Puget Sound Convergence Zone
Encyclopedia
The Puget Sound Convergence Zone (PSCZ) is a meteorological phenomenon that occurs over Puget Sound
in Washington. It is formed when the large-scale air flow splits around the Olympic Mountains
and then converges over Puget Sound. This convergence zone
generally occurs between north Seattle and Everett
and can cause updrafts and convection
, which leads to a narrow band of precipitation
. This is evidenced by the 140-180+ inches (356 cm-457+ cm) of precipitation annually over Glacier Peak
to the east of Puget Sound.
A second, weaker convergence zone can occur between approximately Victoria, British Columbia
, and Bellingham, Washington
, over the San Juan Islands
, as a result of southwesterly air blowing from the Strait of Juan de Fuca
meeting northerly air moving southward down the Strait of Georgia
.
and northern King
counties, from Everett, Washington
, to the Northgate
neighborhood of Seattle. The strongest part of the Convergence Zone (where the heaviest precipitation falls) tends to lie along and adjacent to the King-Snohomish County line so that neither county is left dry. The proximity of the Convergence Zone to the King-Snohomish County line is the reason that cities located just north or south of the line, which are located within the rain shadow
of the Olympic Mountains
, approach Seattle in annual precipitation. The effect of the Puget Sound Convergence Zone nearly offsets that of the rain shadow. Without PSCZ, cities such as Edmonds
, Mountlake Terrace, and Lynnwood in Snohomish County and Shoreline, Lake Forest Park, and Bothell in King County would be noticeably dryer than Seattle.
and surrounding U-District often minimal, despite the fact that the general area comprising the UW community is located only 1-2 miles from what can be reasonably deemed as the "northern third" of Seattle or "North Seattle." To wit, NE 65th and NE 75th Streets, both commonly used as dividing lines between "North Seattle" and "Central Seattle," lie 20-30 blocks from the University of Washington's northern border, on NE 45th Street. However, the meager amount of city blocks between the University of Washington/general U-District area and the southern extent of North Seattle means everything as far as the Puget Sound Convergence Zone is concerned--for the Zone generally does not extend beyond the generally-agreed upon streets separating North Seattle from Central Seattle (this is not always the case, however, and both January 10, 2007 and January 14, 2008 serve as counter- examples). Therefore, it is entirely possible for areas from Shoreline to the Roosevelt neighborhood of Seattle (which encompasses NE 75th and NE 65th Streets) to be berated with heavy rainfall and strong winds, while areas not even a mile to the south are subjected to nothing more than overcast skies, due to the "calm zone" often present immediately outside the PSCZ.
, and spread south throughout the course of the afternoon and evening. By evening, the Zone had spread into Northern King County, dumping 3.5" of snow in Shoreline, and 6.5" of snow in Woodinville. As the Zone slowly sank south of Shoreline into Seattle (past NE 145th Street), snow amounts began to taper off. The snow-producing part of the Zone ended abruptly at Roosevelt High School
, a mere ten blocks north of the beginnings of the University District and the University of Washington
community. Just north of Roosevelt High School
, an inch of snow coated the ground, and due west of the school in the Greenlake
neighborhood of Seattle, an inch of snow had also fallen. In line with the known "abrupt edge" of the Puget Sound Convergence Zone, areas to the south of this Greenlake
-to-Roosevelt High
line (marked by NE 68th Street), including the U-District, witnessed only a dusting of snow.
Puget Sound
Puget Sound is a sound in the U.S. state of Washington. It is a complex estuarine system of interconnected marine waterways and basins, with one major and one minor connection to the Strait of Juan de Fuca and the Pacific Ocean — Admiralty Inlet being the major connection and...
in Washington. It is formed when the large-scale air flow splits around the Olympic Mountains
Olympic Mountains
The Olympic Mountains is a mountain range on the Olympic Peninsula of western Washington in the United States. The mountains, part of the Pacific Coast Ranges, are not especially high - Mount Olympus is the highest at - but the western slopes of the Olympics rise directly out of the Pacific...
and then converges over Puget Sound. This convergence zone
Convergence zone
Convergence zone usually refers to a region in the atmosphere where two prevailing flows meet and interact, usually resulting in distinctive weather conditions....
generally occurs between north Seattle and Everett
Everett, Washington
Everett is the county seat of and the largest city in Snohomish County, Washington, United States. Named for Everett Colby, son of founder Charles L. Colby, it lies north of Seattle. The city had a total population of 103,019 at the 2010 census, making it the 6th largest in the state and...
and can cause updrafts and convection
Convection
Convection is the movement of molecules within fluids and rheids. It cannot take place in solids, since neither bulk current flows nor significant diffusion can take place in solids....
