The rapid intensification of the storm is evident in two sea-level pressure analyses from Environment Canada - see below. The first is from 4 pm AKDT yesterday and the second is from 10 am today; in the 18 hours between these maps, two distinct low pressure centers merged and a rapid deepening was observed.
The more southerly of the two low pressure centers at 4 pm yesterday was the remnants of Tropical Storm Fengshen, which passed to the south of Japan on Monday as a 60-knot storm. I'm no expert on the extratropical transition of tropical cyclones, but it's likely that the deep tropical moisture associated with the southern storm helped feed the intensification of the combined system. Ironic as it may seem, the climate of Alaska is closely tied to events in the tropics.
Update: here's a wonderful infrared satellite image of the storm on Saturday morning, as linked by Gary in a comment. The image is from the water vapor band, so light areas show regions of cloud and/or high humidity in the middle to upper atmosphere, while darker areas represent low humidity aloft (not close to the surface).