After a brief reprieve here in Seattle (and an inch or so of rain last night), another shortwave trough is moving up through the area and rain is on its way in again. So the rain will continue. But, it's going to be a rainy week or so ahead as well. Why is this? A particular weather event known as a Pineapple Express (not the movie...the real thing). Let's look at what makes a Pineapple Express event.
In simplest terms, a Pineapple Express is a heavy rain event here on the west coast due to a plume of moisture moving northeastward out of the tropics. Such moisture plumes are often referred to as atmospheric rivers. In stylized form, you'll often see these events depicted as giant arrows of moisture running into the coast:
Of course, since this air has a tropical origin, it tends to be much warmer than usual. Here's a forecast of 850mb temperatures for that same time next Tuesday:
Pineapple Express events are often set up by a deep trough aloft over the central Pacific. This helps orient the pressure field aloft to promote southwesterly flow out of the tropics and into the west coast of the United States. There is even some evidence out there that suggests this sort of pattern may in turn be linked to a phenomenon in the tropical Pacific and Indian Ocean known as the Madden-Julien Oscillation, or MJO. However, that's a topic for another post. In the meantime, there's a lot of water to look forward to in Seattle.