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Seattle Snow Cliff Mass University of Washington November 9, 2011.

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Presentation on theme: "Seattle Snow Cliff Mass University of Washington November 9, 2011."— Presentation transcript:

1 Seattle Snow Cliff Mass University of Washington November 9, 2011

2 Reminder: Seattle Can Get Big Snow Events

3 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.

4 December 2008: Two Weeks of Cold and Snow--But No Big One-Day Snowstorms

5 Seattle December 2008 12-24 inches of total snowfall (18 inches SeaTac)

6 Typically, once a decade we get a big event, with a foot of snow or more…like the last week of December 1996

7 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.

8 Lowland snowfall was greater in the 50s, late sixties and early 70s Lots of year to year variations.

9 Seattle Snowstorms 101

10 Freezing Level Snow Level 1000 ft Important terminology: snow level and freezing level 32F

11 For many of our snow events temperatures are marginal

12 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.

13 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

14 During the winter the mountains block the cold air from the interior. Cold Air

15 Our air and weather systems generally move west to east: Thus, our weather comes from off the mild Pacific.

16 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.

17 Moisture Cold Air

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

19 The Pressure Pattern That Does It


21 Some Seattle Snowstorms are Very Localized

22 Puget Sound Convergence Zone


24 A snowstorm associated with the Puget Sound Convergence Zone December 18, 1990 The surprise snowstorm

25 The city was crippled Trees fell Power failures were widespread Classic problems with buses

26 Sometimes Convergence Zone Snow is Extremely Localized

27 Why do we often get sheets of ice with Seattle snow? November 27, 2006 Ice during Seahawk Game

28 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.

29 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

30 Seattle Times Feb 1999

31 We are getting many of the snowstorms generally right now, but not all. Better computer models Better understanding More observations


33 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.

34 The Only Useful Tool for Seasonal Prediction of Snow: El Nino/La Nina


36 This Year: Weak to Moderate La Nina

37 Niño Region SST Departures ( o C) 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

38 Pacific Niño 3.4 SST Outlook Figure provided by the International Research Institute (IRI) for Climate and Society (updated 19 October 2011). 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).

39 Typical US Temperature, Precipitation and Jet Stream Patterns during La Niña Winters

40 Snow and La Nina Typically more snow in La Nina years--but no guarantees. This is a statistical relationship. Major impact after January 1



43 Implication for this year: higher than normal probability for lowland snow in Seattle

44 Better Information Can Help SDOT and the Region to Cope with Snow. One tool: SnowWatch Developed with Support from the City of Seattle







51 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.


53 Snow Hype on TV

54 The End

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

56 Upper Level Chart for Snow


58 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.


60 Jim Foreman: The king of TV snow hype …but all TV stations do it.


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