Saturday, July 17, 2010

Recapping Saturday's Tornado Outbreak (MUCH nicer Sunday)

Supercell. I snapped this shot of the massive, long-lasting "supercell" storm (sometimes called a "mesocyclone") that tracked all the way from Long Prairie to Chaska, dropping a few tornadoes along the way. The advancing "shelf cloud" is a battering ram of rain/hail-cooled air - warm, moist air rising up into the thunderstorm, producing a smooth, almost laminar, lens-like shape to the advancing wedge of severe weather. This photo was taken from Tonka Bay, looking due west. More background on supercells, how and why they form, and why these are the storms most likely to spawn tornadoes, here.

Spinning Bulls-Eye. Doppler "velocity" field showing air moving toward the MPX radar site in Chanhassen (bright green) while moving away from the site near Kimball (in bright red). This was evidence of strong, sustained rotation, a hallmark of a potentially violent supercell thunderstorm that went on to produce as many as 2-3 tornadoes as it tracked from near Long Prairie to Watertown.

Mushroom Cloud. That smudge of white over central Minnesota is "moist exhaust", a blossoming thunderstorm anvil, escaping out the top of a severe, rotating thunderstorm that tracked 100-150 miles, from near Alexandria to the southwest suburbs of the Twin Cities. This image is from NASA's "MODIS" satellite, with 1,000 meter resolution, taken around 1 pm Saturday afternoon, as the severe storm was just spinning up.

Hail & High Water. I only saw pea-size hail at my home, but there were numerous reports of 1-3" diameter hail, as large as baseball size near Long Prairie. Hail that large hits the ground at nearly 100 mph!

"Hail Core" A 3-D slice of the severe storm hovering over Cold Spring showed evidence of massive amounts of ice suspended in the thunderhead. Sure enough, spotter reports came in of 4.5" diameter hail in the Cold Spring area - that's bigger than baseball size!

* Saturday: biggest outbreak of severe weather since June 17? It's possible. Between tornadoes, funnels, wall clouds, damaging hail and straight-line winds, the severe weather episode probaby impacted more Minnesotans than the June 17 severe weather spree.

* Unofficial estimate: 7 tornadoes from Saturday's outbreak. NWS teams being dispatched to the towns affected - we should know more within 24 hours.

* 3 additional funnel cloud sightings (details below).

* Isolated "supercell" thunderstorm tracks from near Long Prairie to Watertown, spawning funnels and tornadoes along the way, typical for Kansas, a little unusual for Minnesota in mid July.

* 4.25" diameter hail (baseball size) reported at Cold Spring at 3:37 pm. Scores and scores of damaging hail reports.

* 2-4" rains soaked a huge swath of land extending from Long Prairie and Wadena on east to Nisswa, Brainerd, Garrison and McGrath, nearly a month's worth of rain in some towns, falling in less than 4-5 hours.

* "Bow echo" moves across the north metro, whipping up 70 mph+ straight-line wind gusts.

Hot & Bothered. Saturday's highs ranged from a reasonable 76 at Hibbing, on the cool side of a frontal boundary, to a blazing 96 in Redwood Falls. The mercury peaked at 88 in St. Cloud, 92 in the Twin Cities, before the storms struck, the 4th day above 90 so far in July, the 7th 90-degree day of the summer season so far.

Welcome to Wichita! What took you so long? Instead of listening to loons down by the lake yesterday my wife and I were serenaded by the new state song: the emergency outdoor siren song! Good grief. I'm tempted to move all my earthly possessions down into the basement and lock the door. Call me in October - when we get the all-clear, ok?

We had a friend in the house, visiting from Boston, and this poor gal was FREAKED OUT by all the Doppler-babble, the warnings, the sirens blaring, the ugly, mutating sky scraping against the western horizon. "Come on out to Minnesota - we'll take in a sunset together!" Right. On paper that sounded pretty good. The reality? "Come on out to Minnesota - we'll spend some quality time huddled down in the basement together?" You really get to know a person chatting with them, under the stairs in your basement. Their true persona shines through. You can't fake it, under the stairs in your basement, while the sky rages overhead.

I don't know about you, but I was watching the Doppler radar screen and the clock much of Saturday. "If we leave now we can get in 22 minutes on the lake."Nothing like dodging severe storms as you try to salvage a little quality lake-time. The first "supercell" thunderstorm that dropped tornadoes near Sauk Centre, Delano and Watertown produced a monstrous, rotating thunderhead that looked like a massive UFO, like something out of the movie "Independence Day." Did you see that thing? In-between lightning strikes I dashed outside to snap a few pictures - a remarkable sight, tangible, visible evidence of the powerful forces in play, a stark reminder of the violent wind shear rippling overhead. The storm held together for 2-3 hours, surviving the trip from near St. Cloud to the southwestern suburbs of the Twin Cities. We got a 2 hour break, a reprieve, before the second (more widespread) wave of severe storms approached from the west: reports of tornadoes near St. Cloud (twice in one day!), then Dayton, Anoka, Blaine and (about an hour later) Lino Lakes.

The storms merged and mutated into one massive "bow echo", a boomerang-shaped squall line, as it roared across the north metro, whipping up straight-line winds estimated at 70 mph or more (and some ping-pong size hail). The sun is setting a little earlier now (sorry to have to remind you of that painful fact) - that made it even harder for law enforcement and SKYWARN spotters to track these wall clouds, funnels and tornadoes, in many cases relying on lightning to see the funnel or debris field.

