Snowstorms of the Pacific Northwest

LowlandsCliff Mass

University of WashingtonJanuary 21, 2009

Seattle Feb 1-2, 1916The greatest 24 hr snowfall (21.5 inches) since official record keeping in Seattle (1890). 29 inches on the ground. 4-5 ft drifts.

The dome of theSt. James Cathedral collapsed

Seattle December 2008

Seattle December 2008

12-24 inches of total snowfall (18 inches SeaTac

Understanding and dealing with local snowstorms have become safety, economic, and political issues.

We know a lot more about local snowstorms and are better at predicting them.

Lowland Snowstorms 101

AnnualSnowfall

Greatest Annual Snowfall Totals in the World in our MountainsThe Mt. Baker Ski Area in northwestern Washington State reported 1,140 inches of snowfall for the 1998-99 snowfall season

Lowland snowfall was greater in the 50s, late sixties and early 70s

Lots of year to year variations.

One Possible Explanation: the Pacific Decadal Oscillation (PDO)

Decadal Oscillation (PDO)

PDO is thought to be a natural mode of atmospheric variabilityNegative phase of PDO associated with greater snowpack in NW.

Freezing Level

Snow Level

1000 ft

Important terminology: snow level and freezing level

32F

Sometimes you can see the snow level around here

Not unusual in marginal situations for there to be much more snow on hill tops

Why are snowstorms rare over the western Washington lowlands?

• To get snow you need cold and wet.• It is easy to be mild and wet here• Sometimes we are cold and dry (this week!).• To get cold and wet is very hard…but why?

Seattle 2007

The weather of the Pacific Northwest is dominated by local weather features, most forced by our regional terrain.

During the winter the mountains block the cold air from the interior.

ColdAir

Cold Air from the Continental Interior Has a Hard Time Reaching Western Washington

Our air and weather systems generallymove west to east:Thus, ourweather comes from offthe mild Pacific.

The secret of Northwest snow is usually to bring in cold air from the north and interior while moist, cool Pacific air moves in aloft.

Moisture

Cold Air

The Fraser River Valley is an important conduit of cold air into western Washington.

The PressurePattern ThatDoes It

Upper Level Chart for Snow

Many Regional Snowstorms are Very Localized

Puget Sound Convergence Zone

A snowstorm associated with the Puget Sound Convergence Zone

December 18, 1990

The surprisesnowstorm

The city was crippled

Trees fell

Power failures were widespread

The same problems with buses

Sequim-Port Angeles Snowstorms

Port Angeles, March 2007

Kitsap County and the Hood Canal area is another focus for heavy snow

Why do we often get sheets of ice with snow over the Puget Sound lowlands.

November 27, 2006

Why so icy?

• Since it is usually mild here, the surface ground temperatures are generally above freezing.

• Snow falls on the roads and is not removed or plowed to the side (you know who to talk to about that!).

• The snow starts to melt, but then cooler temperatures behind the weather disturbance freezes the air slush into ice.

• Now it is impossible to remove without salt or a warm up.

Seattle and vicinity generally gets 6-9 inches of snow a year in 2-3 events.

Not unusual to have year with nearly no snow.

On the other hand, roughly once a decade we get a big event, with a foot of snow or more…like December 1996

Heavy snow (nearly two feet north of Seattle), followed by heavy rain caused the collapse of buildings throughout the region--1/2 billion dollars damage

Melting snow plus heavy rain led to landslides throughout the area

Snowstorms are the most difficult forecast problem for

meteorologists

• Why? Have to accurately predict temperature and precipitation amount to get the forecast right.

• Not much practice!• Often right on the edge of rain or snow

Seattle Times Feb 1999

We are getting many of the snowstorms generally right now,

but not all.

• Better computer models• Better understanding• More observations

Communication of snow forecasts is still an issue.

There is a major problem for local meteorologists…..

Snowstorm Hype

Jim Foreman: The king of snow hype

…but all TV stations do it.

The Book

• My attempt to write a comprehensive introduction to Northwest weather suitable for the layman.

• Includes everything from the big storms and local weather features, to climate, weather prediction, and how to read the skies.

• Full of color illustrations and photos.

The End