Usually, when you think of Hawaiian weather, it is visions of a tropical paradise with endless beaches as a warm breeze sways the palm trees amid infinite sunshine.
But Hawaii can bring the rain, too -- as those along the West Coast can attest a few times each fall and winter.
There is a specific atmospheric river setup where the jet stream will dip south into the tropical Pacific near Hawaii, scoop up a bunch of warm, moist air and carry it north and east across the rest of the ocean. This train of moisture will dump heavy rainfall where it reaches landfall, in either the Pacific Northwest or California.
Many locals dub this weather event a "Pineapple Express" due to the storm's Hawaiian origins and the Aloha State's penchant for pineapples.
These storms can bring torrential rains for as long as 2-3 days and are a well-known culprit of many flooding events. The more significant storms can carry as much as the equivalent of 10-15 inches of rain. The accompanying warm air will push snow levels well above average -- perhaps as high as 6,000-8,000 feet, leaving all that moisture to fall as rain in the mountains instead of snow and putting tremendous pressure on mountain-fed rivers.
Pineapple Express.— NWS Seattle (@NWSSeattle) December 20, 2019
That's the tweet. pic.twitter.com/IM6q9tK72S
On the surface, Pineapple Express days are marked by unusual warmth, with temperatures reaching well into the 50s and sometimes 60s in the Pacific Northwest when 40s are typical. Many record-high temperatures in the fall and winter are set during dreary, rainy Pineapple Express events instead of the more logical sunny days.
The Pineapple Express is always considered an atmospheric river event, but not all atmospheric rivers are Pineapple Expresses. The source of tropical moisture must originate or pass near Hawaii to earn the fruity moniker.