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Column: Trick to storm forecasting isn’t clairvoyance, but ‘the cone’
Rudi Kiefer

When Tropical Storm Eta brought 3 to 5 inches of rain to Miami and the rest of southern Florida, forecasters talked a lot about “the cone”. Hurricanes and their weaker brethren, tropical storms, aren’t cone-shaped. The cone refers to the area where a storm is predicted to go. 

Weather forecasters aren’t clairvoyant. The question came up how they could possibly predict the zig-zagging path that Eta was taking after causing severe damage in Nicaragua and Guatemala. The tool for this is a cone of probabilities for anticipating the storm’s possible movement. One could also say it’s funnel-shaped on the map. The narrower the cone, the greater the likelihood that the storm’s center will strike a given area. Farther ahead in the cone means farther into the future, and predictions become more fuzzy. This is why the cone widens along the forecast path. The more we move into the future, the less certain we can be about where the storm might go.

Winter storms are steered across the continent by the jet stream, a giant river of air in the upper atmosphere that drags frontal systems along its path. That’s a strong force, and winter forecasts turn out to be quite dependable. Hurricanes and tropical storms are less accommodating. They are steered by surface winds because the main jet stream isn’t present this far south. Building a forecast consists of analyzing past situations and comparing similarities in surface winds. When Eta was leaving Cuba and heading for Miami, the trade winds were strong. They are a persistent pattern of windflow from the northeast. This was pushing the storm toward the west. At the same time, its enormous store of energy was nudging it northward to deposit all that heat in the cooler regions. After all, the job of tropical weather systems is to move extra energy away from the tropics. 

Like a chess player, the computer building a forecast model must anticipate thousands of different combinations of windflow, of moves and counter-moves. High pressure over the continent can work like a bumper and divert a storm eastward and into the Atlantic Ocean. Trade winds can weaken or strengthen their push. The next few moves are reasonably certain but the ones that follow aren’t. In their minds, chess players form a cone of possible moves similar to computers building storm forecasts.

Rudi Kiefer, Ph.D., is a professor at Brenau University, teaching physical and health sciences on Brenau’s Georgia campuses and in China. His column appears Sundays and at