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    End of Southwest Monsoon, Less Flash Flooding in Sight

    By By Alex Sosnowski, expert senior meteorologist.
    September 15, 2013, 12:38:55 AM EDT

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    An end of the pattern bringing frequent showers and thunderstorms to the Southwest United States is on the way.

    According to Western Weather Expert Ken Clark, "A dry flow of air from the west will break through this weekend into next week."

    The wind shift will shut off the flow of tropical moisture and high humidity, largely responsible for the flash flooding experienced in recent weeks.


    The perennial flow of moisture from the tropical Pacific and the Gulf of Mexico contributes to the thunderstorms and is known to locals in the Southwest as the Monsoon.

    This pattern occurs on average spanning late June, August and early September but can start and end earlier and later.

    Southwest U.S. Regional Radar
    Photos: Boulder, Colo., Under Water
    Severe Weather Watches and Warnings

    "The Monsoon this year began and will end close to average," Clark said. "By official definition, in Arizona, it starts when there are three consecutive days with high humidity and ends when there is a considerable length of time featuring low humidity."

    Showers and thunderstorms will continue to focus over New Mexico, Colorado and Wyoming into the weekend with a few pockets of showers and thunderstorms farther west over the Great Basin.

    The eastward shift of dry air should end or greatly lessen the risk of flash flooding in the Boulder area and other communities, but not until later next week.


    "The arrival of dry air from west to east this weekend into next week does not mean that it cannot shower or thunderstorm for the balance of the next couple of months, but such events should be far less frequent moving forward," Clark said.

    This year's Monsoon has delivered near-average rainfall to many areas from New Mexico to Arizona, southeastern California, southern Nevada, much of Utah and Colorado. There have been a few pockets, like Flagstaff, Ariz., and Thermal, Calif., where rainfall has been more than double that of normal.

    The arrival of the dry air is the preliminary step toward the Santa Ana season in Southern California. While a Santa Ana event is not expected through next week, there will soon be high pressure areas building inland from the Pacific Ocean that hook up with strong winds aloft.

    "When winds produced by these systems line up from the north and northeast, a Santa Ana can be produced," Clark said.

    Santa Ana events are most common from the middle of autumn to late in the winter and cause wildfires to spread rapidly.

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