Puget Sound Convergence Zone

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 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 over Glacier Peak to the east of Puget Sound.

Most common locations

Puget Sound Convergence Zones, although often variable and erratic in both location and strength, tend to form in the general vicinity of Central/Southern Snohomish and Northern King counties. [ [http://www.komotv.com/weather/faq/4306427.html What is the Puget Sound Convergence Zone? | KOMO News - Seattle, Washington | F.A.Q ] ] Thus, a typical Convergence Zone will range anywhere from Everett, Washington, to the Northgate neighborhood of Seattle. Oddly enough, the strongest part of the Convergence Zone (where the heaviest precipiation falls) tends to be located directly along and adjacent to the King-Snohomish County line (the line that separtes Snohomish County, to the north, from King County, to the south), ensuring that, when a PSCZ forms, neither county is left dry. The proximity of the Convergence Zone to the King-Snohomish County line also allows cities located just north or south of the line (such as Edmonds, Mountlake Terrace, and Lynnwood in Snohomish County and Shoreline, Lake Forest Park, and Bothell in King County)to come close to matching the city of Seattle in terms of yearly rainfall, despite the effects of the Olympic rain shadow on the cities. This is because precipitation from the Puget Sound Convergence Zone generally offsets the lesser amounts of precipitation these county-bordering cities receive, when compared to Seattle proper, due to the rain shadow. [ [http://www.komotv.com/weather/faq/4346936.html Does The Convergence Zone Mean It Rains More In Lynnwood? | KOMO News - Seattle, Washington | F.A.Q ] ] Absent the presence of the PSCZ, cities such as Lynnwood would receive noticeably lighter amounts of yearly rainfall than Seattle and other cities to the south.

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 Washington 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. [ [http://www.komotv.com/weather/faq/4306427.html What is the Puget Sound Convergence Zone? | KOMO News - Seattle, Washington | F.A.Q ] ]

The example of April 18, 2008

On April 18, 2008, a strong snow-producing Puget Sound Convergence Zone formed around Everett, Washington, and spread south throughout the course of the afternoon and evening. [ [http://www.komotv.com/news/17940574.html Waking to a springtime wonderland | KOMO News - Seattle, Washington | News ] ] By evening, the Zone had spread into Northern King County, dumping 3.5" of snow in Shoreline, and 6.5" of snow in Woodinville. [ [http://www.komotv.com/news/17920384.html Summer is 63 days away. Really? | KOMO News - Seattle, Washington | News ] ] 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.

References