Seattle Snow

Cliff MassUniversity of Washington

November 9, 2011

Reminder: Seattle Can Get Big Snow Events

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.

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

ClimatologySeattle 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

Freezing Level

Snow Level

1000 ft

Important terminology: snow level and freezing level

32F

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.

ColdAir

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

Some Seattle 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

Classic problems with buses

Sometimes Convergence Zone Snow is Extremely Localized

Why do we often get sheets of ice with Seattle snow?

November 27, 2006Ice 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

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).

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 variabilityNegative phase of PDO associated with greater snowpack in NW.

Jim Foreman: The king of TV snow hype

…but all TV stations do it.