10:30 a.m. Monday, October 24, 2011

If you live around Seattle, chances are you had an early morning wake-up call from Mother Nature around 4:40 a.m.  An isolated thunderstorm popped up bringing lightning, thunder, and heavy rain.  There was a lot of standing water on the roads around Seattle which didn’t make it easy for commuters trying to get around the viaduct closure. 

Looking at the radar image I captured just before 5 a.m. you can see a line of showers stretching from Seattle to Fall City.  You might recognize that radar signature; it’s a Puget Sound Convergence Zone.   It was this convergence zone that brought the heavy showers and helped spark the thunderstorm over Seattle. 

A trough of low pressure is sitting over us, bringing cool moist air off the Pacific Ocean into Western Washington.  As the air comes off the ocean it wraps around the Olympic Mountains, coming together (converging) over Puget Sound.  As the winds collide, they have nowhere to go but up, creating a line of heavy showers.

Another ingredient that helped spark the thunderstorm was very cold air above the surface.  As I mentioned earlier, a trough is sitting over us.  Think of that as a cold pool of air sitting above our heads that plays an important part in destabilizing the atmosphere.    

In order to get heavy showers and thunderstorms to form, you need vertical development in the clouds.  This is achieved when that cold pool of air is in place.  As an air parcel is forced up, in this case by the convergence zone, it will continue to rise as it remains warmer than the environment.  As the parcel rises, the water vapor in the air cools and condenses creating a cloud.  Through the process of condensation, heat is released allowing it to rise farther up into the atmosphere.  This lifting is referred to as convection. 

As you can imagine, convection is more likely during the afternoon when you have daytime heating.  As the sun warms the surface it is much more effective in causing air to rise.  Even though it was before sunrise early this morning, the combination of the convergence zone and cold air aloft provided strong enough convection to fire up that thunderstorm over Seattle.

The convergence zone is still holding together as I write this blog just after 10:30 a.m.  A line of heavy showers stretches west to east from North Seattle to Lake Sammamish.  As we progress through the afternoon, the trough will be moving east.  Drier air working in will allow the showers to start tapering off with more sun breaking out. 

-Sam Argier, KIRO 7 Morning Meteorologist