Regional Surges of Monsoonal Moisture into the Southwestern United States

Episodic surges of moisture into the southwestern United States are an important attribute of the North American monsoon. Building upon prior studies that identified mesoscale gulf surges using station-based diagnostics, regional surges in monsoonal moisture are identified using precipitable water and integrated water vapor flux from the North American Regional Reanalysis. These regional surge diagnostics exhibit increased skill over gulf surge diagnostics in capturing widespread significant multiday precipitation over the state of Arizona and are associated with the northward intrusion of moisture and precipitation into the southwestern United States. Both tropical and midlatitude circulation patterns are associated with identified regional surge events. In the tropics, the passage of a tropical easterly wave across the Sierra Madre and through the Gulf of California facilitates a northeastward flux of moisture toward the southwestern United States. In midlatitudes, the breakdown and eastward shift of an upper-level ridge over the western United States ahead of an eastward-propagating trough off the Pacific Northwest coast helps destabilize the middle troposphere ahead of the easterly wave and provides a conduit for subtropical moisture advection into the interior western United States.