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 to the right show the barometric pressure field.  Lower pressures are indicated in dark blue.  Higher pressures are indicated in red.  Hurricanes and tropical storms, as well as non-tropical low-pressure areas, are associated with low pressure (blue). 

The white lines are isobars (lines of equal pressure).  The closer the lines are to each other, the stronger the winds.

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.