What a Convergence Zone!

Monday, November 28, 2011
4:22 P.M.

05:31 pm PST Sun 27 Nov 2011
Sorry for the lack of posts over the Thanksgiving weekend, I was at home and the internet there is dreadfully slow… so slow that it is impractical to use. And forget about using blogger…
Last night, we saw an incredibly well-defined convergence zone sweep through the region. Cliff Mass talked about this in the weather class I have with him, and he mentioned that one place north of Seattle recorded an inch of rain in an hour. That’s an incredible amount of rain. The last time I witnessed something like that was on December 14, 2006, when a very strong squall dumped an inch of rain in an hour over Seattle.
The video below shows the rainfall in Seattle that night. As you can see, rainfall rates of an inch per hour are incredibly heavy, and are rare in the Seattle area.
You don’t want to be biking through that!
This convergence zone could have been thought of as a “grand finale” for active weather in the Seattle area. November is coming to a close, and the last week of November, which is climatologically the stormiest time of the year for our area, is nearly past us. The next 10 days will be incredibly boring, as a HUGE ridge of high pressure parks just off our coast and stays there.
The ridge is already building right now, but after a weak system rolls through on Tuesday, this ridge will be in full-force.
Look at the size of this ridge by Wednesday afternoon.
Valid 07:00 pm PST Wed, 30 Nov 2011 – 63hr Fcst – UW 12z 36km WRF-GFS 1000-500mb thickness, SLP
This model shows the “thickness” of the atmosphere between the 1000 and 500 millibar levels in decameters. The thickness can be thought of as the difference in height between the two levels. This big ridge of high pressure warms the air aloft, making it less dense. Since the air is less dense, there is a greater distance between the 1000 and 500mb levels of the atmosphere, and hence, a greater 1000-500mb thickness. At the surface, the ridge is expected to be around 1040 millibars, which is disturbingly high pressure. I say “disturbingly” because pressure that high generally means that this ridge ain’t going anywhere for awhile. Unfortunately for weather nuts like me, that looks to be the case.
Valid 01:00 pm PST Sun, 04 Dec 2011 – 153hr Fcst – UW 12z 36km WRF-GFS 1000-500mb thickness, SLP
Later this week, the ridge will grow even larger, pushing the jet stream up to the arctic circle. This spells atrociously boring weather for our region. Get ready for inversions, deteriorating air quality, and pretty bad sailing
Stay dry out there! It shouldn’t be too hard…
Charlie
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