Arctic Outbreak Forecast      Issued:   08:00 PM Saturday November 24, 2001   Surge of Arctic Air Possible    Cold air under strong high pressure is poised to move south from the Arctic regions of Canada and Siberia this coming week. By normal standards, this air will not be all that cold, considering it is of Arctic origin. But, considering the time of year and how unseasonably warm it has been recently, this punch of Arctic air will bring an abrupt change for most of the week ahead.   A deep trough of low pressure, currently digging into the southwest U.S. will traverse the central states early this week, opening the gates behind to the air that is pooled in northern Canada. Beginning late Sunday or early Monday, the Arctic front will begin its plunge into the lower 48, reaching the Gulf Coast overnight Monday night.   The potential is for temperatures, across all the eastern half of the U.S., except Florida, to plummet as much as 30 degrees from Tuesday to Thursday, but the coldest nights could be later in the week as the skies clear. For Florida, the greatest temperature changes won't come until the weekend.   On a more uncertain note, there are also early indications of possible winter weather after midweek from east Texas across northern Louisiana, Mississippi, Alabama and northward into the Midwest.   We will continue to monitor this developing weather situation and the potential for winter weather. You may view an updated version of this Arctic Outbreak in your personal web page's long range section each evening.   Jeff Reichelt