The recent storms affecting California and the West Coast have brought an uncommon term to the weather scene: Pineapple Express. The term refers to a strong flow of atmospheric moisture, and the attendant heavy rains, from the waters near the Hawaiian Islands to the west coast of North America. This pattern is often associated with a ridge of high pressure in the Arctic. As the moisture laden air runs into the western mountains, the air is forced up, resulting in condensation. That in turn produces heavy rains and snow. If the onshore flow falls further south, more of the moisture makes it into the Central Plains. This enhances our chance for precipitation, during an otherwise dry pattern. Unfortunately, the systems don’t generally tap into the moisture from the Gulf of Mexico, so precipitation amounts in the Central Plains are much more limited.
Mary Knapp, Weather Data Library