Seattle Snow
Cliff Mass
University of Washington
November 9, 2011
Reminder: Seattle Can Get Big
Snow Events
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.
December 2008: Two Weeks of Cold and
Snow--But No Big One-Day Snowstorms
Seattle
December
2008
12-24 inches of
total snowfall (18
inches SeaTac)
Typically, once a decade
we get a big event, with
a foot of snow or
more…like the last week
of December 1996
Climatology
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.
Lowland snowfall was greater in the 50s, late
sixties and early 70s
Lots of year to year variations.
Seattle Snowstorms 101
Important terminology: snow level and freezing level
Freezing Level
32F
1000 ft
Snow Level
For many of our snow events temperatures are marginal
Seattle Snow
• In the frequent marginal cases there can be
several inches on top of Queen Anne Hill,
Capitol Hill, Lake City, West Seattle, and on the
top of View Ridge (and other high areas), with
nearly nothing near sea level.
• Less snow near water due to warmer
temperatures.
Why are snowstorms rare over Seattle and
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
To get cold and wet is very hard…but why?
Seattle 2007
During the winter the mountains block the
cold air from the interior.
Cold
Air
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
Some Seattle
Snowstorms are Very
Localized
Puget Sound Convergence Zone
A snowstorm
associated
with the
Puget Sound
Convergence
Zone
December
18, 1990
The surprise
snowstorm
Classic problems with
buses
The city was crippled
Trees fell
Power failures were
widespread
Sometimes Convergence Zone
Snow is Extremely Localized
Why do we often get sheets of ice with Seattle snow?
November 27, 2006
Ice during Seahawk Game
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.
• 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.
Snowstorms are the most difficult
forecast problem for
meteorologists
• Why? Have to accurately predict temperature
and precipitation amounts 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
Forecasting Snow
• Although we may not get the details right,
meteorologists can usually tell you when you
need to worry.
• In other words, we can tell whether the basic
ingredients are available—temperature,
precipitation, etc. The question is whether
they will in the necessary way.
The Only Useful Tool for Seasonal
Prediction of Snow:
El Nino/La Nina
This Year: Weak to Moderate La
Nina
Niño Region SST Departures (oC)
Recent Evolution
The latest weekly SST departures are:
Niño 4
-0.5ºC
Niño 3.4
-0.9ºC
Niño 3
-1.0ºC
Niño 1+2
-0.9ºC
Pacific Niño 3.4 SST Outlook
• The majority of ENSO models predict the continuation of La Niña through the Northern
Hemisphere winter (Niño-3.4 SST anomalies less than -0.5°C).
Figure provided by the
International Research
Institute (IRI) for Climate
and Society (updated 19
October 2011).
Typical US Temperature, Precipitation and Jet
Stream Patterns during La Niña Winters
Snow and La Nina
• Typically more snow in La Nina years--but no
guarantees. This is a statistical relationship.
• Major impact after January 1
Implication for this year: higher
than normal probability for
lowland snow in Seattle
Better Information Can Help SDOT
and the Region to Cope with Snow.
One tool: SnowWatch
Developed with Support from the
City of Seattle
The other game changers:
upgraded weather
• The new coastal radar will substantially
improve forecast skill.
• All of the NWS have been upgraded to dual
polarization—which can tell us a lot about
what kind of precipitation is falling, where the
snow level is, etc.
Snow Hype on TV
The End
Cold Air from the Continental Interior
Has a Hard Time Reaching Western
Washington
Upper Level Chart for Snow
One Possible Explanation for the snowy 50s and 60s: 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.
Jim Foreman: The king of TV snow hype
…but all TV stations do it.