Tuesday, May 21, 2013

May 20, 2013

An historic tornado occurred yesterday in Moore, OK.

More on that in a bit.

We started off the morning in Omaha, NE, looking toward OKC as an initial target.  A dryline/cold front intersection was forecast to sit just west of that city.  All the parameters--moisture, instability, shear, lift--were in place, so we figured it was a lock.

We left at about 8 AM, and OKC was reachable by 2 PM.

Then things went wrong.  An 18Z sounding went up from Norman (they actually sent it at 17:15Z, 12:15 PM) and it showed moist and very unstable atmosphere, basically uncapped.

Uh oh.

Soon after that, the MD went out and then a tornado watch.  As expected, it was for serious storms with big hail and damaging winds, in addition to the expected tornadoes (with a few long track/damaging ones possible).

As luck would have it, storms started firing up shortly after 18Z, right along the dryline and the cold front.  We were north of it at this point, still in Kansas, so we had to hurry.


A storm initiated near Newcastle, just west of Moore.  It got tornado warned almost immediately.  Which was a good thing, as it started to produce a tornado fairly quickly.

An aside about the normal behaviour of supercells.  Usually they initiate and take a long time to wind up and produce tornadoes.  On the order of an hour or so, sometimes 2.

Also as an aside, storm initiation in this part of the world (most of North America, aside from the Gulf Coast) tends to be during the mid afternoon.  3 or 4 PM.  Or sometimes 6 or 7.  So when storms went up in the early afternoon, we weren't ready, and nor were we in position.

This was our eventual downfall, as the storm was crossing our projected path, and there was no way we could get in front of it before it crossed I-35, so we had to cut east and then south, to try and get ahead of the storm without it hitting us and getting the vehicle damaged or destroyed by hail.

After some driving slowness through some towns, we eventually ended up straight east of the storm.

Which was, of course, exactly when the tornado decided to lift.  All the time we had been listening to radio from OKC talking about how big and nasty the tornado was, and how it was grinding through parts of Newcastle and Moore.  The TDWR at KOKC was showing a tightly coiled spiral of reflectivity (note to self: download that later) and the the WSR-88D showed something that I never like to see: a region of high reflectivity in the hook, a huge rotational couplet and associated very low correlation coefficient.  This combination of signatures is fairly well correlated with tornado debris, so much that this combination is called a tornadic debris signature or a debris ball.  (Usually the reflectivity and CC are fairly circular.)

Anyhow, long story short, we didn't see a tornado, and the storm structure, such as it was, was obscured a lot by haze.

We went to our hotel in Ada, OK, to prepare for today.

Speaking of which, it looks like the same kind of conditions are coming together this afternoon (but hopefully not *early* this afternoon) around Dallas, our initial broad target.  I hope it's not a repeat of yesterday.

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