To understand the setup for this event, one has to look to the Hawaiian Islands where two streams of tropical moisture are being injected into the mid latitudes. In particular, there is a subtropical ridge centered just to the east of Hawaii that is pumping moisture directly from the tropics over the islands that is confluent with a potent river of subtropical moisture associated with a trough to the northwest of the Islands (click to enlarge).
|1600 UTC 16 Jan 2012 precipitable water (contours every 5 mm) and
925 mb wind vectors (click to enlarge).
|1200 UTC 16 Jan 2012 precipitable water (contours every 5 mm) and
925 mb wind vectors forecast loop valid through 0000 UTC 21 Jan
Bottom Line: The atmosphere is locked and loaded. Buckle up for a wild ride.