The Origin of the Puget Sound Tornado

Around 3 PM on Wednesday, a tornado was spotted over Puget Sound (see picture below).  Technically, this rotating wind feature is known as a waterspout since it developed over water.

Such Puget Sound twisters have occurred before, and in this blog, I will describe their origins.

Monday's waterspout was produced by a thunderstorm associated with cold, unstable air forced to rise by a Puget Sound convergence zone.

As I noted in a previous blog, we had unusually cold air aloft on Monday....a gift from the Arctic Express from Alaska.  The figure below shows the temperature at around 5000 ft at 5 PM on Wednesday, with blue colors indicating below-normal temperatures.

With warmer air near the surface, this created a large change of temperature with height, which produces the potential for great instability in the vertical, leading to convection with towering cumulus clouds and even thunderstorms.

Such thunderstorms are aided by having low-level air convergence, which produces upward motion (see below).  This gives an upward kick to the air to rise.

On Wednesday, we had a very effective source of low-level convergence:  a Puget Sound Convergence Zone, forced by air forced around the Olympic Mountains (see schematic below).   

This combination of low-level air convergnce and and an unstable atmosphere produced a band of cumulonimbus clouds (thunderstorms), which were evident on an infrared satellite image with lightning observations  (see below, red crosses show lightning strikes)

The weather radar image at 3 PM indicated the strong thunderstorm cell associated with the Puget Sound convergence zone (red colors show the heaviest rain).

The Space Needle Cam indicated very heavy precipitation with the thunderstorm cell.  Wow.

But why a tornado?    Where did the rotation come from?  

There is inherent rotation from the converging winds in a convergence zone (see below), and that rotation can be increased by the strong vertical motion in a thunderstorm (see schematic below).   

The increasing spin is analogous to the increasing spin of a skater when they bring their hands in (see below)

Puget Sound is hardly a place where storm chasers gather to view tornadoes.    But there is a long history of weak tornadoes associated with Puget Sound Convergence zones, something I describe in my Northwest Weather book.

For example, a bunch of kids were lifted into the air on June 14, 2001, over West Seattle!  And Bill Gates' childhood home in North Seattle was seriously damaged by a tornado in 1962.