On the heels of these past two days, I thought I would do a brief analysis of why we are seeing this type of sudden downpours-to-sunshine type weather. For starters, we know that in the Northern Hemisphere we are currently transitioning from winter to summer. This means that the dominating cooler air mass we had over the winter is slowly being replaced by the warmer summer air mass. The clash between the two air masses causes some pretty interesting weather (see: the past few days here in the NW, almost every spring in the Midwest and South).
To get a little more specific, here in Oregon we get a cool onshore flow from the coast that can sometimes gets mixed in with some warmer air that is being forced up from California due to a low that would be located off the Northern California coast. What that leaves us with are these spotty-type clouds which indicate cool, but unstable air. These spotty clouds allow for sun to shine through the clearer areas, and the sun heats the ground surface. As the warmth from the sun heats the ground, it causes the air at the ground to rise. This is called convection: heated air that is forced to rise because it becomes lighter than its surroundings. As that air is warmed and rises, it becomes condensed and clouds form--leading to rain and in our case, downpours. Just how unstable does the atmosphere become? Check out this model representation of yesterday around 5 p.m. This model represents CAPE or Convective Available Potential Energy and measures how unstable and volatile the atmosphere is. As far as CAPE goes, the values expressed are not that high, only in the 100-400 range. Areas in the Plains can see CAPE in the 2000-3000 range. But when mixed with that marine air, what we saw late yesterday afternoon was plenty to trigger some thunderstorms and heavy downpours with hail mixed in. In fact, at 4:50, our radar indicated at least 6 lightning strikes. So you may have noticed that by 7 p.m. last night the showers stopped and there was clearing and some sunshine out there. Why does that happen? We lose our heat source, the sun. With nothing to warm the air, the atmosphere cools. When it cools off, air sinks as it becomes dense and sinking air usually results in high pressure and fair weather. That is exactly what happened yesterday! Look for it to happen again today.