Spring Weather Extremes: It's Just Life in the Convergence Zone

If you don't like the weather, wait 10 minutes.

A sunny Edmonds morning can change to a hailstorm, bring winds that topple trees, and then back to sun again ... all before lunch. You get on the phone to tell your friend in Seattle about it and learn that it's been sunny and calm all day.

What's going on?

Yes Virginia, there is an explanation. It's called the Puget Sound Convergence Zone, and it stretches down the I-5 corridor between roughly Arlington and Northgate, with Edmonds right in the middle. And while it can affect our weather at any time during the year, it is typically most extreme in spring when it frequently results in hail and sometimes tornados.

OK, so just what is this Convergence Zone thing and how does it happen?

It's the combined result of our topography and latitude.

The Puget Sound region lies smack in the middle of the zone of prevailing westerly winds. These winds are continually blowing onto our shores, bringing with them weather systems that originate over the Pacific. The evening weather forecast always shows weather developing over the Pacific and moving onto the Washington coast ... well, the prevailing wind is what moves it here.

When it hits our shores, the wind also hits a wall: the Olympic Mountains. Some of the air will try to go up over the mountains, but most stays close to the ground and splits into two streams, one going around the mountains to the north through the Strait of Juan de Fuca and the other to the south toward Olympia.

As the winds enter the Puget Sound area, they hit another wall: the Cascades. This turns these two streams of air toward one another and sets them on a collision course.

When they collide, watch out.

The collision will typically happen somewhere between Northgate and Arlington. They hit like two Sumo wrestlers, and there's nowhere to go but up. This updraft creates a low pressure zone, drawing even more air into the rising current and creating high winds close to the ground.

But back to the updraft. As the air rises the temperature drops, and the water vapor it carries condenses into rain, sleet or hail. As it falls to earth, the high surface winds caused by the updraft catch it, often whipping it into a violent maelstrom of water, wind and ice. Winds topple trees, torrents of rain flood roadways in minutes, and hailstones sometimes as big as golf balls damage cars, homes and buildings.

At the top of the updraft the air, now colder and devoid of its moisture, begins to sink back to earth. This typically creates sunny, dry conditions on the north and south sides of the convergence. Within half an hour of the storm, the sun shines on flooded roads, and reflects off the white hailstones covering cars, lawns and buildings.

To help visualize this, the National Oceanic and Atmospheric Administration (NOAA) has produced a wonderful animated slide show. See it here: http://www.wrh.noaa.gov/sew/PugetSoundConvZone/player.html

Because the Convergence Zone tends to be most active in spring and early summer, Edmonds and other convergence zone communities experience more rain in April, May and June than the rest of Puget Sound, where rainfall is actually declining during these times. So if it seems that Spring takes a long time to come here, you're right, it does, and the Convergence Zone is to blame.

Happily, violent convergence events are very localized and tend to break up quickly.

So again, "If you don't like the weather, wait a few minutes...or travel a few miles."