In what might qualify as one of the top understatements of the week, the weather has been a little active over the past two days. With a windstorm and ensuing Convergence zone on Monday, followed up by impressive hail storms on Tuesday, the atmosphere has been one big bubbling cauldron of activity
Lucky for us, there are some well-placed time lapse cameras around the Puget Sound area that did a great job in capturing some of the zaniness.
Just watch as the skies come alive as cold air moves in and the atmosphere becomes unstable. If you look over the mountains, you can see cumulus clouds erupt into cumulonimbus clouds as if it's taking a page from the Icelandic volcano that no one can pronounce or spell.
Here is the video taken from the UW Atmospheric Sciences Building on Monday. This features a fantastic Convergence Zone which waged its battle just north of the UW. You can see the clouds frequently shift direction as the upper winds swirl over King County. (In fact, someone wrote that at one point Monday, the flag atop Swedish Medical Center on Seattle's First Hill was pointing one way, while the flag atop Harborview a short jaunt to the south was pointing the opposite direction.)
And here are two videos from Tuesday. From the UW one, keep an eye toward the horizon about 1/2 way through after the first rain squall passes and watch that cumulus cloud just explode over the Olympics.
And here is the video over Silverdale. Again, watch the cumulus clouds just bloom off the mountains -- and you can even see it snowing up there!
For those that missed it, these storms brought quite a bit of hail. Look what it did to I-405 in Mill Creek:
A couple of notes about the hail Tuesday:
The atmosphere was really unstable due to extremely cold air moving in to the upper atmosphere. How cold? Aside from temperatures struggling to break 50 here on the ground, temperatures are very, very cold in the upper atmosphere.
In fact, readings from about 18,000 feet up were around -36 Celsius Monday -- the second coldest reading of the "winter" season and colder than anything observed in January and February. (See Paul Deanno's excellent blog entry on why cold air makes for such thunderous weather.)
We also had a question of whether that was snow on the ground in the video, but no. The atmospheric makeup, while cold, was not cold enough in lower levels to snow. It was hail, which can also accumulate and look like snow if it the stones are large enough and you get a big burst in a short period of time.
Another question was why isn't it called sleet? Hail and sleet are two different animals. Hail is formed by water that is carried by strong updrafts into the upper reaches of tall cumulonimbus where it freezes into ice. That ice falls, picks up more rain on the way down, then gets blown back up by the updraft where it refreezes and grows. The process repeats until the stone is heavier than the updraft can support. The stronger the storm, the stronger the updraft, and the larger the hail can grow.
Sleet is when rain falls from a cloud but then encounters freezing air hugging near the surface and freezes the rain into an ice pellet. Sleet needs freezing temperatures under the cloud to occur. Hail can occur even if it was 90 degrees on the ground when the storm hit.