Garrett’s Blog: Factors For Localized Flash Flooding

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Additional flash flooding will be possible this week with a slow moving cold front approaching from the north.

On Sunday, July 5th, a handful of slow moving thunderstorms developed with peak daytime heating. A cluster of cells in particular, near Van Buren, dropped 6.87″ of rain in a few hours causing extensive flooding problems.

The core of the heaviest cell over North Fort Smith and most of Van Buren was about 11 miles across and 8 miles wide.

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While most of the area was dry Sunday you can see a handful of storm tracks near Clarksville, Booneville, Greenwood, and of course Van Buren & Fort Smith.

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The observations from around Crawford County are striking; 6.87″ near Van Buren but only 0.05″ a few miles east in Franklin County and less an 1/2″ near Natural Dam.

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Several inches in a short amount of time caused flooding across most low lying areas and especially near industrial park.

Many factors caused the extreme flash flooding:

  • No steering winds. With jet stream winds from the north at only 10-15mph there was nothing to move the storm.
  • High water content in the atmosphere. Precipitable Water is the amount of liquid in a column of air if you were to squeeze it all out. These values are running around 1.80″ which is extremely high. With the storm regenerating new updrafts it was a very efficient rain maker.
  • Low Lifting Condensation Level Heights (LCL). This is also why the clouds look so low to the ground with so much motion. Low LCL heights have been shows to limit dry air entrainment and moist downdraft formation, limiting the cell’s movement (Goff & Hanson, 2011).
  • Wet Bulb Zero Height (WBZ) The height at which the wet falls below 0º. High WBZ have been shown to correlate with flash flooding events. WBZ heights currently are around 12,000ft.

These same factors are in place this week with a slow moving cold front arriving from the north.

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Through the next 5 days estimated rainfall totals are in the 2-4″ range with locally heavier amounts likely.

-Garrett

 

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