Tuesday, February 12, 2013

Hattiesburg MS EF-4 tornado environment

I had the day off Sun Feb 10 (and was watching a TWD marathon) and not paying a lot of attention to the weather until after darkfall, when I began to catch photos and video of a fairly large and certainly destructive tornado which had struck Hattiesburg, MS.

A quick review of data indicated a slow-moving upper level trough in place from ND-MT southwesteward to the lower CO river valley.  An occluding/stacked deep layer low would move from wrn NE to srn MN throughout the day.  Along/just ahead of the trailing cold front, a surface-based squall line had already initiated by midnight local time across nearly the entirely length of TX (SPS to DLF) along and just ahead of the trailing cold front... supported by ML dew points of 52-60F and high-level flow that would gradually become very difluent and divergent through the remainder of the day.  By dawn, rather rich moisture well downstream was in position to advect from the wrn Gulf of Mexico into the lower MS river valley... with ML dewpoints of 64-68F, and a moist layer depth of 1.5 to 2.5 km (with the remnant EML having already lifted nearly entirely for surface-based storms soon after).  The threat for severe storms would naturally be enhanced along and just south of the retreating warm front as storms within and ahead of the squall line swept into the destabilizing warm sector.  Some of the more destructive tornado outbreaks with several discrete tornadic storms do seem to be associated with a slow-moving large scale trough and a band of marked difluence/divergence ejecting out to the southeast over a deep moist layer... although in the end, convective mode did become a bit on the muddled side with lots of storms on Feb 10.

As early as 1 PM local time, pre-squall line storms were already plaguing northern portions of the LA "panhandle" and the adjacent southern 1/3 of MS.  The upswing in pre-squall line convection was further supported by a considerably strong corridor of increasing low-level moisture convergence/advection along the gradually veering 50-kt synoptic LLJ axis.  Accordingly, by 22-23Z a number of storms evolved into discrete supercells along and south of the effective warm front.  Interference among various supercells and individual seeding from upstream convection may have kept a more widespread/damaging tornadic event from occurring, but the supercell which tracked ENE through Hattiesburg was one storm which obviously thrived in a somewhat less messy local area on the southernmost flank of the training convection.  The fact no lives were lost is a wonder and a blessing, as this was a EF-4 rated tornado.

The LIX RAOB was launched about the same time the tornado initiated and 60 nm to its south--free of convection.  Although the Hattiesburg area was affected by convection for much of the~5 hours prior to the tornado... during nearly an hour preceding the arrival of the tornado, the inflow environment "cleared out" aside from some possible "sprinkles" from the storm's own anvil business given 8-10 km flow averaging 280 degrees.  However, the surface layer via the ob had remained relatively mild (near 70F) all afternoon considering the preceding thunderstorms... and didn't really climb all that much during the period of pristine inflow--so a moot point, perhaps, esp given the rather rich/deep moist layer. 

Anyway, I modified the lowest ~35 mb of the LIX RAOB very slightly based on obs from KHBG; e.g., the surface temp was reduced ~1F and the surface dew point ~1.5F.  The RAOB surface wind, 21006, was perhaps a bit too light/veered--with 18510 being more appropriate based on the "clear inflow" period as well as obs closest to the general south--but I didn't adjust the hodograph and present it in a different form because the hodograph shape and vertical shear parameters would have changed very, very little.  A final note on the shear environment... the near-surface hodo (wholly around 180-190 degrees in the surface to kink layer, if "modified") was relatively backed considering 900 m AGL flow having veered all the way to 225 degrees (from due SW).  Also, the kink height was quite low for a U.S. significantly tornadic supercell occuring near or east of the MS river: 113 m AGL.  Then again, the most well-defined kinks are not characterized by as much windspeed increase above the kink as this one, so this is not a particularly classic case of a sharp low level hodograph kink (which are most common west of the MS river).  Below are the surface-based and lowest 100 mb mixed-layer thermodynamic parameters from the modified LIX RAOB as well as the vertical shear parameters from its unmodified hodograph (both of whose images appear below).

Sfc T/Td: 71/66.5°F
SBCAPE: 567 J/kg
SBCIN: 73 J/kg

ML T/Td: 75/67°F
MLCAPE: 1083 J/kg
MLCIN: 8 J/kg

0-3 km MLCAPE: 91 J/kg
MLLCL: 566 m
MLLFC: 1419 m

0-1 km SRH: 305 m2/s2
0-3 km SRH: 360 m2/s2
0-6 km bulk shear: 67 kt
storm motion: 245° @ 40 kt

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