Forecast Center

January/February 2005



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 © 2005 Copyright Taylor & Francis. Weatherwise magazine is available online at: http://www.informaworld.com/openurl?genre=article&issn=0043-1672&volume=58&issue=1&spage=82.


PART ONE: The Puzzle



California often makes the weather headlines during the winter months, especially during an El Nio year. The oceans become warm relative to the land, and strong weather systems spring into life in the Pacific Ocean. As these systems make landfall, they can cause severe beach erosion, landslides, flooding rains, and wintry mountain snows. In this example we'll take a look at such a system.

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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PART TWO: The Solution



The system that came ashore November 4, 2004, as shown in this issue's puzzle, brought the third major storm of the winter season to California. The region was already experiencing one of its wettest Octobers on record. The Sierras got up to ten inches of new snow, and chain restrictions were in effect for Interstate 80. The rains hit homebuilders hard, shutting down many construction projects in the Sacramento area.

As with the November 4 storm, many of the fronts that pass through California have characteristics of an active front, or an anafront. Thus behind the front, we find extensive low and mid-level clouds, showery precipitation, and gusty northwest winds. As the systems entering California are often mature or decaying, they are closely followed by pools of very cold air aloft. This provides an unstable atmosphere behind the front and enhances the postfrontal precipitation, which is typically in the form of rain in the valleys and snow in higher elevations. The destabilization can be compounded by heat originating from the Earth's surface, as California tends to get significant sunshine and warmth before the Pacific systems arrive.

Ahead of the front are fair skies, westerly winds, and sometimes blowing dust, while over the coastal waters the front tends to make rapid progress south owing to the lack of friction. The inland mountain ranges impede the progress of cold air. This results in a distinctive north-south orientation to many surface fronts as they cross the region. The weather systems dry out as they enter Nevada, especially in the south central portions of the state where moisture is blocked by the Sierras.

As with most weather systems in hilly and mountainous terrain, weaker storms can be very difficult to analyze and the front may be difficult to find. Winds may not match the conceptual model expected with a particular weather system. For example the front extending through Utah is weak and readers might very well be justified placing it in southern Idaho or even southern Utah. The key to locating fronts and tracking them is to carefully monitor trends at stations throughout the region, paying attention to wind shifts, temperature drops, and pressure rises. In many cases these are more important than the isobaric patterns themselves.




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