The anxiety level isnt as intense as it was in the 1970s when natural gas was in shortage. Still, the arrival of winter--which blew in officially Dec. 21--inevitably stirs concern among manufacturers over the availability and price of the fuel, which is heavily used by industry, for the next three months. This winter, theres nothing particular to worry about, says the American Gas Assn. (AGA) in its year-end forecast. Although the notorious El Nino effect will have an impact on winter temperatures in the U.S., says the Washington-based trade group, the effects will be regional, with warmer-than-normal temperatures in the Midwest and colder-than-normal in the Southeast. As a result, there will no overall effect on demand for natural gas. However, notes AGA, "there is some evidence that winters subsequent to a warm El Nino event tend to be colder than normal." Thus, it warns that a warmer first quarter of 1998 may be a prelude to a colder November and December.