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May-June 2011

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Forecast Center

During the cool season, the weather is almost never quiet in the northeast United States for very long. Much of the cold air that originates in the Canadian arctic eventually spills southeastward into the eastern United States and Quebec. Meanwhile, Pacific weather systems that cross the country tap the stout moisture supply in the Gulf of Mexico and the Gulf Stream waters. In extreme cases, “bombs” form off the Atlantic Coast, marked by extreme deepening of frontal systems and vast plumes of wintry precipitation that move inland. In this example we'll introduce a less dramatic example of an East Coast weather system, though one that is much more common.

This weather map depicts an early morning situation in March. Draw isobars every four 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 one station reports 074 and a nearby one shows 086, the 1008 mb isobar will be found halfway between the stations. Then try to find the locations of fronts, highs, and lows.

TIM VASQUEZ is a former Air Force forecaster and author of Digital Atmosphere, a weather forecasting software program. He lives near Norman, Oklahoma, where he keeps busy as a weather consultant and software developer.

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