The Global Forecast System is a global numerical computer model run by NOAA. This mathematical model is run four times a day. It is an excellent model in the one- to five-day range. The accuracy drops significantly after day five, and significant long-range forecast changes are noted from run to run.
The graphics above show the accumulated precipitation over a 6 hour time span. The solid gray lines are isobars expressed in millibars (lines of equal pressure). The closer the lines are to each other, the stronger the winds. Hurricanes and tropical storms, as well as non-tropical low-pressure areas, are associated with low pressure. The lighter gray lines with arrowheads are known as streamlines. These lines indicated the direction of flow of air at the surface. The spacing of these lines does not indicate the strength of the wind.
Remember that these are mathematical models with varying initial assumptions. Thus, different models produce different final results. They do not necessarily reflect the "official" hurricane track issued by the National Hurricane Center. Forecasters review all of the model data but use their own experience and scientific expertise to arrive at a final forecast.
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