Anatomy of a supercell: Huttner Weather Lab takes direct microburst hit

Tree branch on power lines in Deephaven. Image: Paul Huttner MPR News

First Hand Weather Reporting

Everyday I sit in the "Huttner Weather Lab" in the scenic out of the way little west metro suburb of Deephaven. I forecast, blog and broadcast the wild swings of our Minnesota weather -- usually about someplace else in Minnesota or the Upper Midwest.

Tuesday Minnesota's extreme weather found me.

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Scary supercell races toward Spring Park on Lake Minnetonka. Image: Candi Stabeck

At 7:05 p.m., the sky turned an eerie shade of green. At 7:10 p.m., the rain and hail started. At 7:12 p.m., the swirling winds suddenly shot up, and the bedlam began.

It was that moment we all know too well in Minnesota. Things outside are getting downright scary, and we make the mad dash for the basement. Time for me to take a dose of my own severe weather advice and get below ground.

Racing from the weather lab and 5 steps before the basement stairs a bright neon flash and explosion outside the kitchen window as a transformer blows across the street. Power out. The first basement step, and the distinct sound of big trees snapping in the severe gusts.

Two minutes later, peeking up from the basement stairwell the winds begin to subside below the "scary" threshold. The now storm darkened weather lab driveway is littered with leaves and sticks, and a large tree branch is snapped off and laying on sagging power lines across the street. It will be the first of many I see in the neighborhood in the next half hour.

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Tree branch on power lines in Deephaven. Image: Paul Huttner MPR News

Damage Report: Worse than June 21st Derecho

The power is out at the Weather Lab. That means no doppler, NWS storm reports no high res satellite or lightning data. The MPR studio digital connection is dead...but the good old back up "hard line" phone connects with producer Sam Choo in the MPR studio. I have been tracking the storm since it blew up near Wheaton, Minnesota near the South Dakota border 4 hours earlier, and I'm not giving up now.

"I have a first hand storm report Sam."  I spend the next few minutes describing the storm and damage I can see on the air by phone with host Steven John and my weather colleague Craig Edwards who is backing me up due to the power loss. Craig is always Johnny on the spot. As the storm lashes Eden Prairie the power flickers at Craig's house and we lose him for a minute. Then Craig's power comes back.

A few minutes later I head for Craig's "Eden Prairie Weather Lab" to finish the storm coverage. I can't get out of my neighborhood on the usual routes because huge trees are down across about half the roads. The damage is worse than I thought, and is far worse in Deephaven than the massive June 21st derecho.

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Massive tree blocks Lake Avenue. Image: Paul Huttner MPR News

Here are some shots of the extensive damage in Deephaven from the Tuesday evening storm.

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Storm images: Paul Huttner MPR News
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Many trees were snapped off a few feet above ground level.
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Large tree vs. tennis court at Deephaven Beach.
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Near miss: Huge tree fall between 2 houses on Lake Avenue. Note the trellis standing survived.

 

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This tree was snapped at the base.
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A good dad with a sense of humor after flash flooding rains.

Tornado or microburst?

At first when I surveyed the damage last night, I saw a few downed branches lying in different directions. That lead me to initially suspect the possibility that I had just survived a direct hit from a small tornado.

Radar signatures looked very suspect west of Lake Minnetonka, and the storm showed frequent signs of strong rotation during its trek across Minnesota. I am amazed it did not actually drop a tornado. In terms of sheer rotation and storm structure, It was "90 percent tornadic" for about 6 hours.

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Trained SKYWARN storm spotter John Wetter captures the "mesocyclone" west of the metro. Image: John Wetter

Here's the radar shot from 6:51 p.m. shows a possible developing hook like appendage west of Lake Minnetonka, indicating a strong "mesocyclone" with the possibility of a rotating wall cloud. That mesocyclone would pass right over Deephaven on the east end of Lake Minnetonka a few minutes later.

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Image: Weather Underground

Further investigation of the storm damage in the neighborhood shows nearly all trees blown down in the same direction. From the NNW toward the SSE. That leads me to conclude that it was more likely a microburst that hit Deephaven Tuesday night. Ultimately the result is the same.

In my experience it takes sudden wind gusts of at least 70 mph to snap trees off like those in Deephaven Tuesday night. Either way you cut it the damage is consistent with an EF0 tornado.

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The EF scale still is a set of wind estimates (not measurements) based on damage. Its uses three-second gusts estimated at the point of damage based on a judgment of 8 levels of damage to the 28 indicators listed below. These estimates vary with height and exposure. Important: The 3 second gust is not the same wind as in standard surface observations. Standard measurements are taken by weather stations in open exposures, using a directly measured, "one minute mile" speed. Image: NOAA

Anatomy of a supercell:

The storm that raced across Minnesota Tuesday evening was a classic " rotating supercell." The storm had it's own rotation, inflow and developed a strong rear flank downdraft by the time it approached the Twin Cities.

The Twin Cities NWS captured the progress of the storm as it made a straight line beeline from Wheaton to the metro.

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Lead supercell races across Minnesota Tuesday. Image: Twin Cities NWS

The storm's updrafts likely approached 100 mph as it produced violent "hail cores" with numerous hail reports between 1.5 and 3 inches in diameter. Damage to crops and cars was reported along the path.

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Image: Twin Cities NWS

Large hail falling from several thousand feet up in a storm can race to earth at speeds of 100 mph. That's what an Eden Prairie police vehicle found out Tuesday evening.

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Image: Eden Prairie Police

This may not have been the most damaging storm to pummel Minnesota this summer overall. But it was in my neighborhood. And it is certainly one of the most interesting storms I have covered in a 30-year weather career.

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