of brush and scattered pine at lower elevations, and fir and spruce on higher ridges and plateaus. Many peaks extend above timberline. As in the Northern Rockies, winter temperatures are quite low, and summer temperatures are moderate for the latitude because of the elevation influence.

Precipitation is generally around 10 to 20 inches annually in the valleys and on eastern slopes, and 30 to 40 inches locally at higher elevations on the western slopes. The heavier precipitation at higher elevations is caused by the additional orographic lifting of mP air masses as they are forced across the Rocky Mountains. Most of the precipitation in the winter is in the form of snow. Precipitation is light but not infrequent during the summer, mostly as thunderstorms. These storms cause wildland fires, but ordinarily the burned acreage is small.

There are strong chinook winds with associated warm and dry conditions in the spring and fall on the eastern slopes of the mountains. These winds sometimes bring subsiding air from high levels in the atmosphere down to the surface and produce extremely low humidities.

The fire season normally extends from June or July through September, but earlier or later periods of critical fire weather may be caused by the chinook winds.

The synoptic patterns which produce high fire danger are the ridge aloft and dry cold-front passages. In addition, the pattern producing chinook winds is important on the eastern slopes. In this pattern, the airflow aloft is usually at right angles to the mountain range, while at the surface, a High is located in the Great Basin and a front is found east of the Rockies. In the area between the front and the Rockies the air flows downslope, winds are strong, temperatures are high, and humidities are acutely low.