PUGET SOUND COLD AIR OUTBREAKS

Puget Sound Cold Air Outbreaks

* Geographic and topographic maps created from the USGS/ESRI ArcGIS mapping software and database

Introduction

Getting cold air into the Puget Sound region is a difficult task. Mountain ranges block Canadian and Arctic air masses from spreading to the west. A dominant wind flow off the Pacific Ocean keeps temperatures at the surface too warm during the winter for much snow to fall in cities like Olympia, Tacoma, Seattle, or Everett. On a few occasions per year, enough cold air does manage to enter the region so snow can accumulate several inches.

Getting arctic air into the Puget Sound Basin

The Cascades Mountains to the east of the Puget Sound are a massive barrier blocking the westward movement of dense Arctic air, assuming the cold air can even get that far. Other mountain ranges, like the Rockies of western Canada and the western United States, tend to block the cold air before it even gets to the Cascades. If the cold air can be pushed up against the Rocky Mountains long enough, it can spill over the lower elevated passes between the mountain ridges. From there it can weave its way west through the valleys, gorges, and basins toward the coast. A deep cold upper-level low pressure trough positioned over the Pacific Northwest, southwest Canada, and the adjacent Pacific Ocean is favorable for allowing cold air to stay lodged up against the mountains. A strong Arctic high pressure system near 1050 millibars (mb) central pressure or stronger is optimum to push the cold air into the mountain valleys. Not every cold air outbreak will have atmospheric features to the extent described. The upper level trough may be flatter or the high pressure system may only be 1040 mb or less.

A good path for Arctic air to reach the west coast is through the Fraser River Valley in British Columbia, Canada. The Fraser River flows west out of the mountains through Vacouver and into the Strait of Georgia. The Fraser River, as shown on the Wikipedia map, is the longest river in British Columbia. The river starts out in the east-central part of British Columbia near the border of Alberta just south of the western bulge of the provincial line. The river initially flows northwest but then turns south and flows to near the border of Washington State. From there the river turns west into Vancouver where it finally empties into the Strait of Georgia. The Fraser River Valley can be considered the "Arctic Passage". Once over the Strait of Georgia, cold air spreads south into the Puget Sound.

When west to northwest winds run into the Olympic Mountains to the west of the Puget Sound, some of the air moves up and over the mountain barrier as opposed to just going around it. The warming of the air that occurs when it sinks on the downwind side of the barrier results in the development of a lee-side low pressure trough over the western side of the sound. The lower pressure increases the pressure gradient between the trough and an Arctic high pressure system to the east. The difference in pressure helps draw cold air into the basin of the Puget Sound. A low pressure system approaching from the west or southwest from the Pacific Ocean can also increase the pressure gradient and help funnel cold air into the basin.

Click HERE to see a three day series of surface and upper air charts from November 26, 2006, to November 28, 2006, showing an arctic outbreak which affected Washington State. Note the central pressure of the arctic high pressure system is slightly above 1050 mb on the first two surface maps. Also, note the favorable position of the upper-level low pressure trough on the 500 mb maps. The center of the closed circulation of the trough moved from southern British Columbia and the adjacent Pacific Ocean waters to eastern Washington and northern Idaho.

Cold air from over the Pacific Ocean

The air behind Pacific cold fronts is rarely cold enough at the surface for snow to reach the ground before melting completely. Times exist however when a storm system moves southeast from the Gulf of Alaska or south from the Yukon and northern British Columbia, then turns east into the Washington and Oregon coast. This type of storm can ingest cold air from western Canada into its circulation. The water modifies the air but temperatures can still be cold enough to support snow in the lower elevations of the Puget Sound region. The communities in the higher hills, such as east of Seattle, have a better chance of getting snow. Cooling due to melting and evaporation from heavier showers also increases the odds of snow reaching the ground.

References

Garth, K. F., C. F. Mass, G. M. Lackmann, and M. W. Patnoe, 1993: Snowstorms over the Puget Sound Lowlands, Wea. Forecasting, 8, 481-504.

National Weather Service Forecast Office Seattle/Tacoma, WA - Numerous Area Forecast Discussions issued over the years.