When the United States sees a sweeping cold snap like the one we're in now, that cold air doesn't stop at the border. It can continue blowing south into Mexico and race across the Gulf. This cold air can create a powerful jet of wind in southern Mexico known as the Tehuantepecer.

Mexico is roughly shaped like a horn; from its wide, desert border with the United States, the country funnels south and east to a narrow passing before jutting back north as the Yucatán Peninsula. That narrow strip of land connecting the Yucatán to the rest of Mexico is called the Isthmus of Tehuantepec. The southern half of the Isthmus of Tehuantepec is occupied by the Sierra Madre, in the middle of which is a sizable gap known as the Chivela Pass.

When a stiff, cool wind blows south from the Gulf of Mexico, the air gathers in the cup-shaped northern side of the Sierra Madre. Since the air is cool and dense, it can't rise up and over the mountains, so it sits at the foot of the range and pools up. Chivela Pass is the only opening in the mountains through which the wind can blow, so it acts like a drain, allowing the pooled-up air to blow through to the Pacific side of the isthmus.

When this happens, the wind dramatically speeds up. It's Bernoulli's Principle—think of putting your thumb over the end of a garden hose. The wind is being funneled into a relatively tight pass, making it flow faster than it would otherwise. The wind can get pretty strong, with gusts of 50+ MPH not uncommon.

The Tehuantepecer is happening right now and will continue through Sunday. Here's an animation from last night's run of the GFS model, showing mean sea level pressure (the contours) and winds, in knots, ten meters off the surface (the colored gradients).

The wind blows in from the Gulf of Mexico, presses through the Chivela Pass, and sustained winds speed up to 40 knots (~46 MPH) as they emerge over the city of Salina Cruz and head out over the Pacific Ocean. The model shows how far the wind can blow out to sea—this particular event reaches a few hundred miles into the Pacific.

The weather is pretty cool, in more ways than one.

[Images: NASA / Google Maps, with modifications by the author / WeatherBELL]

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