Forecast Center

January/February 2007


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:

PART ONE: The Puzzle

In this issue we present a relatively easy puzzle -- a cold front moving south through the Great Plains. However, looks can be deceiving. Such fronts are often not the harbinger of sunny, cool weather portrayed by introductory weather books. In the solution we'll discuss some of the ways that cold fronts on the Great Plains can evolve into major weathermakers.

This weather map is for the October midday hours. 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 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 Rocky Mountains form a unique north-south mountain barrier, whose effects are unsurpassed on the planet. This barrier allows for the development of what are arguably the strongest and most dynamic continental frontal systems on Earth. North-south barriers allow cold polar air and warm tropical air to be more efficiently funneled southward and northward, enhancing the development of significant temperature contrasts. The strongest cold surges are often referred to by Texans, Oklahomans, and residents of Kansas as "northers" or "blue northers". The strongest northers, such as those that occurred on November 11, 1911 and January 20, 1985, are accompanied by striking arcus clouds, roaring north winds, and temperature falls of 50 degrees or more in six hours. The northers that occurred during the Great Depression often picked up dust from eroded crop fields and became the subject of dozens of ominous photographs now used by publishers to portray the Dust Bowl.

This issue's puzzle, valid for Halloween 2006, shows a more benign example of the same phenomenon. Readers may have noticed the very sharp temperature and wind direction contrasts in Texas, which allows the front to be accurately located. The front's position in Arkansas is further north than what might be expected, owing to the higher terrain and friction of the Ozark Plateau which slows down its movement. Further west, the front suggests that little progress has been made into New Mexico and Colorado. This is due to the barrier effect of the Rocky Mountains -- the cold air mass is simply too shallow to ascend into the mountainous terrain. As this is a very common effect, forecasters often mark the front as stationary through this region as seen here. The front typically remains stationary along the Front Range unless soundings show considerable cold air depth exceeding a few thousand feet of depth or a very strong push of polar air indicated by a polar high whose central pressure exceeds 1030 to 1040 mb.

In Colorado we see overcast skies along the tail end of the front. This is due to an effect known as upslope flow. The terrain in this area slopes upward from east to west, and when long fetches of wind move westward, the air ascends and cools adiabatically. This results in destabilization, increasing humidity, and in many cases the formation of clouds and even precipitation. In the Denver region, upslope flow is a major contributor to snowstorms during the wintertime and to severe thunderstorms during the summertime. While upslope flow is most common in the eastern halves of Colorado and New Mexico, at times it may affect parts of the Great Plains as far east as the 98th meridian.

Damming is another effect that sometimes takes place along the Rockies. While this concept is often associated with winter storms in the Carolinas, damming may occur in a weak weather regime when polar air stagnates in the southern Plains. When damming is associated with an approaching storm system, winter weather may result from two key processes: (1) forced lift due to "overrunning" as tropical air moves northward above the polar air, and (2) the fall of precipitation into the dry, stagnant polar air mass, which causes adiabatic cooling. A few noteworthy winter storms in Texas during the 1980s and 1990s were missed altogether by numerical models, which underforecast the role of adiabatic cooling, catching forecasters off-guard with expectations of rain.

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