Great Basin
In the Great Basin or intermountain region the vegetation consists of generally sparse sagebrush and grass, with some pine and fir at higher elevations. This is largely a plateau region but occupies a significant portion of the Cordilleran Highlands, with their individual peaks and lesser mountain systems, between the Rocky Mountains and the Sierra-Cascades.
The Rocky Mountains generally prevent the westward movement of cold cP air masses from the Great Plains to the Great Basin, so major cold waves with high winds are rare. Winter temperatures are quite low, however, because of the high elevation and good radiational cooling. Summer heating is very effective, and summer temperatures are high.
Annual precipitation is rather low, ranging from 10 to 20 inches in eastern Washington and Oregon and western Idaho to less than 10 inches in Nevada and Utah. At higher elevations, precipitation is higher, generally 20 to 40 inches, as in the Blue Mountains in eastern Oregon and Washington and the Wasatch Range in Utah. The entire Great Basin is in a rain shadow. The mP air masses which enter the region from the west have crossed the Sierra-Cascade Ranges and have lost much of their moisture during the forced ascent.
Much of the precipitation occurs in the wintertime, although some areas have a secondary maximum in spring. Precipitation is more general and widespread in winter, while in spring it is showery and scattered. Summer precipitation is generally light. Intensive local heating produces frequent afternoon thunderstorms, but usually little precipitation reaches the ground.
The fire season normally starts in June and lasts through September and, occasionally, October. Both timber and range fires are common.
Several synoptic weather types produce high fire danger in the Great Basin. Often, the pattern aloft is more distinctive than the surface pattern. One pattern is the same as is described above for the South Pacific coast region; that is, a pattern with an upper-air ridge over the western portion of the United States. At the surface in the Great Basin the pressure pattern tends to be flat, often with a thermal trough extending from the Southwest to the Canadian border. This pattern produces hot, dry days with considerable low-level, air-mass instability during the summer.Subsidence beneath the ridge may result in very low humidities that sometimes reach the surface.
Another upper-air pattern affecting this region occurs when short-wave troughs move through the region from northwest to southeast, steered by northwesterly flow aloft. If the cold front associated with a short-wave trough is dry, the windiness with it will produce a peak in the fire danger. These fronts are more likely to be dry in the southern portion of this region than in the northern portion.
A third weather pattern, which is important as a fire starter, develops whenever the anticyclonic circulation around a closed High aloft has transported moist air from over the Gulf of Mexico across the Southwest and northward into the Great Basin region. Then, daytime heating and orographic lifting of the moist air produces many high-level thunderstorms, which may cause numerous lightning fires.
The Rocky Mountains generally prevent the westward movement of cold cP air masses from the Great Plains to the Great Basin, so major cold waves with high winds are rare. Winter temperatures are quite low, however, because of the high elevation and good radiational cooling. Summer heating is very effective, and summer temperatures are high.
Annual precipitation is rather low, ranging from 10 to 20 inches in eastern Washington and Oregon and western Idaho to less than 10 inches in Nevada and Utah. At higher elevations, precipitation is higher, generally 20 to 40 inches, as in the Blue Mountains in eastern Oregon and Washington and the Wasatch Range in Utah. The entire Great Basin is in a rain shadow. The mP air masses which enter the region from the west have crossed the Sierra-Cascade Ranges and have lost much of their moisture during the forced ascent.
Much of the precipitation occurs in the wintertime, although some areas have a secondary maximum in spring. Precipitation is more general and widespread in winter, while in spring it is showery and scattered. Summer precipitation is generally light. Intensive local heating produces frequent afternoon thunderstorms, but usually little precipitation reaches the ground.
The fire season normally starts in June and lasts through September and, occasionally, October. Both timber and range fires are common.
Several synoptic weather types produce high fire danger in the Great Basin. Often, the pattern aloft is more distinctive than the surface pattern. One pattern is the same as is described above for the South Pacific coast region; that is, a pattern with an upper-air ridge over the western portion of the United States. At the surface in the Great Basin the pressure pattern tends to be flat, often with a thermal trough extending from the Southwest to the Canadian border. This pattern produces hot, dry days with considerable low-level, air-mass instability during the summer.Subsidence beneath the ridge may result in very low humidities that sometimes reach the surface.
Another upper-air pattern affecting this region occurs when short-wave troughs move through the region from northwest to southeast, steered by northwesterly flow aloft. If the cold front associated with a short-wave trough is dry, the windiness with it will produce a peak in the fire danger. These fronts are more likely to be dry in the southern portion of this region than in the northern portion.
A third weather pattern, which is important as a fire starter, develops whenever the anticyclonic circulation around a closed High aloft has transported moist air from over the Gulf of Mexico across the Southwest and northward into the Great Basin region. Then, daytime heating and orographic lifting of the moist air produces many high-level thunderstorms, which may cause numerous lightning fires.
Encyclopedia ID: p379
