Saturday, December 27, 2008

Transition Day

The temperatures in nearly all of western Washington are now in the 40s and warmer southwesterly flow has removed the persistent cold air. Take a look at the Seattle profiler winds and temperatures for the last day. 24h ago there was weak SE low-level winds and temps around 0C (32F) in the lower atmosphere...now there is fairly strong SW flow and temps of 6-7C. The melt-out has begun in earnest. Many primary and secondary roads are passable, with a little slush in places.

A cold front is now making landfall on our coast...you can see that by the switch to NW flow on the coast (see map). General precipitation will drop back in the lowlands...although it looks like a Puget Sound convergence zone will form north of Seattle this afternoon (that usually occurs with NW winds on the coast). However, this is an ideal pattern to get big snow in the mountains. In fact, the heaviest snows often fall there after cold front passage when winds are more westerly. Why? Winds from the west have a stronger wind component up the mountains then prefrontal winds from the south. Also the air behind our cold fronts are cold and unstable...with lots of cumulus-type clouds. Such instability clouds really blossom as they are forced to rise by terrain.
The next several days promises one wet system after another and more seasonal temperatures. The media has been pushing the flooding issue, but I don't think that lowland flooding will be a serious threat this time...assuming everyone clears their drains. Behind the cold front, temperatures will fall and the melt off will be much slower than in 1996..the last major melt-off flood event.

21 comments:

Mike in MLT said...

For the first day I had to engage "four wheel drive low" to get out of my long driveway, down the street, and on to the arterial. The compact snow and ice on our street is now like cream cheese. Bare and wet and back in two wheel drive after those first hundred yards.

I've got a blue tarp over the kid's igloos. But I suppose I'll have to cave them in for safety in a couple of days.

I'm happy to say my wife scored a copy of Cliff's book. There are a few books that are a nice reference to have around--Cascadia is the geology book I keep. My father especially will enjoy his copy of the weather book.

Julia said...

The low on my Max/Min thermometer is the same as the current temperature when I posted yesterday: 32F. The high is the current 38.5F; that's pretty much how it's gone; it was 34.6F at noon yesterday, and 36.1F at 10pm last night.

Unfortunately, the effect of temperatures above freezing and below 40F, abetted by some rain and SW winds on snow and ice on north-facing or shaded surfaces is wet zero-friction ice. This is the point where people get wrapped around power poles, or injured falling on unshovelled sidewalks.

Anonymous said...

Hi Cliff, I'm a newbie when it comes to reading your blog and understanding the PNW weather so I must ask what is the significance of the first figure, the profiler, and how do I interpret the figure? Great blog and thanks for the information.

Weatherfreak said...

Snow going fast in SE Auburn with temp 43 and strong southerlies gusting to 26mph. We are a snow loving family but my son said yesterday he wants it all to melt so he can ride his new bike. I agree, green is much more useful than slush! ;)

Anonymous said...

I'm with you, anonymous. I was going to ask, Cliff, does your book lend clues about reading some of the charts you post (such as the first one on this blog entry).

nocurling said...

i cannot believe how much snow has melted overnight and this morning. benches that had over a foot of snow are now bare.

light rain and 42* @ 200' in east olympia.

Matt said...

40 degrees at 300' elevation near Indianola. Everything is melting quite quickly; big patches of the lawn that were buried in several inches of slush yesterday afternoon are now clear, and our side street, which was a deep sea of muck, is now bare and wet.

The temperature did indeed climb steadily throughout the evening. From 34 to 38 between 4 pm and around 11 pm last night, and the wind howled throughout the night, to the point that I feared power outages were imminent, but the trees held, and outage happened.

Let it thaw, let it thaw, let it thaw!

Matt said...

And it looks like Cliff was right about not much risk of flooding, as the NWS has canceled their flood watch for most of Western Washington:

http://seattletimes.nwsource.com/html/localnews/2008564681_webwinter27m.html

camco said...

Did I read somewhere (here?) that in 1880 Seattle recorded 70 inches of snow in 8 days?

JewelyaZ said...

Cliff and everyone else frustrated by the Seattle response to snow removal, you'll be amused by this if you haven't seen it already:
http://seattletimes.nwsource.com/html/ronjudd/2008558465_trailmix25.html

Here in East Bellevue, 210' elevation, it's a balmy 45.3F and very windy. Melting is going amazingly fast.

