This study is concerned with early monsoon-like or monsoon onset precipitation occurring over the desert southwest and Great Basin of the U.S.A. during 2002, and the testing of mechanistic ideas recently advanced in J. Climate. The proposed hypothesis argues that the North American (NA) monsoon in this region draws from the Gulf of California (GC) as a moisture source, with monsoon-like precipitation beginning after GC sea surface temperatures (SSTs) reach 26 C, and with relatively heavy rains beginning after northern GC SSTs reach 29 C. Naturally, favorable low-to-mid-level winds are needed for advecting GC moisture into this region. It was postulated that most onset behavior can be explained through these factors: GC SSTs and atmospheric circulation.

The first warm season rainfall affecting the Great Basin and Arizona (AZ) occurred on 31 May and 1-2 June. Satellite imagery suggests this was due to (1) a low pressure system off the coast of San Diego and (2) a decaying hurricane south of the Baja Peninsula. However, just prior to this period, SSTs reached 26 C in the central and southern GC, suggesting a third factor: GC SSTs supplying moisture that was advected northwards in combination with moisture from (1) and (2). This unusual monsoon-like precipitation episode occurred over a month before the monsoon season normally begins.

The NA monsoon officially began in early July in Tucson AZ due to an unusual arm of high pressure advecting moisture from the Gulf of Mexico westwards into AZ. Rainfall was light-to-moderate. As this elongated high became localized over W. Nevada and moved slightly eastward, SSTs in the N. GC warmed from 27 C to 29.5 C over a 2 day period (8-10 July), exceeding the 29 C threshold noted above for heavy rainfall. Low-to-mid-level winds became southerly, and in several days thereafter, moisture advected from the GC region resulted in thunderstorms over much of eastern California, Nevada, and AZ. These were the first heavy rains of the summer. In fact, on July 14th, strong thunderstorm winds forced the closure of the Phoenix International Airport for many hours, causing extensive damage and many flight cancellations. The development of these rains appear consistent with the mechanistic ideas advanced in our previous six season study, namely the dependence of heavy rainfall on the timing of the N. GC 29 C threshold.

Including GC SST information might improve forecasting skill in this region, especially during July when monsoon onset typically occurs. It is noteworthy that the timing of the N. GC SST threshold in 1999 was followed 3 days later by the Las Vegas flood, costing $20 million in property damage.