Picture by Reid Wolcott of low clouds over the Puget Sound lowlands, taken from Newcastle Golf CourseHow many ways can a meteorologists spell "Boring"? For the past several days we have been stuck in the classic mid-winter ridge pattern, when high pressure over the region produces dry, low wind conditions. The problem is that such conditions produce the hated, yes even despised, persistent low clouds and fog. Why? With a ridge there is a lack of clouds aloft and the surface can effectively radiate heat to space without the clouds getting in the way. The atmosphere doesn't radiate as well and the result is an inversion, with temperature increasing with height. Low wind speeds with the ridge result in a lack of good atmospheric mixing in the vertical, and thus a strengthening of the inversion.
Inversions act as barriers to air motion and as the air progressively cools near the surface, fog and low clouds can form. Moisture collects below the inversion, strengthening the fog. And wait! It is even worse than that! The top of the clouds are very effective in radiating heat to space....so you get cooling there, which mixes down into the cloudy layer. If the cloudy layer is thin enough, some sun can get through to "burn" if off. Fog burns off in two ways----from the outside perimeter towards the interior and from the bottom up. We didn't understand how fog burned off from its edges until weather satellites showed this to us. The burning off from the bottom results in fog "lifting" into stratus.
When there is fog around, Sea Tac is often enshrouded (and operations affected), while Boeing Field and Portland are operational. Why? Sea Tac is high--roughly 450 ft up---and when the clouds start lifting in the region, they can still be in the soup while Boeing Fields gets enough low-level cleaning to allow the airport to open. Portland has the advantage of air jetting out the Gorge...which can mix things enough to stop fog formation. In any case, it is always wise to fly as late as possible in the winter from Sea Tac, when fog is least frequent.
And when fog and cold are around, there is another threat to think about ....black ice on the roadways....so be careful.
Computer models indicate this general pattern should hold into next week. Not my favorite weather pattern. And one that doesn't improve our declining snowpack. Right now the snowpack is below normal for increasing portions of the region (see graphic)...and with El Nino effects strengthening after Jan 1, a little pessimism is in order for skiing and water supply.