NWS Damage Reports. The "T" icons mark tornado touchdowns, "H" marks the location of large hail, "W" severe winds > 58 mph.

Preliminary Tornado Reports (as of 11 pm Saturday night)

Cushing (Morrison county)

Foreston (Mille Lacs county) Reports of a brief touchdown of a rope-tornado

Dayton (Hennepin county)

Anoka (Anoka county)

Kingston (Meeker county)

Watertown (Carver county)

Lake Elmo (Washington county)

Funnel Cloud Sightings:

Sauk Centre

Kingston (Meeker county)


* Click here for a full (exhaustive) report of every single severe weather event reported in Minnesota Saturday, courtesy of the Chanhassen office of the National Weather Service.

The reality: we won't know until sometime later today how many tornadoes actually touched down on Minnesota Saturday. Early yesterday I predicted 3-6 tornadoes later in the day. The final number is probably going to be at the upper end of that range. That means nearly 40 tornadoes so far in 2010, during an "average year" (whatever that is) we should have seen about 20-22 tornadoes, to date. Welcome to the land of ALL or NOTHING. No tornadoes between January 1 and June 16 in Minnesota. Since then we've been making up for lost time - in a hurry.

Serious Rotation. Doppler can't pick up individual tornadoes (much too small to show up). All it can do is look for rotation in the parent thunderhead, the massive cumulonimbus cloud that may spawn a tornadic circulation. There was ample evidence of spinning "supercells" Saturday, this 3-D slice of a severe thunderstorm near Long Prairie shows powerful rotation.

"Bow Echo". This is what the Doppler looked like around 8:30 pm as the squall line bulged, this boomerang-shape evidence of violent straight-line winds, possibly exceeding 70 mph, as it ripped across the northern suburbs of the Twin Cities. If you want to learn more about the physics and causes of bow echoes, click here for a good overview. Wonderful light reading on a Sunday.

It's safe to come out of the basement. Violent storms are rumbling toward Milwaukee and Chicago, a wind shift to the northwest will chase the bubbling, boiling clouds to the south and east, puddles will dry up - a MUCH nicer day for Aquatennial events in Minneapolis, Highland-fest in St. Paul, whatever you have going on later today.

The weather should cooperate. Expect highs in the low 80s, a stiff northwest wind (10-20 mph) and a big drop in humidity, dew points dropping into the 50s later today. We all get a chance to catch our breath. Soak up any sun, because the next disturbance may brush far southern MN with more showers/storms Monday, just a few isolated T-storms lingering into Tuesday and Wednesday, before a more widespread outbreak of T-storms returns by Thursday. Yes, another "active" week is shaping up. The Doppler is smoking - wish I could unplug it for a week or so. That isn't going to happen anytime soon, I'm afraid. Heat? After a cool, comfortable (70-degree) Monday temperatures rebound to or above 90 by the end of the week. Not sure if it will turn into a full-blown heatwave, but some Dog Day weather is about 72-96 hours away.

Something to look forward to. Enjoy a well-deserved break in what will probably go down in the record books as one of the stormiest July's in recent memory.

Rainfall Recap. All those counties shaded yellow picked up at least 1.5" of rain Saturday, the red shading represents Doppler radar estimates as high as 2-4". I've also outlined heavy rain put down by the supercell that tracked due southeast, from Long Prairie to Chaska, earlier in the afternoon - before the main, widespread squall line arrived later in the evening.

Wild Sky. Stark evidence of the turbulence around the fringe of the supercell storm that sparked a rash of tornadoes and funnels just west of the Twin Cities around 4 pm Saturday. The sky was bubbling and boiling, an amazing sight.

Weather Stories

* 18 out of 26 tornado sirens fail to sound during last Wednesday's tornado scare. Read the latest from KAAL-TV. More evidence that you just can't count on the sirens to get word of an impending tornado.

Billings Tornado Close-Up. Here is some truly spectacular footage, taken by some guy leaning out his apartment balcony in Billings, MT, where roughly a month ago an EF-2 tornado descended on DOWNTOWN. Footage courtesy of KRTV-TV in Billings. You have to see this.

Cloud Seeding to Suppress Hail? CBC News reports on a Calgary, Canada company, Weather Modification Inc, that uses two small airplanes to "seed" growing cumulus clouds with chemicals, trying to reduce the size of subsequent hailstones. Weather modification is still more magic than science, and the liability issues are enough to have a gaggle of lawyers licking their chops. What happens if, instead of mitigating hail, you wind up inadvertently sparking a flash flood? Tinkering with the weather is still wishful thinking, although cloud seeding is taking place over California's Sierra Nevada range, attempting to enhance winter snowfall and increase the snow pack that will eventually be needed for drinking water in San Francisco and Los Angeles. From time to time it's possible to nudge Mother Nature in a certain direction (on a very small scale). But the ability to alter weather on a large scale is still science fiction, not science.

Paul's Conservation MN Outlook for the Twin Cities and all of Minnesota

Today: Damp start, then partly cloudy, breezy and more comfortable (no sirens). Winds: NW 10-20. High: 83

Sunday night: Clearing, quite comfortable. Low: 63

Monday: Sunny start, chance of a late-day shower or T-shower. High: 82

Tuesday: Partly sunny, another chance of late-day instability T-storms. High: 84

Wednesday: Still unsettled, some sun, dry most of the day, late-day thunder. High: 86

Thursday: Warm and sticky with heavier T-storms developing. High: near 90

Friday: Hot sun, muggy. High: 93

Saturday: Probably the better day. Sticky sun, plenty warm for the lake. High: 89

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