The effects of gulf surges and low-level moisture on precipitation and severe weather in Arizona

Periodic surges of near-surface moisture from the Gulf of California often reach the low-lying deserts of Arizona during the annual North American Monsoon. Past researchers have assumed that the increased moisture introduced by these gulf surges acts to increase precipitation across the region despite no formal research on the topic. Presently, the primary method for detecting surges is to look for changes in the surface observations at Yuma, Arizona. Unfortunately, these surface data are easily influenced by other variables. Therefore, a new method for Assessing Low-level AtmospheRic Moisture using Soundings (ALARMS) was introduced, tested, and applied to create a 12-year (1993--2004) climatology of surges. Intraseasonal variability in surge events is related to the Madden-Julian Oscillation (MJO) with increased thunderstorm activity in the tropical eastern Pacific Ocean tending to increase surge frequency. Interannual variability is loosely correlated with El Nino-Southern Oscillation (ENSO) at three of the sites, as El Nino years tend to experience fewer surges at these locations. Composite soundings illustrate the atmospheric profile of typical surges, and it is clear that surface moisture values at Yuma, AZ are not consistent with moisture content in the low-levels above the surface.; Multiple statistical methods are employed to determine the effects of the increased moisture (surge events and low-level dewpoints), integrated precipitable water (IPW), and convective available potential energy (CAPE) on precipitation and severe weather across the lower elevations of Arizona and southern Nevada. The results show that increased low-level moisture significantly increases the amount of rainfall at each gauge as well as the number of stations receiving measurable precipitation. Further, rainfall is more likely on days with higher dewpoints, and gulf surges appear to significantly increase precipitation across the study area. IPW and CAPE display less pronounced effects on rainfall. The effects of gulf surges and low-level moisture are less pronounced on severe weather events, and IPW and CAPE also display weak relationships to severe weather. Precipitation and severe weather in the Arizona deserts rely on much more than one simple variable, but it is shown that low-level moisture (often associated with gulf surges) is an important factor.