Forecast Center

May/June 2008


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 © 2008 Copyright Taylor & Francis. Weatherwise magazine is available online at:

PART ONE: The Puzzle

In this issue we'll look at an extratropical low with a cold front, warm front, and occluded front moving across the Midwest states. This type of system is actually quite common, affecting the nation during the cool season. However on this particular day it proved to be a prolific snow producer. It brought over a foot of snow to parts of Wisconsin, Michigan, and Ontario and resulted in hundreds of airline flight cancellations in Chicago, Detroit, and Milwaukee.

This weather map is an event during the early evening in February. Draw isobars every four millibars (1008, 1004, 1000, 996, 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.

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Scroll down for the solution

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PART TWO: The Solution

The evening of February 6, 2008 showed two weather regimes across the United States: cold temperatures in the 20s and 30s across the Midwest with gusty west winds, and very warm 60s and 70s along the Atlantic Seaboard and southerly winds. The transition zone between the two air masses defined a front.

Readers of previous Forecast Center issues will recall that the warm side of a transition zone defines the location of the front. For example if we drove a car from Virginia to Arkansas, we would notice temperatures in the 60s at the start of the trip, falling off gradually as we crossed through Tennessee. The front is located at the warm side of this gradient, where we initially crossed into the cooler air, so the front is found in far eastern Tennessee rather than in western portions of the state.

Two low pressure centers are observable. The first is along the Pennsylvania-Ohio state line and it is the deeper system that is easier to find. The second is in northern Indiana. With the lines that we've drawn, there are no closed isobars that define it, but the wind field shows a very definite cyclonic pattern with a pressure value of 998.1 mb at Chicago which is lower than the readings surrounding it.

There two low pressure centers because we are seeing different stages in the life cycle of an extratropical low. Initially, cyclones develop along a front, feeding off the temperature contrasts and deepening from upper-level support. This process is referred to as self-development. Soon the low becomes mature, a stage illustrated by the Pennsylvania-Ohio low. Eventually the cyclonic flow around the low causes the cold air to surround it completely, displacing the warm air aloft and resulting in occlusion. This is the decaying stage. The low found in Indiana is an example of a decaying low.

It might be puzzling that much of the precipitation and bad weather is found some distance north of the fronts, with very little weather actually occurring along them. This is because of a conveyor belt of warm, moist air extending from the Carolinas northward to the Great Lakes, wrapping into both of the low pressure areas and transporting a deep fetch of moisture into the heart of the system. This air is forced to rise due to several factors: isentropic lift (i.e. the "pathway" that air must take slopes upward when crossing from a warm to a cold air mass), lift from the coupling of upper-level divergence and low-level convergence of wind into the surface lows, and the release of latent heat that makes the air more buoyant. The net result is clouds and precipitation. The freezing rain and sleet found in the Lake Erie region confirms the presence of warm air just above the surface from this conveyor belt, but further west, snow is found since the entire troposphere in these regions is below freezing.

This weather system produced heavy snow in the Great Lakes region, with a wide swath of 10 to 20 inch snowfalls across southeast Wisconsin. The 13.4-inch total at Madison, Wisconsin was the second-greatest 24-hour snowfall on record, eclipsed only by an event on December 3, 1990, and resulted in thousands of vehicles stranded on Interstate 90. The region southeast of Toronto saw as much as an inch of ice from freezing rain. On the preceding day, the system produced a record tornado outbreak in the lower Mississippi River and Ohio River valley regions, with 82 confirmed tornadoes and widespread damage.

Computer programs do not produce the Forecast Center solutions. All fronts and isobars are manually drawn by the author using illustration software.

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