What was with the massive snowflakes this afternoon around Seattle?
The serene scene of snow falling in Seattle turned more into a surreal scene Monday afternoon as parts of the city transformed into a virtual snow globe, complete with massive snowflakes that filled the sky with a dizzying spectacle of frozen crystals that would challenge the most celebratory of ticker tape parades in Manhattan.
As we all rushed outside our office to take pictures, we noticed was falling wasn't just really large snowflakes, but instead they were several snowflakes that had stuck together into random patterns. How did that happen?
The massive snowflake clusters are caused when temperatures are right near freezing. As the snow falls from the sky, but then encounters 33 or 34 degree weather, the snowflakes begin their melting process. As they do so, they become coated with a thin layer of water which in this case acts a bit like glue. In the chaos of falling snowflakes, instead of them just harmlessly bouncing off each other, they begin to stick to each other.
If the wind is nearly calm, as it was here, it won't blow apart the clusters, leaving you with "snowflakes" that appeared to be as large as 1-2 inches in rough diameter.
So when you see the big flakes, it's a sign that your snow is just on the fringe of being rain instead. In this particular case, the big flakes indicated we we were warming up, but snow shower passed before it could finish the change into rain.
But then what was up with the rapid melt?
Almost just as surreal, in the moments after the shower passed, our area right by the Space Needle underwent a rapid snow melt. Just as more snow began sticking to the trees and cars, moments later it was all dripping in snowmelt as lumps of dying snow succumbed to gravity and splashed on the ground. Some 15-20 minutes after it looked like a blizzard, most of the new snow had melted away.
That’s because another process had just ended: Evaporative cooling. As the region was warming with the day's heat, the persistent snow band over Seattle was countering that warming process, holding the temperature right at 32-33 degrees. The process of snowfall cools the atmosphere and the greater the intensity of the shower, the more effective tis process is. As the snow falls out of the cloud into the sky, some of that precipitation will evaporate. But the process of evaporation takes energy from the surrounding air, creating a cooling process.
This is how some areas started earlier with their sticking snow Sunday -- they were in the sweet spot of the approaching trough that had more intense precipitation rates, which knocked those areas into freezing sooner (their elevation also helped quite a bit), allowing snow to pile up longer to the tune of 8-15". It took Seattle until well into early Monday morning to get cold enough and get a "snow" intense enough to where snow levels made it to near sea level.
In Monday afternoon's case, the region's air mass had generally warmed to about 35 degrees but the localized snowing process was pushing the temperature down to 32, then drifting up to 33-34 during "big flake" time. Once the snow passed, the cooling engine went with it, the temperature bounced back up to 35-36, and the snow underwent a quick melting process.