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Friday, March 28, 2008

Weird ass weather!

Okay. See the pretty cherry blossoms across the street? They're blooming cuz it is spring. So why the heck is it snowing!?

KOMO news weatherman, Scott Sistek, breaks down the science of why/how it snows at sea level this time of year:


So I've just finished writing about how snow levels are generally 500-1,000 feet. So then, how is it possible to still snow near sea level?

If the intensity of a passing shower is strong enough, it can artificially lower the snow level a few hundred feet through a process called evaporative cooling.

When snow first begins to fall into drier air, that snow will evaporate. But the process of evaporation takes energy. Sapping energy from the air causes it to cool, dropping its temperature. So as this process churns away, the temperature will drop, but the humidity will rise as you add more evaporated moisture to the air content. The drier the air is at the beginning, the farther the temperature can fall during this process. (This is also known as "wet bulb cooling".)

But wet bulb cooling needs some help, and that brings up the second factor: precipitation intensity. We need to have a decent amount of oompf in the precipitation to get that cooling engine going. If it's a really light snow or flurries, it won't evaporate as much or as quickly, which means this cooling engine won't be very efficient and the temperature may not drop as much.

On the other hand, if you get a really heavy snow shower, that can really get that evaporation going and drop the temperature quite a bit. This is typically how snows in the Convergence Zone work when it snows in Everett/Lynnwood despite being in the low 40s across the rest of the region. The snow along the Hood Canal usually benefits from this as well.

That's what we're looking at in these snow scenarios the rest of this week. The one thing about this process is once the precipitation ends and the cooling mechanism goes away, the snow level usually quickly rises back to where it was before, and the temperature near the ground will rise.

Thus, it's not uncommon for the fresh-fallen snow to quickly begin to melt. And that's what we expect with these snow scenarios. Any snow that does fall should begin to melt as soon as the snow stops, as the general ambient temperature is expected to remain above freezing. The exception to this is area above the 500-700-foot snow level at night, where snow could stick around a little longer."

BTW, KOMO is my favorite local news for weather. The KING 5 guy just rubs me the wrong way, and national weather/or online weather places never seem to get the nuances of our weird weather climate down...


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