The last two weeks it has been fairly cold and we had two snowstorms amounting to about 18" total accumulation. A proverbial winter wonderland. The Mt. Baker ski area is in full operation, the store is busy, things are good.
Last week, the weather forecasters started warning about a change and it showed up this evening. We know it as a Pineapple Express -- the jet stream drives moist and warm air from the tropics up to where we are. A couple inches accumulation of rain in one day and temps in the 30's and 40's.
The engine that drives this is called the Madden-Julian oscillation.
From the Wikipedia article:
The Madden-Julian Oscillation (MJO) is an equatorial traveling pattern of anomalous rainfall that is planetary in scale. The mechanism and cause of the MJO is as yet not well-understood and is a subject of ongoing study.The Wikipedia entry for Pineapple Express outlines what we are in for tomorrow:
MJO is characterized by an eastward progression of large regions of both enhanced and suppressed tropical rainfall, observed mainly over the Indian Ocean and Pacific Ocean. The anomalous rainfall is usually first evident over the western Indian Ocean, and remains evident as it propagates over the very warm ocean waters of the western and central tropical Pacific. This pattern of tropical rainfall then generally becomes very nondescript as it moves over the cooler ocean waters of the eastern Pacific but reappears over the tropical Atlantic and Indian Ocean. The wet phase of enhanced convection and precipitation is followed by a dry phase where convection is suppressed. Each cycle lasts approximately 30-60 days.
Pineapple Express is a non-technical, shorthand term popular in the news media for a meteorological phenomenon which is characterized by a strong and persistent flow of atmospheric moisture and associated heavy rainfall from the waters adjacent to the Hawaiian Islands and extending to any location along the Pacific coast of North America. The Pineapple Express is driven by a strong, southern branch of the Polar jetstream and is usually marked by the presence of a surface frontal boundary which is typically either slow or stationary, with waves of low pressure traveling along its axis. Each of these low pressure systems brings enhanced rainfall.So the Mt. Baker ski area is closed Monday and Tuesday, we will loose a lot of our base there as well as the lowland snow (snow levels are due to rise to 6,000' tomorrow). Well crap!
The conditions are often created by the Madden-Julian oscillation, an equatorial rainfall pattern which feeds its moisture into this pattern. They are also present during an El Ni�o episode.
The combination of moisture-laden air, atmospheric dynamics, and orographic enhancement resulting from the passage of this air over the mountain ranges of the West Coast causes some of the most torrential rains to occur in the region. Many Pineapple Express events follow or occur simultaneously with major arctic troughs in the Northwestern United States, often leading to major snowmelt flooding with warm, tropical rains falling on frozen, snow laden ground. Examples of this are the December 1964 Pacific Northwest flood and the Willamette Valley Flood of 1996.
