You can usually see it on a satellite photo, where the band of rainfall stretches from the Pacific Northwest all the way southwest to near Hawaii.
These tend to be the wettest-type of storms we get around here -- bringing between 1-2" of rain per day for Seattle and much more near the mountains -- as it has an abundance of warm, tropical moisture.
It also brings high potential for flooding, as the warm air tends to raise snow levels quite high around here. That has two compounding effects:
1) It means precipitation falls as rain instead of snow in the mountains, increasing the run-off into the local rivers.
2) It will also begin to melt the snow at lower mountains altitudes, adding even more liquid water into the mix.
Pineapple Expresses usually mean wintertime temperatures in the upper 50s or warmer. I remember a December night in the mid 1990s when the temperature reached 63 degrees -- at 2 a.m.(!) -- during a Pineapple Express event.