Part 2: The Crystal Ball, Tuning In

By Sar Perlman — August 2003

More of this Feature • Hurricane Prediction • Crystal Ball, Tuning In • Nature’s Fury • On The Web • Tropical Cyclone Facts
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The Crystal Ball
Inside NHC’s headquarters—a two-story, heavily fortified, above-ground concrete bunker designed to withstand the worst hurricane—forecasters constantly review current observations from weather and military satellites, aircraft reconnaissance, Doppler radars, coastal meteorological stations, and other sources for any signs of trouble.

In analyzing all these data sources, scientists watch for the six conditions that are necessary for a hurricane to form: a pre-existing surface disturbance with thunderstorms near the core, warm ocean water temperature (at least 79ºF), a moist middle troposphere, light winds aloft, an upper-level outflow, and a location of 5º latitude or higher. A cyclone will continue to intensify until one or more of these conditions disappears or changes dramatically, at which point the storm will begin to diminish. However, the cyclone can reintensify rapidly if it moves into an area where all these ingredients are present.

A hurricane’s movement and direction are determined mostly by environmental steering. Much like a leaf floating in a river, a hurricane is guided along its path by surrounding atmospheric currents, such as fronts, high- and low-pressure systems, and the jet stream. In their absence, the cyclone’s internal motion takes over. In this case, a cyclone can move about quite erratically and be difficult to predict.

With these complex factors, as well as statistics and historical information, serving as guidelines, computer models process the current observations for the storm and produce a number of likely forecasts. But some models perform well under certain circumstances and perform poorly under others. “My job as a meteorologist is to absorb the info off the satellites, surface analysis charts, and ship reports, determine which is the best model or models to use in this situation, and then mentally and manually tweak that model’s results to come up with the best forecast,” says Martin Nelson, lead forecaster for the NHC and a man with 26 years’ experience. “In the end you still need a knowledgeable person there to make the final tweaks. A computer can’t really think, and you have to not only think, but also to make crucial judgment calls in order to forecast.”

Tuning In
For mariners, the center’s cornerstone forecast is the Tropical Cyclone Advisory, issued when a new tropical cyclone has formed and updated every six hours subsequently for the life of the storm. A typical advisory includes: the cyclone’s name; pressure; location; present direction; maximum sustained winds; the 64-, 50-, and 34-knot wind radii within the system; and the forecasted position and intensity 12 to 120 hours into the future. Vessels should avoid the storm’s 34-knot wind field, says Eric J. Holweg, an NHC meteorologist and author of the NOAA Mariner’s Guide for Hurricane Awareness in the North Atlantic Basin. At this speed sea state development approaches critical levels that rapidly decrease a boat’s maneuverability.

Extending the 34-knot forecasts by adding forecast error margins is a vital tool. Known as the 1-2-3 Rule, it instructs mariners to chart the 34-knot area forecasted for the next 120 hours on a map, then adds 100 miles to the forecasted 34-knot radius in day one, 200 miles to that of day two, and so on. Connecting a line tangent to each circle results in a cone-shaped area that everyone should avoid (see illustration above).

Never cross the projected track of a tropical cyclone, since high swells ahead of the storm and sudden changes in the cyclone’s speed can place your vessel in unexpectedly rough conditions. Moreover, Stacy Stewart, an NHC hurricane specialist, warns that intensity forecasting can still be off by at least one and possibly two Saffir-Simpson Hurricane Scale categories at day three and beyond.

During hurricane season (May 1 through November 30), the NHC issues Tropical Weather Outlooks every six hours, which give a synopsis of all existing tropical disturbances and cyclones with a brief description for each, along with Tropical Cyclone Discussions and Tropical Cyclone Strike Probabilities, also issued every six hours.

Tropical Weather Discussions, which provide an overview of convective activity and general weather features that might breed tropical cyclone development, are issued four times a day year-round. These reports can be accessed via weather radio, radiofax, e-mail, and NHC’s Web site (www.nhc.noaa.gov).

When a tropical cyclone wanders into an area beyond the tropics—north of 31º north latitude—the NHC coordinates forecasts with the Camp Springs, Maryland-based Ocean Prediction Center, which issues marine warnings, forecasts, and guidance for these regions.

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