The western U.S. is particularly sensitive to variability in climate, and management of water resources in this arid region presents a formidable challenge. An improved knowledge of hydroclimatic variability is a first step in better planning and management of systems which are both directly and indirectly climate dependent.

The North American monsoon system (NAMS)is an important feature of the atmospheric circulation over the North American continent, and plays a key role in the hydrologic cycle of the arid southwest. Knowledge of intraseasonal, annual, and decadal variations in this system is crucial for current and future management of western U.S. water resources.

In this investigation, a 41-year precipitation dataset from NOAA Observing Stations and NCAR-NCEP Reanalysis Data are used to identify regional atmospheric features associated with significant bursts and breaks in monsoon precipitation. We analyze 500 mb geopotential height fields, 500 mb vertical velocity, and precipitable water over the Southwest preceding and following the onset of these events.

Spectral characteristics of burst and break periods within NAMS are examined through wavelet analysis. Fluctuations of warm season precipitation are analyzed to ascertain the spatial/temporal coherence of observed periodicities in precipitation. These analyses reveal that summer rainfall in the southwestern U.S. may be strongly modulated by large-scale, low- frequency (20-60 day) dynamics. Improved understanding of these dynamics can lead to more accurate seasonal to subseasonal forecasting of precipitation.