It’s pouring down rain in one spot, but completely dry just a short distance away. This is a characteristic of a Puget Sound Convergence Zone, or PSCZ for short (because let’s face it, that’s quite a mouthful). So what is it and how does it form?
A PSCZ is a band of clouds and enhanced precipitation that form as winds get deflected around the Olympic Mountains. The most common wind directions associated with PSCZ events is west to northwest (i.e. from the west or northwest). This wind direction is typical after the passage of a front, so the prime time for PSCZ development is after a front passes.
As the air approaches the Olympic mountains, it is forced around them, as shown in the figure above. Once this air reaches the I-5 corridor, it turns; the northern segment turns towards the south, while the southern segment turns towards the north. As a result, the air collides, commonly in the area shaded in the above image.
When it collides, there is nowhere for it to go but up. This sparks rising motion and convection, which creates clouds and eventually precipitation.
A vast range of weather can be associated with a PSCZ. Heavy rain is common, but we can also see hail, thunder and lightning, and gusty winds. If they form when it’s cold enough, convergence zones can also be responsible for significant lowland snowfall, when other areas get nothing. Even if it’s too warm for lowland snow, a PSCZ can lead to significant mountain snow.
As aforementioned, convergence zones tend to form in the region highlighted in Figure 2. Oftentimes they set up in the East/West direction in a narrow band similar to the one shown in Figure 1. However, there is no “one size fits all” approach when it comes to PSCZ formation. They can be oriented in many different ways and come in different shapes and sizes, as illustrated in the following examples.
The movement of convergence zones depends on the strength of each wind component (i.e. which direction of the wind is strongest). Sometimes each component can be relatively equal, meaning that the PSCZ will stall out in one location, pounding the area with potentially heavy rain. When this happens, localized flooding is also possible.
Puget Sound Convergence Zones are a staple to PNW weather and knowing more about them can help in planning your daily life. If there are winds from the north in Everett and from the south in Seattle and you live in shaded region in Figure 2 (especially near Lynnwood), watch out. Some stormy weather may be headed your way.