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July-August 2009

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Forecast Center

E  ven as summer approaches, all it takes is widespread snow cover and a couple of clear nights to generate prolific amounts of cold air in the Canadian Arctic. Once this cold air moves south and interacts with warm spring air, very strong weather systems and powerful fronts can result. For this puzzle, we’ll take a look at such a system, which affected the northern United States in May 2009.

This weather map is an event during the early evening in May. Draw isobars every 4 millibars (996, 1000, 1004 mb, etc) using the plot model example at the lower right as a guide. As the plot model indicates, the actual millibar value for plotted pressure (xxx) is 10xx.x mb when the number shown is below 500, and 9xx.x when it is more than 500. For instance, 027 represents 1002.7 mb, and 892 represents 989.2 mb. Therefore, when 1 station reports 074 and another nearby shows 086, the 1008 mb isobar will be found halfway between the stations. Then try to find the locations of fronts, highs, and lows.

For further information, along with helpful hints and advice, visit

The solution appears on page 65.

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