* WHAT...Visibility one quarter mile or less in dense fog. * WHERE...Portions of southeast Louisiana and southern Mississippi. * WHEN...Until 9 AM CDT this morning. * IMPACTS...Low visibility could make driving conditions hazardous. * ADDITIONAL DETAILS...Patchy dense fog can be more hazardous than widespread due to rapid visibility changes over short distances. Slow speeds well before entering areas of reduced visibility as vehicles behind you may not see you slowing in dense fog areas and vehicles ahead may have already slowed.