WeatherWatcher: Puget Sound Convergence Zone strikes again

Sunday, February 25, 2018

Doppler Radar image at 4:20pm, February 25, 2018.

As seen in the radar image above, the band of precipitation stretching from west to east is a classic Puget Sound Convergence Zone event. We had one runthrough during the morning hours and then this second one that came in during the afternoon hours, more or less parking over the King and Snohomish County line. This event dumped a mixture of everything, from rain, hail, graupel and snow. Accumulations were slushy and wet, up to around a half inch in places.

The Puget Sound Convergence zone is often an overlooked event in the forecast because so few explain or realize that it can be a temperature difference of 10-12°F within a 5 mile north or south direction.

Today was a perfect example of that. I drove from West Seattle at 3:30pm in the afternoon and the temperature was 45°F. When I hit Northgate heading home towards Shoreline the temperature had dropped to 40°F. Once inside Shoreline it was 36°F and dropping.

This huge difference in temperature inside a convergence zone compared to everywhere else is what catches everyone by surprise. You can go from mostly sunny and 45°F and 10 minutes later be stuck in a winter wonderland. The reason for the huge temperature change? The atmosphere is still very cold not far from the surface, so any heavy precipitation brings the snow level down to the surface in a very short time.

The convergence zone forms when moist air moving from west to east is split around the Olympic Mountain range. This causes a mild low pressure zone east of the Olympic mountains and brings the air back together in the middle. The colliding winds downstream of the Olympics forces the air to rise up into colder parts of the atmosphere and condense, forming clouds and precipitation.

Typically this zone can occur anywhere between Mount Vernon and Tacoma. Sometimes it moves from south to north over the course of a few hours, or from north to south. Other times it can park in one spot for many hours, sometimes a full 12-24 hours.

A Puget Sound Convergence zone event parked itself, centered between Shoreline and South Everett, on April 18, 2008. Shoreline received several inches of snow when areas 20 miles north or south were dry and above freezing.

Another notable event occurred December 18, 1990, which left Shoreline buried in 8-12 inches of snow in just a few hours. Sea-Tac in that event only had 2.5 inches of snow, and Paine Field in Everett didn't record any snow accumulation.

Snow in Shoreline from a Puget Sound Convergence Zone April 18, 2008.
Photo by Carl Dinse

Forecast: Our chilly weather pattern is expected to continue for at least the next seven days. Rain or snow showers are expected to continue Sunday evening into Monday, ending by mid-Monday morning.

Tuesday - we are looking at breezy conditions with snow likely in the morning, then changing over to rain showers in the afternoon. Wednesday through Thursday night we are back into rain, then rain showers with temperatures in the 30's and low 40's.

Friday-Saturday snow returns to the forecast, mixed with rain mostly. Saturday night into Sunday is back to rain. Mid to upper 40's is even possible Sunday but this could change.

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