What is the typical scenario linking intraseasonal oscillations to heavy precipitation events in the western U.S.?
The typical scenario linking the pattern of tropical rainfall associated with the MJO to extreme precipitation events in the Pacific Northwest features a progressive (i.e. eastward moving) circulation pattern in the tropics and a retrograding (i.e. westward moving) circulation pattern in the midlatitudes of the North Pacific (Fig. 1). Typical wintertime weather anomalies preceding heavy precipitation events in the Pacific Northwest are as follows: (1) 7-10 days prior to the heavy precipitation event: Heavy tropical rainfall associated with the MJO shifts eastward from the eastern Indian Ocean to the western tropical Pacific. A moisture plume extends northeastward from the western tropical Pacific towards the general vicinity of the Hawaiian Islands. A strong blocking anticyclone is located in the Gulf of Alaska with a strong polar jet stream around its northern flank.
