The thick frost that covered Roseburg this morning made for a slippery commute, but was a good development for fruit trees. The temperature dropped below freezing shortly before 7 p.m. Tuesday at the Roseburg Regional Airport and stayed there overnight, dipping as low as 27 degrees. The low temperature Tuesday was 24 degrees. The National Weather Service attributed the cold to a high-pressure system settling over the valley, trapping the cold air. The weather service extended an air stagnation advisory and predicted the air won’t start lifting until about 4 a.m. Sunday. No precipitation is expected through the middle of …