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of Atlantic Canada, and latitudes in the midforties to -fifties—"the flow of the river is very strong. In the tropics it can be very weak, sometimes so weak that it is no longer the dominant moving force. Then other factors, including rotation of the earth, come into play and can induce secondary rotations within a storm system, and those rotations can themselves become the storm's driving mechanism. So if the atmospheric movement above a hurricane is so weak it is hardly moving, just the difference in rotation between one side of the storm and the other will induce a forward motion and cause the storm to veer. Also, any kind of terrain, such as the mountains of Hispaniola, will change the course of the storm. A steering flow in the upper air is easier to predict because it is generally well defined."32

One of the critical factors in predicting Atlantic hurricane paths is the Bermuda high, the ridge of high pressure more or less permanently anchored in the midlatitudes. How strong it is, precisely where and how stable it is, will often be the critical factor in a storm's course. South of the ridge the prevailing winds are easterly, which is why storms off Africa head west toward America. They will tend to turn north because of the Coriolis force, but the Bermuda high makes it difficult to predict exactly where, or how sharply that recurvature will happen. Once through the ridge, storms hang a right into the south westerlies, and then can move fairly smartly northeastward. Also, the westerlies can accelerate storms very quickly—they can travel from South Carolina to Newfoundland in twenty-four hours, which gives very little warning time to, well, people like me.

Here we have to risk getting lost in a thicket of acronyms: tracking models include CLIPER (from CLI-matology and PERsistence), GFDL (Geophysical Fluid Dynamics Laboratory model), AVN (AViation Run model), NOGAPS (which, if you really need to know, is the U.S. Navy's "global spectral forecast model with 18 sigma levels, a triangular truncation of 159 waves, parameterizations of physical processes and a tropical cyclone bogussing scheme"; it uses complex motion equations to monitor air circulation around the globe) and UKMET (run by the British Meteorological Office, that is, UK MET). CLIPER is perhaps the simplest of the models, merely a historic track of hundreds of previous storms.

Models that attempt to predict a storm's changing intensity include the GFDL, which is also used for tracking storms, and SHIFOR (Statistical Hurricane Intensity Forecast), which uses climatological and persistence predictors to forecast intensity change. SHIPS (Statistical Hurricane Intensity Prediction Scheme) looks at the difference between the maximum possible intensity and the current intensity, the vertical shear of the horizontal wind, the persistence (that is, the previous twelve hour intensity changes), and other factors. A version of SHIPS is available for the Atlantic and East Pacific.33

Some of these models look only at radar data, others look to history, still others are based on broader, global, meteorological patterns. They all do it differently, and the forecasters, or weather analysts, will still have to make the judgment as to which model, or array of models, to follow in making their predictions. A forecaster has to get a feel for what works best in what circumstances, and not so well in others, and come up with a plausible synthesis. For example, most dropsondes measure the winds at the 10,000-foot level, and forecasters have to estimate how far to scale down estimations of surface winds—most multiply the figure by 0.9, but some use other measures, and the NHC has been criticized in the past for underestimating surface winds.

The forecasters are all scientists, with the data-processing training that the term implies, but they have also learned to rely on an ability to sense patterns from scattered information in a way that computers cannot hope to match. This apparent lack of rigor drives engineers crazy. How can you be creative and rigorous at the same time?