Fog has plagued much of Western Washington since the weekend. Why has it been so persistent?
During the longer nights this time of year, with high pressure aloft over the region and light winds at the surface, colder air and its moisture tends to settle into the lowest elevations.
Meanwhile, warmer air is associated with the higher pressure aloft. This situation creates what is called a temperature inversion – warm dry air aloft trapping cool, moist air near the surface.
If you were riding with a weather balloon as it rose from the ground, it would be cool and moist during the first thousand feet or so, then suddenly warm and dry above that point. If you were flying out of Sea-Tac Airport, shortly after lifting off the ground, you would burst into the sunshine and see a blanket of clouds below you.
Freezing levels have been near 13,000 feet since the weekend. Temperatures in the mountains have been in the 60s with a lot of sunshine. Enough offshore breezes along the coast and in the southwest interior have resulted in sunshine and temperatures into the 60s and even lower 70s.
Meanwhile, the rest of the interior has been stuck in the fog with high temperatures struggling to get into the 50s.
This temperature inversion is forecast to break early next week as a weather system moving through western Canada will increase northerly winds through western Washington and scrub out the fog. Look for the fog to give way to sunshine in much of western Washington early next week.