Forecast Center

September/October 2001

by TIM VASQUEZ / www.weathergraphics.com

This article is a courtesy copy placed on the author's website for educational purposes as permitted by written agreement with Taylor & Francis. It may not be distributed or reproduced without express written permission of Taylor & Francis. More recent installments of this article may be found at the link which follows. Publisher's Notice: This is a preprint of an article submitted for consideration in Weatherwise Â© 2001 Copyright Taylor & Francis. Weatherwise magazine is available online at: http://www.informaworld.com/openurl?genre=article&issn=0043-1672&volume=54&issue=5&spage=74.

PART ONE: The Puzzle

Draw isobars every four millibars (992, 996, 1000, 1004, etc.) using the plot model example at the lower left 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.

A hint for beginners: No fronts were located on this map that were considered significant. However you will find several important low and high pressure areas. A trough, which is an elongated area of low pressure, will also be found. It is marked with a thick dashed line.

Click to enlarge

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Scroll down for the solution

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PART TWO: The Solution

Even the rattlesnakes in the Mojave Desert would call July 3, 2001 a sizzler! At least 11 weather stations checked in with 110°F (43°C) or higher, and Salt Lake City broke its record high for the date.

The surface map showed large thermal lows that covered the Arizona and California interior regions. These thermal lows are considered barotropic by forecasters, which means that they are not fueled by the type of air mass contrast that deepens frontal lows. Rather they’re a reflection of the strong heating that is taking place. This heating causes air to rise and diverge aloft, removing mass and causing surface pressures to fall. The exact center of thermal lows is usually ambiguous from day to day and generally corresponds to the areas that get the strongest heating. Clear skies during the day and upper-level patterns that forced large-scale downward motion across the region, causing heating by compression, likely contributed to the record warmth.

Also note the strong high pressure over northern Arizona. It was caused in part by thunderstorm activity, therefore a dotted cold front marking is used to indicate that an outflow boundary divides the outflow from the warmer air mass. These boundaries are often the focus for new thunderstorm development.