[BNAWX] Severe Storm Outlook Narrative

[EMWIN Server]

ACUS11 KWNS 172346
SWOMCD
SPC MCD 172345
OHZ000-KYZ000-TNZ000-INZ000-MSZ000-MOZ000-ARZ000-180115-

Mesoscale Discussion 1892
NWS Storm Prediction Center Norman OK
0545 PM CST Sat Dec 17 2016

Areas affected...Central/eastern AR...northern MS...western/middle
TN...western/central KY...MO bootheel

Concerning...Tornado Watch 517...

Valid 172345Z - 180115Z

The severe weather threat for Tornado Watch 517 continues.

SUMMARY...The risk for damaging winds, a few tornadoes, and isolated
large hail continues across Tornado Watch 517. The tornado threat
will be greatest with any semi-discrete cells ahead of the cold
front. Downstream watch issuance to the northeast is not currently
anticipated.

DISCUSSION...At 2330Z, a mostly continuous line of convection is
ongoing along a cold front running from northern KY into central AR.
Additional shallower convection is ongoing across portions of
western TN, northern MS, and AR, but thus far this activity has
struggled to deepen. A warm and moist airmass remains in place south
of the front, though relatively warm midlevel temperatures are
limiting the magnitude of available buoyancy, with MLCAPE largely in
the 250-750 j/kg range across the watch area per recent
mesoanalysis.

While very strong low-level shear and effective helicity in excess
of 300 m2/s2 continues to support a tornado threat, the convective
mode has thus far been unfavorable, with a tendency for storms near
the front to develop into small bowing segments. With mid-upper flow
largely parallel to the front and a tendency for storms near the
boundary to be undercut with time, it is unclear if any truly
discrete cells can develop and be maintained. However, a damaging
wind threat and brief tornado threat will continue into this evening
with storms along the front, with a greater conditional tornado
threat (and isolated large-hail threat) present with any
longer-lived discrete supercells that can develop.

The severe threat will diminish from west to east as the front
advances this evening. While some organized convection may move
northeastward out of the watch into portions of northern KY/southern
OH with gusty winds possible, very limited instability should limit
the severe threat and downstream watch issuance is not currently
anticipated. Meanwhile, developing convection across southwest AR
may require some counties to be added to the southern portion of the
watch if it continues to deepen with time.

..Dean/Edwards.. 12/17/2016

...Please see www.spc.noaa.gov for graphic product...

ATTN...WFO...RLX...JKL...ILN...LMK...OHX...PAH...MEG...JAN...
LZK...SHV...

LAT...LON   34539234 35839030 36498902 37038809 38628487 39438393
            38668283 36598552 35828654 35248779 34698895 34079011
            33559131 33279239 33399388 34139310 34539234
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