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 answer was that in the heavier convergence showers the snow level was determined to the surface over some of the lowlands.
This morning dawned with a spectacular convergence zone over the north Sound.
Just beautiful.nd there was also snow showers in the mountains, some quite hard at the same time (over a foot today in the passes!)
The convergence zone moved south during the day, producing snow showers during the final afternoon in Brier and Lynnwood, and subsequently farther south on the eastside.
Here is a nice picture of the snow action in Bellevue:
And with the convergence zone there was an additional 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 near to being cold enough for snow, but not quite there, the precipitation from the convergence zone can get as rain and so turning to blow or mixed snow/rain.When precipitation starts there is first strong evaporation, that quickly 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 air has another cooling trick!melting.The snow aloft melts as it falls into above-freezing air and it takes energy to do that.so that air cools further.sometimes enough to get 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 ceiling of my building.At about 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 vivid and short-lived.Then as the precipitation continued melting continued the cooling.Classic traces.
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