LOS ANGELES - Clear, blue skies are turning brown.A historic Saharan dust plume that blanketed the Caribbean over the weekend is now closing in on the Gulf Coast.The densest plume of dust developed off Western Africa and has traveled nearly 5,000 miles across the Atlantic Ocean.According to the National Oceanic & Atmospheric Administration, the plume of dusty air reached the United States Thursday morning, bringing dust particles to the deep Southern United States.RELATED: Sahara dust blankets Caribbean, air quality hazardousFrank Marks, director of the Hurricane Resarch Divsion at NOAA's Atlantic Oceanagraphic and Meteorological Laboratory, says that these plumes are frequent in the summer, especially in June and July, and take about 10-12.