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Weather Blog: Monsoon And Colorado Flooding

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By Kate Bilo

PHILADELPHIA (CBS)--One of my favorite movies of all-time is Monsoon Wedding. Haven’t seen it? I highly recommend, but then I’m a big foreign film buff. Anyway, it takes place in India during the monsoon, a seasonal shift in winds that brings two distinct seasons to portions of south Asia – wet and dry. Most people associate the word “monsoon” with the wet season, the deluge that causes massive flooding across much of India, but also provides much-needed rainfall to the area.

Did you know that the United States also gets a monsoon? Every year, in the late summer, portions of the desert Southwest get hit by drenching rains and thunderstorms that provide a large percentage of annual rainfall in parts of Arizona and New Mexico especially.

The process is formed when a large ridge of high pressure sets up over the central and western plains, and intense heating over the deserts forms a thermal low. The differential between these two pressure systems funnels moisture-rich air from both the Gulf of Mexico and the Pacific Oceaninto northern Mexico and the desert Southwest, where it lifts along the mountains and plateaus. This process, called orographic uplift, causes heavy thunderstorms to form, and often leads to flash flooding.

The intense flooding we saw in Colorado this past week (called “biblical” in one NWS discussion) was partially produced by this “monsoonal” moisture. The ridge of high pressure stayed planted longer than usual, and when it finally moved out, a strong cold front moved in, shoving that moisture eastwards and pushing it up the slopes of the Rockies. This led to unbelievable amounts of rain throughout the mountains and foothills of Colorado.

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