Snowstorms of the Pacific Northwest Lowlands

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Snowstorms of the Pacific Northwest Lowlands

• Cliff Mass
• University of Washington
• January 21, 2009

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Seattle Feb 1-2, 1916
The greatest 24 hr snowfall (21.5 inches) since
official record keeping in Seattle (1890). 29
inches on the ground. 4-5 ft drifts.
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The dome of the St. James Cathedral collapsed
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Seattle December 2008
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Seattle December 2008 12-24 inches of total
snowfall (18 inches SeaTac
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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.
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Lowland Snowstorms 101
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Annual Snowfall
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Greatest Annual Snowfall Totals in the World in
our Mountains
The Mt. Baker Ski Area in northwestern Washington
State reported 1,140 inches of snowfall for the
1998-99 snowfall season
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Lowland snowfall was greater in the 50s, late
sixties and early 70s Lots of year to year
variations.
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One Possible Explanation the Pacific Decadal
Oscillation (PDO) Decadal Oscillation (PDO)
PDO is thought to be a natural mode of
atmospheric variability Negative phase of PDO
associated with greater snowpack in NW.
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Important terminology snow level and freezing
level
32F
Freezing Level
1000 ft
Snow Level
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Sometimes you can see the snow level around here
Not unusual in marginal situations for there to
be much more snow on hill tops
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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 hardbut why?

Seattle 2007
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The weather of the Pacific Northwest is dominated
by local weather features, most forced by our
regional terrain.
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During the winter the mountains block the cold
air from the interior.
Cold Air
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Cold Air from the Continental Interior Has a Hard
Time Reaching Western Washington
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Our air and weather systems generally move west
to east Thus, our weather comes from off the
mild Pacific.
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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.
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Cold Air
Moisture
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The Fraser River Valley is an important conduit
of cold air into western Washington.
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The Pressure Pattern That Does It
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Upper Level Chart for Snow
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Many Regional Snowstorms are Very Localized
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Puget Sound Convergence Zone
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A snowstorm associated with the Puget Sound
Convergence Zone December 18, 1990 The
surprise snowstorm
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The same problems with buses
The city was crippled Trees fell Power failures
were widespread
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Sequim-Port Angeles Snowstorms
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Port Angeles, March 2007
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Kitsap County and the Hood Canal area is another
focus for heavy snow
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Why do we often get sheets of ice with snow over
the Puget Sound lowlands.
November 27, 2006
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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.

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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 morelike December 1996
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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
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Melting snow plus heavy rain led to landslides
throughout the area
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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

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Seattle Times Feb 1999
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We are getting many of the snowstorms generally
right now, but not all.

• Better computer models
• Better understanding
• More observations

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Communication of snow forecasts is still an
issue. There is a major problem for local
meteorologists..
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Snowstorm Hype
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Jim Foreman The king of snow hype
but all TV stations do it.
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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.

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The End
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