Before the advent of numerical forecasting, American meteorologists had a long and enduring preoccupation with storm tracks. Knowledge of where a storm was brewing on the West Coast often gave indications of what would unfold on the East Coast a few days later. The most common development area was in Colorado and New Mexico, a location that sent storm systems northeast into the Ohio River Valley and the Appalachians. But sometimes the storms began their journey in the Pacific Northwest, as is the case in this present-day example. Draw isobars every eight millibars (1008, 1000, 992, 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 lOxx.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.
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