The City of Bellevue plowed our street this morning. Excellent. Except a week late. And thanks a ton for making a three-foot berm of slushy snow at the end of my driveway, so that I can't get my car off the street for the garbage guys on Monday... well, I'm going to go shovel so that they can... on the off-chance that it doesn't all melt by then! At least thanks to these temps I can go shovel the crud in my sweatshirt and sneakers.

Anonymous said...

east woodinville elev 500' - picnic table measuring stick shows snow pack has shrunken from a high of 25" to 16", so we're on our way down. My husband got cocky and ventured forth in the 1945 ford 2n tractor; the front wheel literally broke off, of course blocking two other vehicles in the driveway.... That makes one broken wind shield wiper blade, door handle on car broke, two collapsed costco tents, frozen shower pipe, frozen laundry room pipe, and I fear the gutters are getting pried and deformed.... All kinds of fun things to fix.

Julia said...

Union Mills:
The thermometer (probe in the coldest most sheltered point possible) passed 40F about 1:20PST. Still a lot of snow laying about, but under the apple trees there are growing patches of exposed earth.

JewelyaZ said...

Cliff,

Can you please teach us to read the Seattle profiler grid that you post sometimes? Here's what I've figured out.
Time runs most current (left) to oldest (right).
Elevation? runs bottom of the chart to the top.
Wind ranges from 10 knots (1 line) to 60 knots (triangle plus 1 line).
Wind line points in the direction of the wind.
But what's the red stuff? And what does it really tell us?

I would love to understand that chart better. Thanks! :-)

evilzenscientist said...

Cliff - as predicted the warmer weather rolled in - it's now hovering around 8'C/46'F in downtown Issaquah and the thaw is almost complete.

Anonymous said...

For camco (and other history buffs wondering about record snowfalls), click on http://historylink.org/index.cfm?DisplayPage=output.cfm&file_id=2674

If you haven't heard of historylink.org, take the time to wander. It's a great resource for Seattle, King County, and Washington state history.

emma

allatham said...

At 500' elev. 8 miles NW of Hood Canal Bridge we still have 7" snow on the ground. 40F but not melting very fast - sure could use some rain! Road about impassable with snow not quite slush.

Anonymous said...

If you planning on building any new outbuilding structures north into CZ country: STEEP PITCH!

Anonymous said...

If we're going to have cooler weather with slow melt-off coupled with a fair amount of rain, does that mean my home's peaked roof is at risk? Water dripped off of it all day, but it is still covered with snow. We made it up to 39F today, but are now at 36F. So - again, will the snow on my roof keep melting now in this scenario or just get heavier with rain at these almost freezing temps? FYI, our picnic table measured 23" yesterday before the rains began, to give you an idea of how much snow we've had accumulate on outdoor surfaces.

Kevin Purcell said...

JewelyaZ details most of the important items in a profiler chart: time runs from the most recent on the left with date/UTC time. The vertical axis is the height in meters (or pressure on some profilers charts ... it amounts to the same thing). The wind barbs show the wind speed and direction at a given height a given time. The contours are isotherms (lines of constant temperature) in Celcuis.

So with the profiler data you can see the air temperature (and wind velocity) at a given height above a set geographical location at a over time.

The point Cliff is making is you can see the air temperature increase at the surface and above to well above freezing starting on 26th (with the surface temp being 2C or so with 0C air at 300m or so. The rapid warming near the surface starts at 27/01 (i.e. Dec 27th 0100 UTC or Dec 26th 5pm PST) and proceeds rapidly after that. At the most recent point on that chart 27/16 (i.e. Dec 27th 1600 UTC or Dec 27th 8am PST) the air temp at the surface is 8C with temp > 7C below 600m. Much much warmer air than we've had and good evidence the cold air has been scoured out above Sandpoint (in Seattle where this profiler is located).

Another thing to think about when looking at profiler data is that snow can fall to about 1000 feet (300m ish) below the 0C line so you might be interested in the temp at 300m to see if you are going to get snow or not.

Go back to previous profiler posts and you'll see my comments (e.g. comparing observations to the forecast for the warm air coming in yesterday a few hours earlier than forecast by MM5).

Anonymous said...

Anyone who has been up to the ski areas have a report on conditions by any chance?

Anonymous said...

Thanks for the info on how to use a profiler Kevin. One more question though. What does each barb indicate concerning wind speed? 5/10/20 mph/meters per sec? Thank you.