Sunday, March 28, 2010

The Alabaster AL tornado on 3/25/10 - a small CAPE setting

The Alabaster, Alabama tornado a few days ago (Thursday 3/25/10) was another reminder that tornadoes can and do occur in small CAPE environments, sometimes with only 200-300 J/kg of CAPE. Such tornadoes typically aren't very strong (the well-photographed Alabaster tornado was rated only low-end EF-1). A photo of the tornado showing the rear-flank-downdraft and a little structure is seen above. Alabama storm chaser Brett Adair also captured the tornado on video shown on The Weather Channel.

The setting had a sharp short wave at 500 mb (see 2nd graphic above) moving across Tennessee and Alabama, with only low to mid 50's F dew points at the surface ahead of a Pacific type cool front (see 2nd graphic above). Because online model forecasts typically are presented showing only areas of CAPE 500 J/kg and above, plan-view model forecasts and analyses didn't highlight the CAPE present over Alabama (see the 2nd graphic above). But with an NWS office located at Alabaster (the Birmingham area office) that is also a RAOB sounding site, and the tornado occurring just to the west at sounding time, the Alabaster tornado was one of those rare instances where an actual observed sounding was available in close proximity to the tornado-producing storm (see 3rd graphic above). This sounding shows the CAPE all bunched down below 600 mb (about 4 km in the vertical), very low in the profile. Given that this was a discrete cell in an environment with 250-300 m2/s2 of storm-relative helicity (SRH) and the CAPE was squeezed low in the sounding, low-level stretching and tilting probably facilitated the tornado. An interesting low-topped tornadic supercell case to study, and not associated with a closed cold-core 500 mb low.

As I write this, tornadoes with damage have been occurring over the Charlotte-Greensboro area of North Carolina, in another setting with small CAPE (< 500 J/kg) but very large low-level shear.

As I mentioned earlier this month, I had hoped to get a page up showing some interesting pics and graphics of the 1990 Hesston KS tornado 20 years ago. Unfortunately, things have been busy on my end, so that will have to wait awhile :-(.

- Jon Davies 3/28/10


StormCchaser said...

Fascinating writeup. That sounding looks a lot like what we have been seeing here in Phx this spring, except that the CAPE here (RUC forecast) has been a bit lower. Tomorrow could be interesting in that regard.

John Moore

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