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In the middle of winter, it seems we're always dealing with one weather train or another. The Siberian Express delivers some of the worst cold waves to the nation's heartland. But there's another locomotive that stops at the West Coast with a less frigid cargo. Tired of the train analogy, well that's too bad because today on the Weather Notebook it's all aboard for the Pineapple Express.

The Pineapple Express runs straight from Hawaii to the shores of Washington, Oregon, and California but it doesn't arrive every day. It only runs a few times each winter, when the jet stream settles into a long, straight, fast flow across the eastern Pacific. This allows huge amounts of tropical air to cruise northeast from near Hawaii toward the West Coast.

The Pineapple Express can dump buckets of rain, and sometimes this rain falls on top of snow cover. The result can be severe flooding, especially on the west slopes of the Sierra Nevada and the Cascades.

Seattle's known for its wet, gloomy winters, but every so often the city turns from gray to white thanks to the Pineapple Express. It takes a special setup to cause heavy snow in Seattle. It takes a bitter cold air mass to push over the Canadian Rockies and slide into Puget Sound, which doesn't happen very often. Combine this with just the right amount of pacific moisture and there you have it.

Seattle hit just the right balance on Groundhog Day 1916, when the city of eternal winter drizzle managed to pick up 21 inches of snow. Even Bill Gates can't stop a snowfall like that, not to mention the Pineapple Express.

The Weather Notebook is a production of The Mount Washington Observatory.