2017-04-26 EVENT: AR/LA/TX

Discussion in 'Target Area' started by Quincy Vagell, Apr 26, 2017.

  1. Quincy Vagell

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    Today appears to be coming around for a significant tornado threat in the Arklatex vicinity. The biggest red flags in earlier forecasts were concerns with veer-back signatures in the mid-levels, which would have been a glaring limiting factor in the longevity and severity of any tornadoes. Largely linear forcing also suggested a fairly extensive squall line would be the main show.

    Recent trends and convection allowing models show better wind profiles with only a bit of a kink in hodographs (which is not completely uncommon in significant tornado events) in northern Louisiana and surrounding portions of far East Texas and southern Arkansas.

    Expectations:
    Ongoing convection from northeast Texas to far southern Missouri consolidates into a substantial squall line with multiple segments by midday across Arkansas and adjacent portions of the Middle Mississippi Valley with an EML plume (i.e. 8-9+ C/km 700-500mb lapse rates per mesoanalysis over East Texas) largely putting a lid on convection down around I-20 and points south toward the Upper Texas Gulf coast. By early afternoon, sufficient boundary layer heating and dew-points in the upper 60s to around 70F should erode the cap and result in convective initiation across Far East Texas and northwestern Louisiana. 1500-3000 J/kg SBCAPE, ~50kts deep layer shear and >200 m2/s2 0-1km SRH should be common in the area.

    Convective mode:
    Although there is increasing confidence in at least some supercell tornado threat, it's still not a slam dunk tornado event. The setup favors convection developing on the southwestern flank of the aforementioned squall line to the north and the renewed convection could quickly merge into linear segments, limiting the tornado potential somewhat. Additionally, there is some potential for prefrontal supercells from near the LA/TX border area, eastward into central/northern LA and possibly southern AR. A significant tornado threat will exist for any cells, frontal or prefrontal, that can remain at least semi-discrete.

    I was a bit surprised that the SPC issued 15% tornado probabilities at 6z and their 13z discussion highlighted some uncertainty there. I am also skeptical of the ability for any discrete cells to stay isolated for very long north of the AR/LA border, but that may be splitting hairs when it comes to the latest SPC decision to shift the moderate risk farther north into AR. While wind fields may be a bit more backed (due to proximity to surface low pressure and local geography into central AR), instability will be less strong than points south and the window for discrete activity appears quite narrow. Still, QLCS/line embedded tornadoes are absolutely a concern given the kinematic profiles.

    Today reminds me a tiny bit of 11/17/15, which featured a large 10% tornado outlook in the same general area, but busted due to convective mode being a large squall line. Obviously we're in late April and not November and instability is much more favorable today.

    Chase territory is not optimal in the outlook area, but there are spots in northern LA that have workable road networks. If storms remain even somewhat isolated into late afternoon, the terrain and road networks get much better into NE LA and SE AR.
     
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