Adventures from Back of Beyond

Tuesday, August 01, 2006

Ground Zero for the NAM

We in Arizona are sometimes ground zero in the U.S. for the North American Monsoon.

The NAM is the seasonal change in wind direction. During most of the year, we -- and most of the U.S. -- are affected by the west to east jet stream, which brings us storm fronts from the Pacific. But during summer, beginning usually near the end of June, our prevailaing winds change, often coming from the south. This change of wind direction, or monsoon, often sucks up much needed moisture from 3 sources: the Gulf of California, Mexico, and the Gulf of Mexico. As the moisture enters the U.S., Arizona is often ground zero for the entry point -- and results in our magnificent monsoons.

Monsoonal moisture make July and August our two rainiest months. Dew points suddenly jump from below zero to the 40s, 50s, 60s, and more. This past month, for instance, we've had 4.6 inches of rain...almost triple our total for the previous six months.

Monsoon days follow a typical pattern. Bright, clear, dry mornings, with intense heating by the sun often cause temperatures to rise into the 100s over the Southwest deserts. That intense heating forms a thermal low, and updrafts billow into the higher and cooler levels of the atmosphere, causing the moist air to condense into magnificent thunderheads. These cumulonimbus cells bring strong but localized downpours accompanied by magnificent lightning displays and booming thunder. It is a magnificent time of year.

As the summertime jet stream moves far northward into Canada, this sort of seasonal wind shift affects much of the continental U.S. Much of the country, unless you're right along the west coast, often has a similar daily thunderstorm pattern. And Arizona is often ground zero.

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