Complex sentence: 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.
Split into simpler sentences: 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. Thus, a typical Convergence Zone will range anywhere from Everett, Washington, to the Northgate neighborhood of Seattle.