Forecast Center

July/August 2007

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 © 2007 Copyright Taylor & Francis. Weatherwise magazine is available online at: http://www.informaworld.com/openurl?genre=article&issn=0043-1672&volume=60&issue=4&spage=86.

PART ONE: The Puzzle

Some of the most violent weather on the planet descends on the Great Plains during the spring months. The Rocky Mountains, which stretch north to south, help bring cold Canadian air southward and very moist Gulf air northward, resulting in the oversimplified scenario known as the "battle of the air masses". In this puzzle we'll take a close look at a major severe weather situation that occurred in spring 2007. In addition to fronts it will be important to check for wind shift lines and troughs. Advanced readers are encouraged to find the dryline, a boundary which separates warm air with high dewpoints from warm dry air.

This weather map is for the afternoon hours in May. Draw isobars every two millibars (1008, 1006, 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. Then try to find the locations of fronts, highs, and lows.

Click to enlarge

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Scroll down for the solution

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PART TWO: The Solution

May 4, 2007 brought one of several significant tornado outbreaks recorded in 2007 (touchdowns are highlighted with red inverted triangles). The map in this issue shows the weather for 4 p.m. Central Time that afternoon, a few hours before a series of tornadic supercell storms developed in southwest Kansas. These storms produced at least 11 tornadoes. The tornado which hit Greensburg, Kansas had the rare distinction of reaching the highest category on the Fujita damage scale. It destroyed nearly all of the town and resulted in ten deaths.

A key feature on the charts was the dryline, which is found between warm, moist air and warm dry air and often serves as a breeding ground for thunderstorms. On the afternoon of May 4 it separated 40s and 50s dewpoints from 60 dewpoint readings to the east. The tornadic storms were intensified by the tongue of 70-degree dewpoints stretching from central Oklahoma into south central Kansas. Since instability is highly proportional to the moisture in a parcel, the higher dewpoint temperatures in this zone allowed for much stronger updrafts and much more intense storm activity.

The chart also shows backed winds along a trough in central Kansas. In the lower levels of the atmosphere, backing, a counterclockwise variation in the wind, augments the type of shear which favors tornadic storms. The southeasterly winds shown in central Kansas provides a richer environment for tornadoes compared to the southerly winds in Oklahoma and Texas.

Another prime weather pattern for severe weather is the warm front shown in Nebraska extending into northwest Kansas. The warm front is often associated with backed winds and serves as a source of convergence which allows storms to develop. On this day, however, the warm front was not a major severe weather producer and some storm chasers found theirselves too far north. The storms preferred to develop in the richer moisture to the south near the dryline and in proximity to the weaker features in southern Kansas.

It illustrates the fact that forecasters can't just look at fronts. They have to look at the subtlest boundaries and features using as much data as possible including radar, satellite, and mesonet data. Failure to identify a boundary can lead to a missed forecast. In many respects, severe storms forecasters are the keepers of meteorology's fine-toothed comb.