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.

Announcement

I will hold a special online Zoom session at 10 AM on Saturday for Patreon supporters.  Will answer questions and talk more about the recent Sound tornado and about drought issues.

Intense Cold Front Brings Heavy Snow and Substantial Precipitation to the Mountains

As predicted, a very strong Pacific cold front moved through the region yesterday, bringing substantial precipitation, including heavy snow in the Cascades.  Several mountain observing sites have received over a foot of new snow.

At Crystal Mountain Ski Area, the upper reaches were in white-out conditions this morning:

And the mid-elevations of Alpental in Snoqualmie Pass were a winter wonderland.

The National Weather Service has a Winter Storm Warning for the Cascades and a Frost Warning for the lower slopes:

The visible satellite imagery shows very cold, unstable air approaching the Washington coast (see below)

The white and dark areas offshore show strong cumulus/convective activity resulting from very cold air moving over warm water.  This creates a large temperature change in the vertical, which produces convection:  small-scale upward and downward motions in the atmosphere.

Upward motion produces clouds and rain, and downward motion creates a clear area.

The radar image last night showed the narrow rainband associated with the intense front (see red arrow). There was also a world-class rain shadow over the North Sound area (light blue arrow).

All of this was nearly perfectly predicted by our models.

Precipitation amounts with the front and trailing showers have been impressive, with the totals through 10AM shown below.

Wow...over three inches in some locations!

Importantly, a persistent Puget Sound Convergence Zone has resulted in heavy precipitation around Snoquamie Pass and the headwaters of the Yakima River (see below)

As a result, there will be plenty of water to fill the Yakima Reservoir system as well as to produce high flows in the entire Yakima river system (see forecast below)

Furthermore, the substantial snowfall will help maintain river flows later in the season.

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UPDATE!    Snoqualmie Pass has been closed due to heavy snow!

Announcement

I will hold a special online Zoom session at 10 AM on Saturday for Patreon supporters.  Will answer questions and talk more about the recent Sound tornado and about drought issues.