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September-October 2011

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Forecast Center

Cold waves are often associated with strong weather systems that dominate the weather map. Fronts, highs, and lows are clearly demarcated, and the temperature and pressure changes are tremendous in the days and hours leading up to the cold wave itself. This is not the case with heat waves. The temperature creeps up one day at a time, in some cases so gradually that it might not be recognized by the forecaster until the moment when late morning temperatures are unusually high. A look at the surface chart shows elusive patterns and weak temperature and pressure fields. However, a closer look will in fact reveal a number of important clues, as we will discover in this puzzle.

This weather map depicts a midafternoon situation in July. 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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