Monday, September 10, 2018

September 8, 2018 Kentucky-Indiana tornadoes from remnants of tropical storm Gordon

It's the middle of hurricane season, with dangerous hurricane Florence bearing down on the Carolinas later this week.  Hurricanes and tropical systems can produce tornadoes, particularly in their right front quadrant.  So, I thought it would be interesting to look back at last week's much weaker tropical system (Gordon) that produced some tornadoes (see above) in Kentucky and Indiana on Saturday, September 8.

Gordon never quite made it to hurricane strength before landfall on September 4 in the Mississippi/Alabama/northwest Florida coastal area, and only produced one or two very brief weak tornadoes on the 4th and 5th with little or no damage.  But then, after a couple of days with no tornadoes, the remnants of Gordon produced several tornadoes on September 8 far inland in northern Kentucky (KY) and southern Indiana (see area indicated on the 2 pm CDT surface map below):

This included one tornado of EF1 intensity at Stanley KY, just west of Owensboro, shown at the top of this post.  Why would Gordon suddenly start producing tornadoes again four days after landfall?

By Saturday the 8th, after having moved northwestward in weak upper flow for three days, Gordon's remnants had merged with a non-tropical midlevel disturbance evident at roughly 20,000 ft MSL (dashed heavy red line on the 3 pm CDT SPC mesoanalysis 500 mb map below):

This caused Gordon's remnants to "recurve" to the east-northeast as a surface low along an east-west quasi-stationary front over the Ohio Valley (see the surface map earlier).  The 3-panel map below shows this recurvature of Gordon's path (red squares and dashed lines) during September 4th through the 8th:

This graphic also shows deep-layer wind shear (surface to 6 km above ground, in blue lines) on September 4th, 6th, and 8th.  Notice how the shear weakened and essentially disappeared as Gordon's remnants moved farther inland on the 6th and 7th.  But by Saturday the 8th, when encountering energy from the midlevel disturbance, the wind shear dramatically increased near Gordon's remnants over the Ohio Valley.  This is what helped to produce the tornadoes, because supercell tornadoes, along with an unstable environment, typically require sizable wind shear (a change in wind direction and increase in wind speed with height) to develop.

At 3 pm CDT on the 8th, instability and wind shear together (shown using an enhanced version of the energy-helicity index or EHI) highlighted an area supportive of tornado development on SPC graphics along the Ohio River border area of Indiana and Kentucky , which is where the tornadoes occurred:

Thankfully, the tornadoes weren't that strong, but the one at Stanley KY did do notable damage to several homes, and was widely photographed:

Hurricane Florence (category 4 as I write this) poses a much bigger threat than Gordon's localized tornadoes when it hits the Carolinas on September 13 and 14.   Wind driven storm surge, heavy rain, and flooding will likely be huge and potentially life-threatening issues, and I'm just hoping it won't be as bad as most forecasters are thinking.

-  Jon Davies  9/10/18

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