Forecast Center

November/December 2002


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 © 2002 Copyright Taylor & Francis. Weatherwise magazine is available online at:

PART ONE: The Puzzle

All eyes turn to the tropics in late summer and fall. It is at this time when ocean temperatures are at their warmest and upper level winds have relaxed. Together these conditions nurture the development of hurricanes, a special type of storm that is fueled by the explosive release of latent heat.

Ironically, strong hurricanes are the easiest storms to track. The center of their convective rain bands can easily be pinpointed using radar and satellite imagery. On the other hand, weaker tropical storms often lack an eye, and their rain bands tend to resemble amorphous blobs. The danger from heavy rains and flooding can be just as serious as the strongest of hurricanes, so the forecaster ends up with quite a challenge. For an inland storm, the solution is a careful surface analysis: a paper map and a sharp pencil. By working on this puzzle, you'll follow the ritual of a forecaster unravelling the structure of a Gulf Coast tropical storm.

Although standard convention dictates that we draw isobars every four millibars, forecasters will draw them every two millibars in tropical weather situations, such as that presented in this issue's puzzle. Draw isobars every two millibars (1014, 1016, 1018, 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

This issue's puzzle shows Tropical Storm Hanna just after landfall at 1 am EDT on the morning of September 15, 2002. The most obvious thing to a seasoned forecaster is the cyclonically (counterclockwise) curved appearance of the wind field. Note the winds blowing from the south along the Florida coast, and from the east in northern Alabama. If the circulation was any stronger, Mississippi would show northerly winds.

By drawing isobars every two millibars, we see a definite low pressure area over southern Alabama. The lowest pressure in Alabama (1012.8 mb) was reported at the town of Troy. The calm winds at Troy suggest that the center of the circulation was at this location. This is confirmed by looking at the wind field of neighboring stations, which show a cyclonic circulation centered directly over Troy itself.

The storm was moving northward. Regions along and to the right side of a tropical cyclone's path are most favored for heavy rainfall. Indeed, parts of far western Georgia were inundated with rain. Note the station plot for Columbus, Georgia, which shows a diamond of four dots in its precipitation field. This is the heaviest category of rain that a station can report. Up to 16 inches of rain had fallen in some spots, and 5-inch amounts were quite common through the region.

The two-millibar isobar interval, as we have seen, is one technique used by forecasters in tropical regions. The other technique is called streamline analysis. Deep in the tropics, the weak Coriolis effect tends to equalize the pressure field, making isobars almost useless. The forecaster gets a much better analysis by ignoring isobars and drawing long lines parallel to the wind direction. This yields a streamline map. The waves and vortexes that appear on these maps often reveal the weakest of tropical systems, including the important easterly waves which are the focus of development for many hurricanes.

This puzzle is a clear example of how surface analysis can help hobbyists and media forecasters keep track of a weak inland weather system from hour to hour. It's not necessary to wait on official position reports. Just do a surface analysis, and you'll have the latest storm coordinates on the block!

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