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