Forecast Center

July/August 2002


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PART ONE: The Puzzle

Spring is a time of drastic weather changes on the North American continent. In winter most of the air masses which affect the United States originate in Canada, but during the spring, the Pacific Ocean rises in importance. In this map puzzle we'll look at a spring weather system that has its roots in the Pacific.

Draw isobars every four millibars (992, 996, 1000, 1004, 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.

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Scroll down for the solution

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PART TWO: The Solution

This issue's puzzle shows a Pacific system working its way eastward on the evening of May 21, 2002. Isobar patterns reveal the existence of a deep low in eastern Montana, as well as another low that readers will locate either in western Nebraska or northeastern Colorado.

The first thing that catches the forecaster's eye is the broad southerly flow sweeping northward through the Great Plains, bringing 70-degree temperatures as far north as Saskatchewan and Manitoba. This air mass is classified as maritime tropical (mT) air, and originates from the Gulf of Mexico. The dewpoint temperatures throughout the air mass are a key indicator of what type of weather will occur along the frontal system. In May, dewpoints in the 60s and 70s are suggestive of heavy thunderstorms, however in this example the dry 40s dewpoints suggest poor rain chances and weak instability.

In the Rocky Mountain region, the strong westerly winds and cool 30s and 40s temperatures are easy to pick out. This air mass is a maritime polar (mP) airmass which originated in the Pacific Ocean and has modified after crossing the Great Basin region. As it progresses eastward, it brings raw, blustery weather to the mountains with orographic snow showers. Winds are disrupted somewhat by the rugged terrain, but are predominantly westerly. This Pacific air mass will soon replace all the air throughout the central United States.

At the top of the map in Alberta and northern Saskatchewan, northerly winds are ushering in fresh continental polar (cP) air. This is shallow, frigid air from the Northwest Territories that is being ducted southward along the barrier formed by the Rocky Mountains. Eventually it mixes in with the Pacific air and loses its identity. Not all forecasters will delineate the leading edge of this air mass as shown here (see the dashed cold front), however it is still important enough to bring rapid weather changes in Denver, Rapid City, and Bismarck as it sweeps southward.

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