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    Ken Clark

    Signs of the Monsoon Season Starting

    6/19/2012, 12:20:02 AM

    It is getting to be about that time of year. The time of year the yearly monsoon season starts in the Southwest. It usually begins the last week or so of June or early in July. The last two years the monsoon season has been pretty much a disappointment. Sporadic and not bringing the rain it should. This we can at least partially blame on La Nina. But this year we are trending to an El Nino and this should bring a more robust monsoon season.

    Here is what we are looking at, and while I am showing the GFS I will say the European is quite similar.

    Current 500 mb pattern:

    Current precipitable water:

    As a reminder, precipitable water is a measure of the of the amount of water vapor in the air. The higher the precipitable water the more moisture that is available for showers and thunderstorms. In the above charts you can see there is an upper-level ridge south of Arizona with a westerly flow aloft of dry air with less than a half inch of precipitable water in place.

    By Sunday though the weather pattern looks totally different.

    Sunday 500 mb pattern:

    Sunday precipitable water:

    By Sunday the center of the upper-level ridge has moved to the southern Plains with a huge trough off the Northwest coast extending south. In between is a southerly flow of air directly into the Southwest. And that southern flow is carrying a lot more moisture with precipitable water values over central and southern Arizona over and inch and in the far south over 1.20 inches. These values even go up higher by next Monday.

    It is very hot in Arizona now, but the air is very dry. By the end of the weekend it will not be as hot on the thermometer but the humidity levels will be much higher and therefore it probably will not feel much better. In addition, the added moisture brings the chance of showers and thunderstorms and the potential for dust storms as well.

    The views expressed are those of the author and not necessarily those of AccuWeather, Inc. or AccuWeather.com

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    Western US weather expert