Transcript Snowstorms of the Pacific Northwest Lowlands

Snowstorms of the
Pacific Northwest
Lowlands
Cliff Mass
University of Washington
January 21, 2009
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.
The dome of the
St. 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
Annual
Snowfall
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
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 variability
Negative phase of PDO associated with greater snowpack in NW.
Important terminology: snow level and freezing level
Freezing Level
32F
1000 ft
Snow Level
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?
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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.
Cold
Air
Cold Air from the Continental Interior
Has a Hard Time Reaching Western
Washington
Our air and
weather
systems
generally
move west
to east:
Thus, our
weather comes
from off
the 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.
Cold Air
Moisture
The Fraser River Valley is an important conduit
of cold air into western Washington.
The Pressure
Pattern That
Does 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 surprise
snowstorm
The same problems
with buses
The city was crippled
Trees fell
Power failures were
widespread
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