Granny Clampett isn't the only one who can predict the weather by her body aches. According to several scientific studies, both barometric pressure and humidity influence the human body's pain levels. (One of the latest being a 2005 study presented at the American College of Rheumatology.)Wind, precipitation, and temperature also have an impact. As these increase, folk suffering from arthritis or neurological diseases (e.g., MS) can predict an approaching storm because they hurt more. Others may have an increase in migranes, asthma attacks, or escalating problems with circulatory conditions. In fact, chronic pain specialists ask their patients to log weather conditions and pain levels, to determine the patient's physical relationship with the weather.
This information is not new, of course. During the Civil War, it was documented that amputee soldiers sensed pain in their "phantom" limbs whenever the weather changed.
Online "ache & pain" forecasts are available at www.weather.com and at intellicast.com (which also offers a very nice "bad hair forecast," if you're interested).
Sources: weather.com, usatoday.com, 02/21/2005 intellicast.com