Severe Storm Outlook Narrative (AC)
Issued by NWS
ACUS11 KWNS 212333
SPC MCD 212332
Mesoscale Discussion 0096
NWS Storm Prediction Center Norman OK
0532 PM CST Sat Jan 21 2017
Areas affected...Portions of far northeast Texas...northern
Louisiana...southern/central Arkansas...and western Mississippi
Concerning...Tornado Watch 16...
Valid 212332Z - 220130Z
The severe weather threat for Tornado Watch 16 continues.
SUMMARY...The threat for tornadoes, very large hail, and damaging
wind gusts continues across much of Watch 16. Additionally,
downstream watch issuance will likely be needed across northeastern
LA, southeastern AR, and western MS within 2-3 hours.
DISCUSSION...Multiple supercells have formed late this afternoon
across west-central portions of Watch 16, in response to low-level
confluence near a surface trough/warm front and mid/upper-level
ascent on the nose of an impulse over eastern TX. A surface low was
analyzed near the TX/LA border at 23Z, with a warm front extending
to its southeast towards New Orleans. Near this boundary, backed
surface winds are enhancing storm-relative helicity sufficiently for
low-level mesocyclogenesis. Indeed, a fairly prominent tornadic
debris signature was noted across far northeast Marion County / far
southeast Cass County in TX over the last 20-30 minutes and another
recently observed over northeast Bossier Parish.
More supercells are likely to form farther south, with the potential
for tornadoes continuing as they progress northeast, especially when
these cells approach the aforementioned surface front. Moreover,
steep mid-level lapse rates noted in 18/20Z regional soundings and
ample effective shear will maintain the potential for robust
organized updrafts capable of very large hail. Indeed, recent KSHV
data have depicted several broad/deep ZDR column and ring structures
aloft, suggesting hailstone growth should be quite efficient.
...Please see www.spc.noaa.gov for graphic product...
LAT...LON 32369408 32919442 33269455 34399421 34879347 34799137
34529059 34129016 33229009 31449098 31139176 30909226