, which leads to a narrow band of precipitation
Precipitation (meteorology)
In meteorology, precipitation In meteorology, precipitation In meteorology, precipitation (also known as one of the classes of hydrometeors, which are atmospheric water phenomena is any product of the condensation of atmospheric water vapor that falls under gravity. The main forms of precipitation...
. This is evidenced by the 140-180+ inches (356 cm-457+ cm) of precipitation annually over Glacier Peak
Glacier Peak
Glacier Peak is the most isolated of the five major stratovolcanoes of the Cascade Volcanic Arc in Washington...
to the east of Puget Sound.
A second, weaker convergence zone can occur between approximately Victoria, British Columbia
Victoria, British Columbia
Victoria is the capital city of British Columbia, Canada and is located on the southern tip of Vancouver Island off Canada's Pacific coast. The city has a population of about 78,000 within the metropolitan area of Greater Victoria, which has a population of 360,063, the 15th most populous Canadian...
, and Bellingham, Washington
Bellingham, Washington
Bellingham is the largest city in, and the county seat of, Whatcom County in the U.S. state of Washington. It is the twelfth-largest city in the state. Situated on Bellingham Bay, Bellingham is protected by Lummi Island, Portage Island, and the Lummi Peninsula, and opens onto the Strait of Georgia...
, over the San Juan Islands
San Juan Islands
The San Juan Islands are an archipelago in the northwest corner of the contiguous United States between the US mainland and Vancouver Island, British Columbia, Canada. The San Juan Islands are part of the U.S...
, as a result of southwesterly air blowing from the Strait of Juan de Fuca
Strait of Juan de Fuca
The Strait of Juan de Fuca is a large body of water about long that is the Salish Sea outlet to the Pacific Ocean...
meeting northerly air moving southward down the Strait of Georgia
Strait of Georgia
The Strait of Georgia or the Georgia Strait is a strait between Vancouver Island and the mainland coast of British Columbia, Canada. It is approximately long and varies in width from...
.
Most common locations
Puget Sound Convergence Zones, variable in both location and strength, tend to form in the general vicinity of central and southern SnohomishSnohomish
Snohomish can refer to:* Snohomish , a tribe of Native Americans* Snohomish, Washington, a city located in the county of the same name* Snohomish County, Washington* The Snohomish River in Washington...
and northern King
King
- Centers of population :* King, Ontario, CanadaIn USA:* King, Indiana* King, North Carolina* King, Lincoln County, Wisconsin* King, Waupaca County, Wisconsin* King County, Washington- Moving-image works :Television:...
counties, from Everett, Washington
Everett, Washington
Everett is the county seat of and the largest city in Snohomish County, Washington, United States. Named for Everett Colby, son of founder Charles L. Colby, it lies north of Seattle. The city had a total population of 103,019 at the 2010 census, making it the 6th largest in the state and...
, to the Northgate
Northgate
- Historical structures :* Northgate, Chester, Cheshire, England* Northgate, Essex, England* St Michael at the North Gate, Oxford, England.- Places :In Australia:* Northgate, Queensland* Northgate, South Australia* Northgate Shopping Centre, Tasmania...
neighborhood of Seattle. The strongest part of the Convergence Zone (where the heaviest precipitation falls) tends to lie along and adjacent to the King-Snohomish County line so that neither county is left dry. The proximity of the Convergence Zone to the King-Snohomish County line is the reason that cities located just north or south of the line, which are located within the rain shadow
Rain shadow
A rain shadow is a dry area on the lee side of a mountainous area. The mountains block the passage of rain-producing weather systems, casting a "shadow" of dryness behind them. As shown by the diagram to the right, the warm moist air is "pulled" by the prevailing winds over a mountain...
of the Olympic Mountains
Olympic Mountains
The Olympic Mountains is a mountain range on the Olympic Peninsula of western Washington in the United States. The mountains, part of the Pacific Coast Ranges, are not especially high - Mount Olympus is the highest at - but the western slopes of the Olympics rise directly out of the Pacific...
, approach Seattle in annual precipitation. The effect of the Puget Sound Convergence Zone nearly offsets that of the rain shadow. Without PSCZ, cities such as Edmonds
Edmonds, Washington
Edmonds is a city in Snohomish County, Washington, United States. Edmonds has a view of Puget Sound and both the Olympic Mountains and Cascade Range. The third most populous city in Snohomish County after Everett and Marysville, the population was 39,709 according to the 2010 census...
