Tuesday, November 22, 2011

The Pineapple Express

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:
But we can do better than that.  Here's a 105-hour forecast of total column water vapor (all the moisture in a column above your head) for next Tuesday:
The reds, whites and blues indicate lots of water vapor in the atmosphere over those locations.  You can see that the water vapor is concentrated in a relatively narrow stream stretching from the tropical Pacific up north into southern British Columbia and northern Washington.  This is the hallmark of one of these Pineapple Express events--a "river" of moisture brought up from the tropics to the west coast.  Strong winds usually accompany this plume of moisture, and since it has tropical origins (often near Hawaii), we get the name--Pineapple Express.  Moisture on an express route from the land of pineapples.

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:
 You can see that a plume of warmer air accompanies this moisture.  This means that most of the precipitation will fall as rain--the freezing level will be at several thousand feet.  Only the higher mountains will see snow.  You can also see on this map the forecast from strong, southwesterly winds in the plume, helping advect that warmth and moisture northwards.

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.

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