Intraseasonal precipitation variability over the southwestern U.S., Central America, and adjacent ITCZ regions is analyzed during the monsoon season (July-August) using surface station observations, along with radiosonde, OLR, and NCEP/NCAR reanalysis data. Over much of Mexico, active periods of monsoon precipitation are most commonly associated with "easterly wave" type disturbances which propagate into the region from the Atlantic. Similar to classical easterly waves, these disturbances tilt strongly to the east with height through the troposphere, and precipitation is concentrated in the low level southerly wind perturbation behind the trough. Farther north, episodes of widespread convection over the southwest U.S. also tend to occur in southerly flow anomalies, but the circulations which predominate here have a more nearly equivalent barotropic structure, with little tilt and amplitudes that increase rapidly with height. These perturbations are accompanied by eastward shifts of the mean subtropical ridge to a position over the southeastern U.S. and Gulf of Mexico, resulting in moisture advection northward at mid-tropospheric levels from the source region over Mexico. This shift can be related to Rossby wave activity propagating into North America from the North Pacific, as shown by an E Vector analysis.

Both the easterly waves and the wave activity originating over the Pacific show highly statistically significant signals up to two weeks prior to the convective outbreaks over North America, which may be useful for predictive purposes. In addition, the submonthly activity itself is modulated by lower frequency intraseasonal changes in the circulation, a small portion of which appears to related to the Madden-Julian Oscillation. However, the bulk of this variability does not appear to be forced directly from the tropics, but may simply be due to other sources of internal variability such as wave-mean flow interaction which cause changes in the large scale circulation. Interestingly, there is only a weak systematic relationship between deep convection over the Sierra Madre and subsequent rainfall events over the southwestern U.S.