Interesting Weather Information

Thursday, July 13, 2017

Tornado Warning 2017.07.07 by @NWSILN - No Tornado but a Good Warning Part II

An Outflow Boundary, An Inflow Jet, and Spin Up

In Part I of this multi-part post I showed you what radar and @NWSILN saw Friday evening 2017.07.07 that lead up to the tornado warning they issued at 5:18 PM EDT (21:18 UTC).

Now we want to look how the spin-up occurred.

The Past as a Key to the Future
 At 3:36 PM EDT (17:36 UTC) KILN Doppler Radar showed an outflow boundary north of the Ohio River stretching from southeast Ohio, westward into southeast Indiana south of Connersville. In the second image the boundary is just north of the line I drew. Notice how you can see it taper off south of Connersville.

The video shows the movement of the outflow. Over southern Ohio it is pushing south while to the west from Finneytown to south of Connersville it is stationary.

An outflow boundary (aka gust front) is a mini front. We mostly think of them as mini cold fronts, the leading edge of air, cooled by evaporation in the downdraft moving away from the thunderstorm.  It is that cool air blast you have often felt before the thunderstorm downpour arrives.

In my experience doing point nowcasting, outflow boundaries can become stationary or stop and turn around. Thunderstorms born from the interaction of inflow jets and outflow boundaries can be tough to deal with when rain threatens an outdoor event.

Outflow boundaries often cause thunderstorms system to strengthen by acting as a source of lift. Warm moist air flowing towards the thunderstorm glide up and over the cold air advancing away from the storm. 

Thunderstorms can be boosted from routine to severe as they encounter outflow boundaries.

Outflow boundaries can also help create rotation in the environment surrounding a thunderstorm. When air is transported into the thunderstorm  and tilted to nearly vertical in the updraft, the rotation gained when inflow interacts with an outflow boundary can add enough spin to the total and help create a tornado.

The images below should give you a basic idea of how it works.

The animation below shows the specifics for the tornado warning of 7July2017.

Finally the last two animations: Watch the shower develop south of Connersville, develop explosively as the inflow jet pushes it into the outflow, merge with the main cell and the velocity couplet (right  panel) form north of Oxford.

 Below: Following the couplet

So it looks like all the elements were there and radar indicated spinup, eventhough there was no tornado in my book this is a good warning.

NEXT: PART III - 3D Views of the storm.

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