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| Credit: NASA AQUA Satellite, ~2pm on Easter Sunday, March 27, 2016 |
Yesterday was definitely a crazy day for weather! For much of the day, a pretty strong Puget Sound Convergence Zone set up shop in the Northern and Central Puget Sound regions, waxing and waning in intensity and shifting northward and southward as the day went on. The convergence zone ventured further south than usual, actually making it south of Seattle at times. The above satellite picture not only shows a well-defined convergence zone but also shows clearing to the north and south of the zone due to sinking air on either side of it, and lots of scattered clouds (popcorn) offshore denoting little showers and indicating an unstable atmosphere. An unstable atmosphere means there is a large decrease in temperature with height, and as Mother Nature would have it, Sea-Tac had a high of 55 yesterday while Snoqualmie Pass has picked up 8 inches of snow in the last 24 hours. There’s still a ton of snow up there, go and get it before it all melts away by the end of this week (but that’s for another blog).
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| Credit: Travis Miller |
In addition to bringing gobs of snow to Stevens and Snoqualmie Passes, yesterday’s convergence zone actually brought a funnel cloud to Mill Creek! Interestingly enough, funnel clouds are not all that uncommon with strong convergence zones. However, these are called “cold core” funnel clouds and are due to localized areas of circulation due to wind shear within the convergence zone. They are NOT like the funnel clouds and tornadoes common in the Great Plains, which are formed from supercell thunderstorms and are much larger and stronger. For comparison, take a look at the F5 tornado below from Manitoba. Yup, even Canadians get tornadoes!
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| Category F5 tornado approaching Elie, Manitoba on June 22, 2007 Retrieved from Wikipedia |
Huge difference. Even so, even our little cold-core funnel clouds can become tornadoes, and when they do, they can cause quite a bit of destruction.
Supercells are large, severe thunderstorms with a rotating updraft, or mesocyclone. The mesocyclone forms when wind shear caused by relatively light winds at the surface and strong winds aloft creates a spinning vortex of air, and then strong updrafts tilt this updraft so that it is vertical. Both these pictures were taken from a powerpoint presentation I saved from my atmospheric sciences 452 (advanced synoptic meteorology and forecasting lab) class at the UW, and the credit goes to Nick Bond.
The difference in wind strength in the vertical can be quite high, especially in areas near the surface that are sheltered from the wind. I remember one time in December 2007 (the “Great Coastal Gale,” to be exact), when winds around a mile up were an astounding 100 knots, while winds away from the immediate coast weren’t even at advisory levels. The coast was a different story, and this was arguably the strongest storm on record for some parts of northwest Oregon and southwest Washington – even surpassing the Columbus Day storm!
While supercells get their rotation from differences in wind strength in the vertical, our tiny cold-core funnel clouds and tornadoes are formed from differences in wind strength in the horizontal. With convergence zones, you have winds coming from the south and winds coming from the north, and this can spin up small, localized vortices that manifest themselves as cold-core funnel clouds. I couldn’t find a picture of this online, so I drew one myself. This is a picture of the convergence zone around the time that the funnel cloud was sighted, and the pinwheel represents a vortex formed by wind shear from the converging air streams. In this case, the pinwheel is over Mill Creek, which is where people spotted the funnel cloud.
In case you want to learn more about supercell thunderstorms, I have a much more detailed post here. I must of had a lot of time on my hands when I wrote it!!!
Thanks for reading!!!
Charlie