11/28/2014

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Say that again? Snow fans have to root for dry air, heavy rain

Say that again? Snow fans have to root for dry air, heavy rain
Snow covers the ground on Dec. 10 on Puyallup's South Hill. (Photo by Eric Heintz)
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As you might have heard, we have a rather complex snow forecast coming up for Friday morning. It's a battle between having enough cold air in place for the rain begin as snow and how quick warm, southerly winds will scour out the cold air and change it over to rain.

Here is what is for sure: A warm front moving in from the northwest will spread precipitation to the coast around midnight and push that into the I-5 corridor from northwest to southeast during the predawn hours.

The computer forecast models are still indicating that the air is just cold and dry enough that it will begin as snow in the hours after midnight, and that a warm southerly wind will eventually push temperatures into the 40s and make for a rainy and breezy afternoon.

But if you want to get those 1-4 inches of snow that the models are predicting before the changeover, you have to root for two things:

Dry air, and heavy rain.

Huh? Sounds counter productive but in reality, that is the last hope for snow fans to get at least a few hours of snow before the rain arrives.

Here is why: Dry air actually will help cool temperatures when the precipitation arrives due to a process called evaporative cooling. Since dry air has plenty of room to hold the moisture, the rain or snow will evaporate into water vapor. However, evaporating water requires heat and energy. As more and more drops evaporate, the air is using up more of its heat and energy, thus making the air cooler.

You can get a sense if this is possible by looking at the current temperature and dew point. (Dew points are the temperature at which the air would be saturated. see more about the dew point here. You can also find the current temperature and dew point at this link.)

You can get a rough idea of the atmosphere is ready for this by taking the mid point of the dew point and temperature, and if it's around 32 degrees or less, then the precipitation could begin as snow. For example, as of Thursday afternoon in Seattle, the temperature was 38 -- seems too warm to snow, right? -- but the dew point was 24. Thus, when the precipitation arrives, the temperature would drop and the dew point would rise (as the air gains more moisture) but they usually meet somewhere around the middle -- in this case, it would probably be snowing and 31 or so at the start.

As for heavy rain? Or at least, heavy precipitation? The greater the precipitation intensity, the more efficient this evaporative cooling process can become and the better the chance of getting cooler air and snow. The process can drop snow levels a good 500 feet, bringing a temporary snow to a spot that was below the snow level before the event began, or create a more intense snow where it was already cold enough. This is a frequent occurrence in the Puget Sound Convergence Zone where that area around the King-Snohomish County line can get a snow on a day when everyone else is getting light rain under a 700-1,200-foot snow level.

So snow fans have to bank their hopes on strong evaporative cooling because there will be many factors working against it such as an increasingly warm wind out of the south and an air mass that, overall, really isn't that super cold to begin with. We usually like to see 850mb temps at least -8, better -10 and they're around -6, and atmosphere thickness readings of 522 or lower and they're around 525-530.

But the modes are still convinced there will be anywhere from a Trace to as much as 4 inches of snow during this process before the rain takes over and dew points are certainly low enough now to suggest evaporative cooling will be a factor. Main question remains how long it can go.

So tell your snow-fan friends to root for dry air and a heavy rain at the start tonight and if we get enough of both, perhaps we'll be able to squeeze out enough snow for a snowman.

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