The heavy rain is over and today and tomorrow we will transition to a week-long period of high pressure and dry conditions. Sounds like heaven? Well, there is a little detail we have to talk about...
Anyway, right now the pressure over us is already extraordinarily high....about 1040 mb (30.70 inches of mercury). In fact, it really is rare to get rain (as we are right now in some areas) with such high pressure.
All the computers models agree that we are about to transition to an extended period where high pressure will build along the West Coast....take a look at the attached figure for Tuesday at 4 PM--the figure shows the flow at 500 mb...about 18k feet aloft). Like sea level pressure better? The next figure shows that at the same time.
Such high pressure will bring absolutely dry conditions and MUCH warmer temperatures aloft. If this was late spring or summer, we could be looking forward to record-breaking temperatures...and some might cancel that trip to Hawaii. But before you do so, remember this is January in Seattle.
The problem this time of the year is that when high pressure builds over us during midwinter, we often develop low-level inversions and fog. Warming aloft is certain. But the sun is still weak and it can't heat up the ground much and there is good radiational cooling to space during the long nights. Fog can form at night and can become very persistent...and in fact it helps to maintain itself for reasons I won't get into now. So there is a danger we can fog in and stay cool at low levels. Such fog can mess up air travel...so fly out as late as possible in the day if you want some insurance.
Our big hope is that the high pressure will slip sufficiently east to give us offshore and downslope flow...which can mix up the lower atmosphere and destroy the inversion. This is what local meteorologists will be analyzing in great depth this week.
And there is another threat...air pollution. Inversions act to prevent mixing of air in the vertical and pollutants can be increasingly concentrated. My colleagues at the Puget Sound Clean Air Agency are already concerned enough to provide initial warnings. There might be burn bans later in the week...so keep that in mind before you fire up your fireplace or woodburning stove.
So we had snow and ice and then heavy rain and floods. Now we may have fog, murk, and air pollution. But there is one good thing about the latest plague...you will be able to escape it by going up: the conditions will be sunny and warm in the mountains. Last time this situation occurred it was in the 30s and lower 40s in Seattle and 60F at Paradise near Mt. Rainier.