Convergence zones

Puget sounders, especially those who ski near Stevens and Snoqualmie Passes, have grown to love convergence zones. Convergence zones are a direct result of the local topography, namely the Strait of Juan de Fuca and the isolated Olympic Mountains. Two things are needed to form convergence zones: the atmosphere needs to be convectively unstable, i.e., once a parcel of air near the surface is given sufficient lift, either from topography or heating near the ground, it continues to rise on its own, usually releasing precipitation, just like a thunderstorm. The winds also need to be westerly or northwesterly so they interact with the topography. When the wind is westerly or northwesterly, it splits around the Olympics; a northern branch flows through the Strait and a southern branch south the of the mountains. They meet back up on the other side and can bring intense local snowfall to the Central Cascades. Think of a large boulder in a stream. It's the same idea.<br><br>Typically, after a cold front passes, both conditions are satisfied and convergence zones form. Their location is mostly dictated by the wind direction. With westerly winds they tend to form farther north, near Stevens, and with northwesterlies they tend to form farther south near Snoqualmie. As a cold front passes the wind typically shifts from southerly/southwesterly to westerly. Then, as the upper trough settles in, the wind typically begins to shift more towards northwesterly. A common scenario is convergence initially near Stevens Pass, then a shift southward toward Snoqualmie Pass a half day or so later as the winds shift. This has happened several times already this winter.<br><br>Last Thursday, Mar 4, two spectacular twin convergence zones formed bookending Seattle. It was sunny in the city, but looming in the southern sky were dark, ominous storm clouds. I think two convergence zones like this are rare, and I don't know exactly caused two of them to form. Perhaps someone out there with more meteorological knowledge can fill us in.<br><br>Convergence zones are easy to spot on the radar. Last Thursday, because the weather was clearing elsewhere, they were also obvious on the satellite. Here are snapshots of each taken at nearly the same time (sorry folks on dialup, the satellite file is large, but it's a pretty picture and worth the wait)<br><br> <br><br>

Cool stuff, Matt.<br>Thanks.<br>I'd never seen a double before.

Matt, thanks for the informative graphics. I remember when those double convergence zones were mentioned on the NWAC mountain weather forecast last week. You seem to know a bit about meteorology, so maybe I'll go ahead and ask you (and everyone else here) the following questions . . .<br><br>I've been thinking about convergence zones quite a bit lately, but on a slightly smaller scale than the Olympic or Vancouver Island convergences: specifically, about convergence zones downwind of large isolated stratovolcanic cones. Is there any info available on this subject at all? The idea first hit me during my snowpack-related research , when I found out that the three highest-snowdepth measurement sites in the entire Cascade Range (Jasper Pass, Cayuse/Chinook Pass, and Blue Lake) were each located 8-12 miles due east of a major stratovolcanic cone (Baker, Rainier, and St Helens, respectively). Could this be mere coincidence, or is there a plausible explanation? Small-scale convergence (not quite microscale, but at the very short end of mesoscale) seems to be the best candidate. Unfortunately, the bureaucrats at NRCS stopped collecting data at the Blue Lake site in 1978, otherwise a significant decrease in snowdepths at that site following the 1980 decapitation of Mt St Helens could have provided strong support for the convergence theory. <br><br>Looking at the NWAC telemetry during the past several years, I have often wondered why Chinook Pass typically has slightly greater snowdepths than Paradise, since they are at the same elevation and same latitude. Meanwhile, only 3 miles N of Chinook Pass, Crystal Mtn gets about 40-50% less snowdepth than Chinook, even higher up at 6300 ft in Green Valley. I had always attributed Crystal's lack of snow to a rain shadow from Rainier (and I've enjoyed the benefits of that rain shadow on many sunny ski days there while clouds covered everything else all around). But could the huge snowdepths at Chinook Pass be due to a convergence zone formed during major westerly storm cycles? Or is it all just because of local terrain and topography, unrelated to the looming presence of Rainier only 10 miles to the west?<br><br>Has any research been done on this subject? I was unable to find any relevant papers about stratovolcanoes and convergence zones during a quick search of the "Meteorological & Geoastrophysical Abstracts" database. Is there a better database for finding weather/climatology papers? Someone in UW Atmospheric Sciences must know the answers.<br><br>Amar<br>

That's an interesting observation and question, Amar. I haven't heard of any studies, but I don't study mesoscale meteorology (and moreover, don't have any formal training in the field either). I'm not sure what journals the database you were searching included, but I'd imagine papers on this topic would be published by the American Meteorology Society. They have a website, ams.allenpress.com/amsonline/?request=index-html with most of their journals online. UW subscribes to their online service, so you should be able to download the journals from any campus computer. Evidently there was a field experiment in 1993, the Coastal Observation and Simulation with Topography (COAST) experiment that examined interactions between winter storms and topography in the Northwest. A search for this may turn up some leads. There was at least one numerical study of PSCZ's using this data; the abstract mentions they cut the height of the Olympics in half, but nothing about shrinking the horizontal scale (I haven't read the paper).<br><br>One would think the process would be the same for PSCZ and convergence zones around volcanos, but I wouldn't be surprised if there is something deeper related to the drastically different horizontal scales. But what do I know. I'll ask some folks in the Atmospheric Sciences department and see what they say.