An Overview Of Seattle’s Crazy Snowstorm!

Before I explain Seattle’s crazy weather on Monday, I must apologize for not keeping you updated on the situation throughout the day and night! Since I’ve moved down to Portland, I’ve definitely been more out-of-touch with Seattle weather. Still, that’s no excuse for at least writing a post-storm blog about the insane and unexpected thundersnow that Seattle saw!

On Monday, two convergence zones formed over the Central Puget Sound area, one in the early morning and one in the afternoon. The morning convergence zone gave several inches of snow to the higher hills above 500 feet and a dusting of snow at my parents’ house in Madrona at only 45′ feet, very close to Lake Washington. But it was the afternoon convergence zone that will go down in history as one of the more impressive convergence zones of the 21st century, and the last thundersnow since 5 AM, December 18th, 2008. And before you start assuming that I’m some sort of weather savant, that was one of the most memorable weather events of my life… there’s no way I couldn’t know exactly when it happened!

The setup was classic for a convergence zone. We had a pool of cool, unstable air associated with a deep upper-level trough just off our coast, and a low-pressure system had just passed to our north. In its wake, cold NW winds off the coast split around Cape Flattery and converged again over the Seattle area, creating clouds and precipitation. Because snow levels were already very low and the precipitation was intense, snow fell all the way down to sea level, coating the area with a beautiful but quite disruptive and slippery layer of snow.

The picture below shows the surface winds at the time of the stronger convergence zone in the afternoon. You can clearly see westerly surface winds hitting the coast, traveling through the Strait of Juan de Fuca and Chehalis Gap, and meeting in north-central Puget Sound over Snohomish county, creating an area of relatively light winds.

As the 3-hour snow accumulation ending at 4 pm shows, the convergence zone was located at the southern boundary of this area of light winds. The modeled zone was only slightly north of the real zone the timing was spot-on. This was an excellent forecast by the WRF.

Here are some radar images showing the evolution of the convergence zones. I had to split the images up into two for size purposes, and unfortunately much of the data later Monday evening has errors and cannot be displayed. Still, this does a good job showing both the morning convergence zone and the stronger one in the afternoon that brought thundersnow. In the evening, yet another, third zone formed, though it was much weaker than the previous two.

One of the more amazing things about these convergence zones was how dramatically they cooled temperatures via a phenomenon known as dynamic cooling. When snow melts to rain, it takes heat energy out of the atmosphere because it takes energy to make something change phase from solid to liquid. Looking at the chart of observations from the UW Atmospheric Sciences Building below, you can see the dip in temperature at 00Z (4 pm our time) as the convergence zone came through. Since temperatures decreased so rapidly with height, this precipitation easily fell as snow even though temperatures were in the mid 30s at UW.

This large decrease with temperature in height also made air that was forced to rise via convergence very buoyant, causing it to rise freely via a process known as convection. This convection was intense enough to give us some incredible thundersnow that will be long remembered.

I’ll leave you with one of the coolest weather videos you’ll find ANYWHERE – a video of lightning striking the Space Needle as the afternoon convergence zone swung through.

Oh, and one more thing. Some models are hinting at a chance of snow Sunday and Monday. If anything occurs, it should be minor. But we can always fantasize about a March snow day!

Have a nice evening,
Charlie