Forecast Center

May/June 2006


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PART ONE: The Puzzle

As early as 1743, the American statesman Benjamin Franklin was the first to notice that American storm systems often moved steadily from southwest to northeast. There is probably was no better place to observe this characteristic than in the northeast United States during the transition seasons. While the winter months are often dominated by cold Canadian air masses spilling southward, the transition seasons place the region directly in the track of storms aggressively exiting the nation's interior. They arrive in the northeast with surprising regularity, and in this example we'll take a look at such a storm.

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 morning of March 10, 2006 showed deep low pressure across the Great Lakes, hinting at a storm system taking aim at the northeast states. Residents of New York and Pennsylvania woke up to cloudy skies, a falling barometer, and wind increasing out of the south. The map solution reveals a textbook model of a frontal system and extratropical low.

The singularly most challenging feature to find is the warm front extending east from the low pressure region. With temperatures near freezing in Quebec and warm 60s in the Carolinas, it's obvious that a front is somewhere in the analysis area. However the puzzle contains a trick: the wind shift line that roughly follows the St. Lawrence River through southern Quebec to Lake Ontario. It's very tempting to define this as the warm front, especially at first glance.

However there are two cardinal rules of frontal placement. First, a front is always defined by a temperature gradient. Second, a front always lies on the warm side of a temperature gradient. Changes in wind direction, humidity, and weather across a zone can help to find a front, but they do not imply the presence of a front. It's temperature that's the key indicator. Temperature fields have an enormous effect on the air mass above, shaping what forecasters call "isentropic surfaces". These surfaces undulate up and down across a region and tend to guide air as it flows with the wind. The shape of an isentropic surface, combined with certain patterns in the wind field and humidity, can lead to significant changes like deteriorating weather or rapid clearing.

The feature found in Canada has characteristics of both a wind shift line and a trough, more specifically an "inverted trough" since it extends poleward from a low pressure system. To the north, bad weather prevails, with low overcast and light precipitation. This is associated with a combination of tropical air moving northward and lift ahead of the warm front. With sufficient moisture and lift, the warm front can become a hotbed for severe thunderstorm activity. It was a pattern almost identical to this one that led to the historical May 31, 1985 tornado outbreak that ravaged Pennsylvania, Ontario, and adjoining states. The easterly surface wind ahead of the warm front, juxtaposed with the southwesterly winds aloft, sets up strong shear profiles northeast of the surface low that favors the development of tornadic storms.

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