The Convergence Zone that Keeps on Giving

Monday, November 14, 2011
6:12 P.M.

Weather Radar from the NWS at 9:06 A.M. PST
The Puget Sound area commonly gets strong convergence zones in the winter, but I have never seen one as persistent as the one that happened today. This convergence zone brought tons of snow to the central slopes of the Cascades, and eastbound Snoqualmie Pass was even closed at one point. Meanwhile, places like Crystal Mountain, outside of the convergence zone, received very little snow today.
Look at some of these tables for the weather today as recorded at Crystal Mountain and Alpental, near Snoqualmie Pass.  Snow levels were below pass level all day today, so to estimate the amount of snow recorded, I took the water equivalent and multiplied it by 10, since one inch of rain is approximately equal to one inch of snow most of the time.

11-14-2011
Northwest Weather and Avalanche Center
Crystal Mountain Ski Area, Washington

Total snow may read bushes

MM/DD Hour Temp Temp RH RH Wind Wind Wind Wind Hour Total 24 Hr Total
PST F F % % Min Avg Max Dir Prec. Prec. Snow Snow
6830' 4570' 6830' 4570' 6830' 6830' 6830' 6830' 4570' 4570' 4570' 4570'
------------------------------------------------------------------------------------
11 13 1900 24 30 99 98 6 15 39 248 0 0 5 14
11 13 2000 25 30 100 99 8 21 40 285 0 0 5 14
11 13 2100 25 30 99 99 6 17 33 173 0 0 6 14
11 13 2200 24 30 99 99 7 18 42 294 0 0 6 13
11 13 2300 25 31 99 99 7 19 44 306 0 0 6 14
11 14 0 23 31 99 99 11 23 46 319 .01 .01 6 13
11 14 100 22 30 99 97 8 22 42 346 0 .01 6 14
11 14 200 23 31 99 89 11 29 49 318 0 .01 6 13
11 14 300 23 31 99 88 11 30 54 339 0 .01 6 13
11 14 400 22 30 99 95 11 26 54 316 0 .01 6 13
11 14 500 23 30 99 96 9 22 37 265 .01 .02 6 14
11 14 600 22 30 99 96 9 23 49 298 .02 .04 6 14
11 14 700 22 30 99 96 15 30 50 292 .01 .05 6 14
11 14 800 22 30 99 95 11 29 54 140 .02 .07 6 14
11 14 900 22 30 99 93 13 29 52 315 0 .07 6 14
11 14 1000 23 31 99 88 13 28 46 249 .01 .08 6 14
11 14 1100 23 30 99 90 16 31 44 258 .01 .09 6 15
11 14 1200 21 29 98 86 16 30 47 266 0 .09 6 14
11 14 1300 21 30 98 82 12 29 54 263 0 .09 6 14
11 14 1400 20 29 98 85 10 27 52 47 .01 .1 6 14
20 98 2 19 39 296




Total
Prec.
4570'
------------------------------------------------------------------------
.1 
.1 inches of precipitation is not that impressive. The winds, on the other hand, are pretty strong! Mission Ridge, on the eastern side of the Cascades, had a wind gust to 96 miles per hour yesterday. Cliff Mass did a fabulous blog post on these strong winds, and you can find this post here.
Snoqualmie Pass, in the convergence zone, received much more snow. The gauge at the pass was malfunctioning, so I used the gauge from Alpental, which is a ski resort adjacent to the pass.

11-14-2011
Northwest Weather and Avalanche Center
Alpental Ski Area, Washington

Wind gages unheated and may rime
24 hr snow gage not yet iunstalled
Total snow may read bushes

MM/DD Hour Temp Temp Temp RH RH Wind Wind Wind Hour Total 24 Hr Total
PST F F F % % Avg Max Dir. Prec. Prec. Snow Snow
5470' 4350' 3100' 3100' 5470' 5520' 5520' 5520' 3100' 3100' 3100' 3100'
------------------------------------------------------------------------------------
11 13 1900 27 31 34 98 98 0 0 288 .03 .03 2 19
11 13 2000 27 31 34 99 99 0 0 287 .02 .05 2 20
11 13 2100 27 31 34 99 99 0 0 287 .04 .09 2 20
11 13 2200 27 31 35 97 98 0 0 287 0 .09 2 18
11 13 2300 27 31 33 99 99 0 0 287 .06 .15 2 19
11 14 0 27 30 34 98 99 0 0 287 .01 .16 2 19
11 14 100 27 31 34 98 99 0 0 287 .01 .17 2 19
11 14 200 27 30 33 98 99 0 0 287 .04 .21 2 20
11 14 300 26 30 33 100 99 0 0 287 .08 .29 2 20
11 14 400 26 30 34 96 99 0 0 287 .02 .31 2 20
11 14 500 26 29 33 99 99 0 0 287 .07 .38 3 20
11 14 600 25 29 33 99 99 0 0 287 .06 .44 3 19
11 14 700 26 29 32 100 98 0 0 285 .1 .54 3 17
11 14 800 25 29 32 100 99 0 0 283 .15 .69 0 14
11 14 900 24 28 32 99 97 0 0 285 .15 .84 1 19
11 14 1000 22 26 30 99 98 0 0 287 .1 .94 2 19
11 14 1100 22 26 30 99 97 0 0 290 .07 1.01 3 20
11 14 1200 22 26 32 99 98 0 0 287 .05 1.06 4 19
11 14 1300 22 26 31 98 97 0 0 287 .02 1.08 4 -25
11 14 1400 22 26 31 98 97 0 0 287 .03 1.11 5 21




Total
Prec.
3100'
------------------------------------------------------------------------
1.11

1.11 inches of precipitation! That is astonishing, even for a strong convergence zone. The temperatures for the most part were slightly above, at, or slightly below 32 degrees, so I have a feeling the snow here would be wetter and have more of a 1:8 inch ratio of water to snow. Even though the temperatures were above freezing for a while, it was likely still snowing at the pass, as snow will often continue to fall 1,000 feet below the freezing level, especially when the atmosphere is unstable and the temperature varies rapidly with height, as it has over the past day.
The main convergence zone action occurred from around 6 A.M. this morning to 2 P.M. Believe it or not, a convergence zone is still going on and is located slightly south of Snoqualmie Pass. Take a look at the radar loop from 6 A.M. to 2 P.M. today, and notice the persistent convergence zone action over Snoqualmie Pass.
The Summit at Snoqualmie, which is typically one of the last places to open for the ski season due to its low elevation, may end up being one of the first areas to open because of this convergence zone dump. In addition, we will likely see plenty more convergence zones in the following week as we will be under a moist, cool northwesterly flow that is conductive to these zones. The snow at Snoqualmie will continue to pile up.
Many people have asked me about snow in the lowlands this coming weekend, and although the mountains will get heavy snow down to low elevations, I doubt we will see snow to the surface. We will not see enough arctic air in our area to see snow in the lowlands. This could change though, and if it does, I’ll let you know!
Back to homework. Thanks for reading!
Charlie
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4 thoughts on “The Convergence Zone that Keeps on Giving

  1. Yo Charlie! Your blog posts are quite enjoyable to read. Keep up the good work! But yeah, this next wed and beyond looks a little interesting. First off the gradient looks pretty impressive with the cold front that passes. Looks to be at least wind advisory level with the new GFS pass. Strong storm indeed. What will be even more interesting is how much cold air that low that drops from the golf of AK afterwards. Latest run of the GFS was pretty cold. But the NAM was pretty warm. I bet we see pretty cold temps with possible snow/rain showers. But prob not a big snow event of any means.

    Like

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