, Mountlake Terrace, and Lynnwood in Snohomish County and Shoreline, Lake Forest Park, and Bothell in King County would be noticeably dryer than Seattle.
Effect on the University of Washington
The Convergence Zone's prime location in the southern third of Snohomish County and the northern third of King County, including North Seattle, make the Zone's presence on the University of WashingtonUniversity of Washington
University of Washington is a public research university, founded in 1861 in Seattle, Washington, United States. The UW is the largest university in the Northwest and the oldest public university on the West Coast. The university has three campuses, with its largest campus in the University...
and surrounding U-District often minimal, despite the fact that the general area comprising the UW community is located only 1-2 miles from what can be reasonably deemed as the "northern third" of Seattle or "North Seattle." To wit, NE 65th and NE 75th Streets, both commonly used as dividing lines between "North Seattle" and "Central Seattle," lie 20-30 blocks from the University of Washington's northern border, on NE 45th Street. However, the meager amount of city blocks between the University of Washington/general U-District area and the southern extent of North Seattle means everything as far as the Puget Sound Convergence Zone is concerned--for the Zone generally does not extend beyond the generally-agreed upon streets separating North Seattle from Central Seattle (this is not always the case, however, and both January 10, 2007 and January 14, 2008 serve as counter- examples). Therefore, it is entirely possible for areas from Shoreline to the Roosevelt neighborhood of Seattle (which encompasses NE 75th and NE 65th Streets) to be berated with heavy rainfall and strong winds, while areas not even a mile to the south are subjected to nothing more than overcast skies, due to the "calm zone" often present immediately outside the PSCZ.
The example of April 18, 2008
On April 18, 2008, a strong and very unseasonable snow-producing Puget Sound Convergence Zone formed around Everett, WashingtonEverett, Washington
Everett is the county seat of and the largest city in Snohomish County, Washington, United States. Named for Everett Colby, son of founder Charles L. Colby, it lies north of Seattle. The city had a total population of 103,019 at the 2010 census, making it the 6th largest in the state and...
, and spread south throughout the course of the afternoon and evening. By evening, the Zone had spread into Northern King County, dumping 3.5" of snow in Shoreline, and 6.5" of snow in Woodinville. As the Zone slowly sank south of Shoreline into Seattle (past NE 145th Street), snow amounts began to taper off. The snow-producing part of the Zone ended abruptly at Roosevelt High School
Roosevelt High School (Seattle)
Roosevelt High School is a public school in the Seattle Public Schools district of Seattle, Washington, USA. Founded in the 1920s, Roosevelt continues to be one of the largest schools in the greater Seattle area....
, a mere ten blocks north of the beginnings of the University District and the University of Washington
University of Washington
University of Washington is a public research university, founded in 1861 in Seattle, Washington, United States. The UW is the largest university in the Northwest and the oldest public university on the West Coast. The university has three campuses, with its largest campus in the University...
community. Just north of Roosevelt High School
Roosevelt High School (Seattle)
Roosevelt High School is a public school in the Seattle Public Schools district of Seattle, Washington, USA. Founded in the 1920s, Roosevelt continues to be one of the largest schools in the greater Seattle area....
, an inch of snow coated the ground, and due west of the school in the Greenlake
Green Lake (Seattle)
Green Lake is a freshwater lake in north central Seattle, Washington, USA, within Green Lake Park. The park is surrounded by the Green Lake neighborhood to the north and east, the Wallingford neighborhood to the south, the Phinney Ridge neighborhood to the west, and Woodland Park to the southwest...
neighborhood of Seattle, an inch of snow had also fallen. In line with the known "abrupt edge" of the Puget Sound Convergence Zone, areas to the south of this Greenlake
Green Lake (Seattle)
Green Lake is a freshwater lake in north central Seattle, Washington, USA, within Green Lake Park. The park is surrounded by the Green Lake neighborhood to the north and east, the Wallingford neighborhood to the south, the Phinney Ridge neighborhood to the west, and Woodland Park to the southwest...
-to-Roosevelt High
Roosevelt High School (Seattle)
Roosevelt High School is a public school in the Seattle Public Schools district of Seattle, Washington, USA. Founded in the 1920s, Roosevelt continues to be one of the largest schools in the greater Seattle area....
line (marked by NE 68th Street), including the U-District, witnessed only a dusting of snow.