The visible satellite image at 1:20 PM yesterday (Friday, May 15th) shows the story. The lowest elevations are essentially clear. Generally more cloudy even in eastern Washington…but look closely…there are some narrow clear zones there too.
So what is going on? Why are the clouds avoiding Puget Sound? And why are there no clouds immediately offshore?
Interestingly, whatever is going on was captured by our high-resolution weather forecast models. The UW WRF model run at very high resolution (1.3 km spacing between the grid points) had a 21h cloud forecast that showed the effect.
Yesterday was a great day to see this effect: We are now in a period with a very strong sun and the large scale winds were light.
The strong, mid-May sun heated the ground, causing the difference in the temperatures in the vertical (the lapse rate) to increases, and the atmosphere started to convect or mix in the vertical. Just like your hot cereal pot when you turn on the burner. And as the air rose, it cooled and eventually became saturated, producing clouds.
Certainly, there were lots of clouds in the mountains, as shown by the wonderful Crystal Mountain 360 degree cam at 1 PM Friday (see below).
In contrast, the Seattle Space Needle cam at the same time shows clear skies over the lowlands and clouds toward to foothills of the Cascades.
So what is going on here? It has to do with the local atmospheric circulations associated with the land-water contrasts (see breeze) and with the mountains (upslope flow on terrain).
During the day, the strong May sun warms the mountain slopes and land surface. In contrast, there is far less heating of the water surface (which has a large heat capacity and allows some of solar radiation to warm below the surface). As a result of the solar heating over land, a sea breeze wind develops that is directed from water to land around Puget Sound and upslope flow begins on the mountain slopes (air tends to rise up mountain slopes….something all hikers know about!). With air moving away from Puget Sound at low levels, air must sink over the Sound to takes its place, resulting in a three-dimensional air circulation shown below (as illustrated by the arrows).
Sinking air is the enemy of clouds, preventing their formation or dissipating any clouds that are around. In contrast, there is lots of upward motion over and near the terrain or away from the water, which helps produce clouds.
Thus, we get a pattern of clear or nearly clear skies over and near the Sound (and the Straits of Georgia and San Juan de Fuca) but plenty of clouds inland. And little clouds over the ocean where the surface can’t warm.
Similar effects, but on a smaller scale, occur over eastern Washington: clouds over ridges and terrain features, but clear skies in the valleys. A good example of this is shown below for the region around Lake Chelan.