January 04, 2009

Diabatic snow

Meteorologists have a term for it..."diabatic" effects. Two major examples of this are evaporation and melting...and both can cause cooling. And both may be important later today.
A strong Pacific system is coming onshore this afternoon and evening and during the afternoon precipitation will begin over the region (see satellite picture). You will see that this is NOT a pineapple express type event where the moisture extends into the tropics. We will get wet, but only for 6-12hr this evening and tomorrow am.
The air mass over us is cool and relatively dry, but not quite cool enough for snow as is. But that could and will change in some areas due to the "diabatics." Precipitation nearly always starts as snow aloft around here...even during rain events. Consider what happens when precipitation falls into above-freezing air below. If the air is not saturated then there is evaporation, which is a very powerful and rapid cooling agent. You know how powerful it is when you step out of the shower or get wet outside. But evaporation only lasts a few minutes to a half hour..until the air saturates. The temperature that air cools to when it falls into dry air is called the "wet bulb" temperature...you guessed it...we measure it by putting a wet cloth on the bulb of a thermometer and see what the reading is.
Now if that doesn't cool the air enough to reach the surface then melting can do the job. Snow falling into above-freezing air melts...and that takes energy out of the air...and thus cools it. Melting can be VERY effective when precipitation rates are large..and can drive the snow level to the ground. Melting is often associated with Puget Sound convergence zone snows--with snow where the precip is heavy and rain elsewhere.
Well, now you are diabatic, savvy what about today? Our air is marginal for snow, but is unsaturated. There will be moderate precipitation rates associated with the incoming system, which eventually will cause substantial warming aloft. Later tonight it WILL rain. But there could be 1-3 hr at the initial portion of the event when there could well be snow...and in fact the computer models are suggesting it. (see graphic). Greater threat will be the Kitsap area, SW interior and high elevations...but everyone will probably see at least some wet snow. Right now 4-7 PM is the best bet for the white fun. But no plows probably needed for this...warming and rain will follow (but be ready for a wintry mix for a few hours if you live on a hill)....cliff

33 comments:

  1. Thank you for the lesson Professor Mass, I certainly enjoyed it.

    Going to read your book for the second time tonight.

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  2. Cliff,
    On 1/02/08 you included a clickable link for a weather forcast survey. In the future would you please highlight web sites that you suggest for further information. In the past I have not been able to access that you have suggested. Thanks

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  3. Cliff,

    You said:

    The temperature that air cools to when it falls into dry air is called the "wet bulb" temperatures

    Did you mean that water cools to?

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  4. I love to learn more stuff like this, and it's one of my favorite parts of this blog. Thanks!

    Grunertom, you can always copy and paste un-clickable links into a new browser tab's address bar.

    Sailorbyfate, my understanding of wet bulb temp is that it is an air temp reading, not a water temp reading. But the temp of the cloth on the bulb of the thermometer will be by definition the same temp as the surrounding air. :-)

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  5. I fixed a few typos...sorry...had to run to give a lecture this morning...cliff

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  6. It just started snowing in the Haller Lake area of Seattle. Just as you said!

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  7. I'm seeing snow flakes here in west seattle. Yep, just as you said.

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  8. Snowing hard in West Olympia at 3:34..I can enjoy this kind of snow because it isn't sticking!

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  9. It's snowing on Capitol Hill now too. Your predictions are awesome!

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  10. [Jamie from Bainbridge here. How do I retrieve a forgotten password?!]

    You called it, Cliff!!! Started snowing on Bainbridge Island at about 3:15 this afternoon! Please guarantee me the snow over the pass and just east of Snoqualmie will stop during the night sometime, as hubby is planning to drive over there Monday morning and needs good weather when he gets to Richland!

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  11. Snowing fairly heavily in Seabeck near Hood Canal. It's beginning to stick too. Breezy as well.

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  12. Breezy here east of North Bend, but no snow yet at 3:42PM

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  13. OK -- I lied. Left the last post, looked up and sure enough it is snowing very lightly here east of North Bend.

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  14. Cliff, if you're here: Do you think there will be a good chance of having avalanche danger increase over Snoqualmie, and thereby needing to close the pass for awhile Monday morning?

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  15. That made my brain hurt a little.

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  16. Snowing on Bainbridge for a couple of hours now -- fairly heavy now and definitely sticking. Ground and trees covered.

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  17. Snowing in portland oregon - started about 4:30 - sticking because the ground is still very cold.
    I do hope it turns to rain in the night.

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  18. 5:18pm and steady snow on Bainbridge... sticking too.

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  19. one half inch accumulation at NE 78th St and I-5 in Seattle. Moderate to heavy with smaller flakes, things looking very white.

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  20. so much for that warmup! Almost six pm and just started snowing about a half hour ago on Capitol Hill.

    wish I had read of the change earlier today.......

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  21. We have at least 2 inches on Bainbridge at 6:15pm. Please tell me it will all melt by 4am, so school starts back up Monday?!?! :)

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  22. Started snowing in Union Mills about 2:30 this afternoon- temperatures hovering around freezing, wet snow that turns to slush when driven on and ice underfoot. Anout a half to three quarters of an inch accumulation, and 31.7 F.

    Ugh.

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  23. Cliff, you nailed it. Here on Kitsap, I left Kingston and drove to Silverdale at 3:30, and it started snowing lightly on the way.

    By the time I left around 4:45, it was snowing heavily, and accumulating quickly. The roads were nasty all the way back to Kingston, which was a slippery drive at 20-25 mph with some skidding.

    Now, a little after 6, it is snowing quite heavily, with 2" accumulation on my front porch. It's a wet snow, which made the driving particularly fun!

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  24. Well, it started snowing at 4:05pm in Sammamish, so Cliff, your prediction was a bit too early ;-).

    Sticking on the roads here, and slick as slug's snot (and yes, that is SLICK!). People are having problems getting up our hill right now.

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  25. 1.5 inches in Magnolia at 6:35 pm. Hanging in there at 32.5 degrees or so. Any chance we're not going to see a warming prior to midnight?

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  26. We just had a wild drive from the Kingston Ferry to Port Townsend. A lot of snow sticking to the road. What is really wild is the wind! Branches on the road and the snow blowing in circles. By the time we made it to Port Townsend at 5:30 it was still snowing but wet roads..... the wind is still wild.

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  27. Snow sticking to everything, about 1/2 inch.

    Location: In between Redmond and Woodinville

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  28. Downtown Issaquah - 6.50pm - 1" of snow and it's settling in the garden.

    We're having bets on whether this is another "snow day" for Issaquah..

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  29. Still snowing hard here in the Central District at 7pm.

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  30. Heavy steady snow in Fairwood area (toward Maple Valley) for the past 2 hours, accumulating nicely.

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  31. 7 p.m. Bainbridge, our snow's starting to fall off the tree branches. Not much of anything, rain or snow, falling.

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  32. It's been snowing since 5pm in the Central District. We've got about 3 inches on the ground now. I hope it clears up by morning, there are a lot of steep hills in the neighborhood. Great explanation of what's going on to cause this!

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  33. It is no longer snowing in Shoreline. I think I just saw the sky flash a few times with lightning.

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