Forecast Center

January/February 2003



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 © 2003 Copyright Taylor & Francis. Weatherwise magazine is available online at: http://www.informaworld.com/openurl?genre=article&issn=0043-1672&volume=56&issue=1&spage=58.


PART ONE: The Puzzle




Quite often when we think of winter weather, we think of snow in the forests of Vermont, icebergs in the Great Lakes, and ski season in Colorado. However, the Midwest and southeastern United States are subject to another type of weather phenomena altogether. This puzzle will examine this type of weather in more detail.

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



The afternoon of November 10, 2002 brought one of the largest tornado outbreaks in recent history to the eastern United States. More than fifty tornadoes touched down in a span of several hours. The weather patterns associated with this outbreak serve as the puzzle for this issue.

The surface chart shows the weather conditions at 1800 UTC (1 pm EST). Most prominent and easiest to find among beginners is the long frontal boundary that extends from the Great Lakes region to Texas. It represents a push of polar air southeastward. The thermal contrasts across this zone are intimately linked to the upper-level patterns, which in turn deepened the system as a whole and enhanced shear and instability across the storm region. The surface boundary is marked by a very sharp wind shift and a strong temperature contrast. Ahead of this feature is deep southerly flow, bringing tropical dewpoints northward. Dewpoints here rise to well over 70 degrees Fahrenheit. This further amplified the instability over the region.

More challenging to find, however, are the east-west warm fronts located near Tennessee and in southern Canada. Ironically, the most important feature is the hardest to locate: the warm front just south of Tennessee. In the solution, it can clearly be seen that this warm front separates lower-80's from mid-70's temperatures, and delineates a slight difference in wind direction. Zooming in and plotting more data would reveal even more detail and make it easier to find.

This warm front was a key factor in the tornadoes that affected eastern Tennessee. Not only did the front serve as a focus for surface wind convergence and lift, but the subtle wind differences across the boundary helped set the stage for rotating thunderstorm updrafts, which in turn provided an environment favorable for tornado development. On the other hand, the cold front and occluded front to the west served as a massive wedge of forcing, which did more to create a solid squall line than to enhance tornado development. Most of the tornadoes occurred in isolated thunderstorms occurring ahead of this squall line, and were associated with the warm front and other small-scale boundaries not resolvable on this map.

Looking at the Great Plains, a 1006-millibar intermediate isobar was drawn. The intermediate isobar helped pinpoint a weak trough that extended from Oklahoma to New Mexico. This trough separated mild Pacific air from colder polar air, and brought changes to the day's weather in Texas. Such troughs are a very common feature in the wake of Great Plains frontal systems. There's nothing wrong with using intermediate isobars to draw more detail out of the map. After all, it's not the completed analysis that's important; it's what has been learned in the process of analyzing the chart. The analysis chart is a tool to make us think. It takes away the shortcuts of modern technology and forces us to consider all ingredients that are affecting a region and how they are interlinked. And that's what builds a solid, successful forecast!


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