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 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.

Most common locations

Puget Sound Convergence Zones, variable in both location and strength, tend to form in the general vicinity of central and southern Snohomish 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.

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.

The example of April 18, 2008

On April 18, 2008, a strong and very unseasonable snow-producing Puget Sound Convergence Zone formed around Everett, Washington, 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.