Heavy Precipitation Project
Heavy precipitation events do occur over the desert southwest. However, there has been limited research into the
character and distribution of these events. With the rapid population growth in desert communities the threat of these
intense storms has become a concern because of the threat they pose to people and property.
People seeking new life styles have been moving to the mild, dry climate of the desert southwest. For this and other reasons the population of the southwestern United States has been increasing rapidly for the past 25 yr. For example, the population of the metropolitan Las Vegas area is nearly 1,000,000; that of Phoenix, Arizona, nearly 2,000,000; and Tucson, approximately 1,000,000. Smaller desert communities such as St. George, Utah, Laughlin and Mesquite, Nevada, Bull Head City, Arizona, and many others are showing steady population growth and economic development. These factors create a vital need for knowledge of extreme rainfall events for input to the identification of potential flood hazards, design of flood mitigation systems, development of flood control strategies and design standards, and for improving our ability to forecast these significant natural events.
Tourism adds another noteworthy dimension to this problem. Annually, millions of families visit the recreational attractions of the mountainous desert southwest. Many people like to tour, hike, and camp in isolated and sparsely populated areas of Nevada, Arizona, Southern Utah, and in the desert area of southern California. Communication of weather warnings to these individuals is difficult, but vital to their safety.
The level of experience and awareness of the potential dangers of desert thunderstorms may be minimal with many people who have recently moved to the southwestern desert area. Upon seeing the dry desert, some may assume that heavy rain, flash flooding, intense lightning, and strong winds with blowing dust don't occur often, or, are not a serious threat to life and property. An educational program may be needed to increase public awareness.
Research at SORD has included the documentation of some of these extreme events and the characterization of their frequency distribution. Preliminary results show that 72% of all warm- season, set-hour precipitation events in Las Vegas, NV, result in totals of 0.05 in (1.3 mm) or less. However, hourly precipitation rates of 0.5 in/hr (12.7 mm/hr) up to nearly 1.5 in/hr (28.1 mm/hr) have been measured. Precipitation rates of as much as 2.0 to 4.0 in/hr (51 to 101 mm/hr) are thought to be possible in the more extreme cases and in very small areas.
Data clearly illustrate that daily precipitation totals of 2.0 to 6.0 in ( 50.8 to 152.4 mm) have occurred in southern Nevada. Dangerous flash flooding occurs with these intense storms, threatening life and damaging property.
Preliminary frequency analyses of extreme daily precipitation events for eleven different sites in southern Nevada (Yucca Flat-Desert Rock-Well 5b-Jackass Flat and Searchlight-Boulder City-Las Vegas-Desert Game Range) have been completed. Both normal and log Pearson Type III frequency analyses (Yucca Flat-Desert Rock-Beatty, Jackass Flat, Southern NTS and Las Vegas) were constructed. In a similar manner, extreme hourly precipitation events are analyzed for Las Vegas. The results have been compared with those found in NOAA Atlas 2.
Projections for the 100-yr 24-hr extreme precipitation events found in NOAA Atlas 2 match most of those calculated by SORD. However, there are important exceptions. Large differences were found over the southern part of the NTS and in the vicinity of the Yucca Mountain Project (YMPO) area. Precipitation observations and frequency analyses both show that NOAA Atlas 2 underestimates the 100-yr 24-hr/daily precipitation event for this area by approximately one inch (25.4 mm). The reason for this difference is due to the fact that the data base used to make the estimates in NOAA Atlas 2 did not contain critical, historical-type, events that have occurred since the Atlas was prepared in the early 1970s.
Research continues...
People seeking new life styles have been moving to the mild, dry climate of the desert southwest. For this and other reasons the population of the southwestern United States has been increasing rapidly for the past 25 yr. For example, the population of the metropolitan Las Vegas area is nearly 1,000,000; that of Phoenix, Arizona, nearly 2,000,000; and Tucson, approximately 1,000,000. Smaller desert communities such as St. George, Utah, Laughlin and Mesquite, Nevada, Bull Head City, Arizona, and many others are showing steady population growth and economic development. These factors create a vital need for knowledge of extreme rainfall events for input to the identification of potential flood hazards, design of flood mitigation systems, development of flood control strategies and design standards, and for improving our ability to forecast these significant natural events.
Tourism adds another noteworthy dimension to this problem. Annually, millions of families visit the recreational attractions of the mountainous desert southwest. Many people like to tour, hike, and camp in isolated and sparsely populated areas of Nevada, Arizona, Southern Utah, and in the desert area of southern California. Communication of weather warnings to these individuals is difficult, but vital to their safety.
The level of experience and awareness of the potential dangers of desert thunderstorms may be minimal with many people who have recently moved to the southwestern desert area. Upon seeing the dry desert, some may assume that heavy rain, flash flooding, intense lightning, and strong winds with blowing dust don't occur often, or, are not a serious threat to life and property. An educational program may be needed to increase public awareness.
Research at SORD has included the documentation of some of these extreme events and the characterization of their frequency distribution. Preliminary results show that 72% of all warm- season, set-hour precipitation events in Las Vegas, NV, result in totals of 0.05 in (1.3 mm) or less. However, hourly precipitation rates of 0.5 in/hr (12.7 mm/hr) up to nearly 1.5 in/hr (28.1 mm/hr) have been measured. Precipitation rates of as much as 2.0 to 4.0 in/hr (51 to 101 mm/hr) are thought to be possible in the more extreme cases and in very small areas.
Data clearly illustrate that daily precipitation totals of 2.0 to 6.0 in ( 50.8 to 152.4 mm) have occurred in southern Nevada. Dangerous flash flooding occurs with these intense storms, threatening life and damaging property.
Preliminary frequency analyses of extreme daily precipitation events for eleven different sites in southern Nevada (Yucca Flat-Desert Rock-Well 5b-Jackass Flat and Searchlight-Boulder City-Las Vegas-Desert Game Range) have been completed. Both normal and log Pearson Type III frequency analyses (Yucca Flat-Desert Rock-Beatty, Jackass Flat, Southern NTS and Las Vegas) were constructed. In a similar manner, extreme hourly precipitation events are analyzed for Las Vegas. The results have been compared with those found in NOAA Atlas 2.
Projections for the 100-yr 24-hr extreme precipitation events found in NOAA Atlas 2 match most of those calculated by SORD. However, there are important exceptions. Large differences were found over the southern part of the NTS and in the vicinity of the Yucca Mountain Project (YMPO) area. Precipitation observations and frequency analyses both show that NOAA Atlas 2 underestimates the 100-yr 24-hr/daily precipitation event for this area by approximately one inch (25.4 mm). The reason for this difference is due to the fact that the data base used to make the estimates in NOAA Atlas 2 did not contain critical, historical-type, events that have occurred since the Atlas was prepared in the early 1970s.
Research continues...