| Description |
Regions within the western United States often experience extreme meteorological events that threaten lives and property, such as those associated with rapid temperature falls as well as frequent, prolonged periods of dry and windy conditions that elevate the risk of wildfires. The magnitude, frequency of occurrence, and seasonality of such extreme events are investigated on the basis of surface weather observations at over seven thousand locations in the western United States. A series of checks were developed and applied to the data within the period from 1997-2017 to remove erroneous reports and identify physically plausible extreme conditions. Roughly 25% of large temperature falls greater than 7°C h-1 at National Weather Service stations exceed 10°C h-1 and 5% exceed 15°C h-1. Falls as large as 19°C h-1 at Laramie, WY, and 21°C over 3 hours at Salt Lake City, UT have occurred. These rapid cooling events occur most often in the High Plains and in the Intermountain West. Road conditions can be seriously impacted when temperatures fall rapidly below 0°C. The most extreme events occur in the plains of Montana, although such events occur more frequently along Interstate 80 and 25 in Wyoming and northern Colorado. Meteorological criteria (low relative humidity and high wind speed) are used to define elevated fire risk at the Remote Automated Weather Stations in the western United States. Interior southern California experiences these conditions year-round, with some stations observing such conditions during 70% (20%) of the summer (winter). Elevated fire iv risk is more common during spring and early summer in the southwestern region (Arizona and New Mexico) and later during the summer in the Pacific Northwest. |
