Today is forecast to be a very warm day in Seattle--probably our last time to get into the upper 60s this year. So what's causing this climatologically unusual day of warmth?
It starts with the upper-air pattern. We're under a ridge at the moment, and with that ridge is a pool of very warm temperatures aloft along the coast. Here's this morning's 700mb analysis:
That map is a bit cluttered, but you can see the area of lower pressure long the coast. Contrasting with that, there is an area of higher pressure in the interior of Washington on the Columbia Plateau. See the large number of isobars (the black contours) that are running north-south along the axis of the Cascade Mountains? That implies a very strong pressure gradient between the high Columbia Plateau and the Puget Sound lowlands. The pressure is much higher to the east of the Cascades than it is to the west. In fact, this morning the pressure in Yakima (east of the Cascades) has gotten to be almost 6 mb greater than the pressure in Seattle (west of the Cascades). That's quite a difference.
The result is that air wants to push from the east to the west across the mountains. Of course, the mountains make it difficult for air to move through. As air crosses the Cascades, it's at a higher elevation than down here near sea-level in Seattle. As a result, the air pushing across the Cascades this morning has stayed above the cold air that pooled in Seattle last night. We can see this on the wind profiles from this morning in Seattle:
But what's interesting to watch is how the height of the transition zone has been decreasing as this day has begun. If you watch over time, the level where the winds switch from northerly to easterly has been dropping rather steadily. At 11 AM it was only measured at around 200 meters above ground. The winds aloft have really picked up, too--there are easterly winds measured at around 40 knots just 500 meters above the ground while at the surface they are barely registering out of the north.
As the sun warms the land below, it will basically destroy the cold pool of air that has been near the ground all night in the lowlands. As the air warms, we'll get more movement and mixing. The constant high winds just above the ground are also helping to promote this mixing. Eventually, that warmer air mass with its easterly wins should mix all the way down to the surface. Not only will this bring the slightly warmer air to the surface, but that air will warm even further as it decreases in elevation. Since the pressure is higher near the surface, descending air compresses and, consequently warms. So, as soon as this air aloft reaches the surface, we expect a big spike in temperatures.
Some higher elevations have already experienced this warmer air. Here's the 11 AM surface map for the Puget Sound area: