Rain lingers, tornado survey, “oddball” storms Tuesday

Summer began Tuesday, but it will feel more like April by tomorrow.

It feels like somebody put a big weather target over Minnesota.

A pesky low pressure system is spinning overhead today.

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Surface low pressure over swirling southeast Minnesota Wednesday.

The severe threat has shifted east today, but scattered bands of rain continue to rotate around the low from east to west. Many areas can expect another .25″ to .50″ of rainfall today as occasional shower bands roll through. Some areas in northern Minnesota may pick up another 1″ of rain through Thursday.

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The system will mercifully moving east by Friday and allowing a return to sunshine.

Tornado damage survey in progress today:

Our local NWS office is in the field today to investigate possible tornado touchdowns in Coon Rapids & Blaine today.

The details from Twin Cities NWS:

Rotating Thunderstorms on Tuesday; Damage Survey on Wednesday

“Tuesday afternoon saw numerous rotating thunderstorms across southern and central Minnesota as well as western Wisconsin. Some of these had deeper rotation in favorable environments, prompting tornado warnings. Numerous funnel clouds and one tornado were reported through early Tuesday evening. In addition, thunderstorms briefly trained over St. Cloud producing very heavy rainfall over a short amount of time, creating flooding in the downtown.

PUBLIC INFORMATION STATEMENT

NATIONAL WEATHER SERVICE TWIN CITIES/CHANHASSEN MN

734 PM CDT TUE JUN 21 2011

…STORM DAMAGE SURVEY TO BE CONDUCTED WEDNESDAY MORNING…

A NATIONAL WEATHER SERVICE STORM DAMAGE SURVEY TEAM WILL BE IN

THE BLAINE AND COON RAPIDS AREAS ON WEDNESDAY MORNING TO SURVEY

REPORTED DAMAGE CAUSED BY ONE OF THE STORMS ON TUESDAY AFTERNOON.

A PUBLIC INFORMATION STATEMENT CONTAINING DETAILS OF THE FINDINGS

WILL BE ISSUED UPON COMPLETION OF THE ASSESSMENT.”

When NWS survey teams hit the field they look for several key damage indicators to assess the causes of damage.

1) Is the damage pattern circular or straight line? Do downed trees and other debris all point the same way, or are there signs of rotation within the damage pattern?

2) What kind of structural damage was done if any?

3) What did local residents see and hear as the damaging wind event rolled through?

4) How does damage in the field compare with other sources such as doppler radar images, photos etc?

We should know by late today if the damage in Coon Rapids was caused by a tornado. My hunch is there was at least one brief tornado touchdown.

Oddball storms:

Tuesday’s (tornadic?) storms were pretty odd in a few ways. These were not your classic tornadic supercell thunderstorms.

-The storm that rolled through Dakota County, St. Paul and into Anoka County was much smaller than your average “tornadic supercell.”

-There was virtually no hail detected in any of the storms Tuesday.

-The storms were moving from SE to NW as they rotated around the low pressure system in southwest Minnesota. Most tornadic supercells move from SW to NE or from W to E in Minnesota.

The atmpsoheric “sounding” (weather balloon data) showed storng rotation with ” speed and dirctional wind shear” present in the atmpsohere Tuesday.

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The main factor in Tuesday’s storms was that they showed strong rotation. The bottom line is that’s all you need to produce a tornado. The atmosphere was not capable of supporting violent EF3+ tornadoes or long lasting “long track” tornadoes Tuesday, but there was enough rotation to cause a few weaker spin ups.

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NWS “storn centric” doppler velocity loop shows storm as it rotated from Dakota County through St. Paul into Anoka County.

Remember there’s no such thing as a “weak tornado.” The damage photos in Coon Rapids prove the point that while very localized, even one small tornado can ruin your day!

PH

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