Convergence Zone Snow in April!

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Bellevue, WA snow around 8 PM at 200 ft ASL. Picture courtesy of Julia Ziobro

Today we had a strong convergence zone and with cool air aloft the result was that in the heavier convergence showers the snow level was driven to the surface over some of the lowlands.

This morning dawned with a spectacular convergence zone over the north Sound.

Just beautiful. And there was also snow showers in the mountains, some quite heavy at the same time (over a foot today in the passes!)

The convergence zone moved south during the day, producing snow showers during the last afternoon in Brier and Lynnwood, and subsequently farther south on the eastside.

Here is a nice video of the snow action in Bellevue:

And with the convergence zone there was an extra bonus of lightning and thunder. My bike ride home tonight around 7 PM was enhanced by such atmospheric pyrotechnics in the vicinity.

Why does a convergence zone help produce snow?

When temperatures are close to being cold enough for snow, but not quite there, the precipitation from the convergence zone can start as rain and then turn to snow or mixed snow/rain. When precipitation starts there is first strong evaporation, that rapidly cools the air to the wet bulb temperatures. The air rapidly humidifies and after 15 minutes to a half hour the air is saturated and evaporation doesn't do much anymore. But the atmosphere has another cooling trick!---melting. The snow aloft melts as it falls into above-freezing air and it takes energy to do that air cools further...sometimes enough to bring the snow level down to the surface. This happened today.

Want to see this in action? Here are the weather observations on top of the roof of my building. At roughly 5 PM (00z, time in red), you will see the precipitation started, the temperature fell, and the relative humidity rose (click to expand!). The initial evaporative cooling was very intense and short-lived. Then as the precipitation continued melting continued the cooling. Classic traces.

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