[BNAWX] Severe Storm Outlook Narrative

[EMWIN Server]

ACUS11 KWNS 280712
SWOMCD
SPC MCD 280711
GAZ000-ALZ000-TNZ000-MSZ000-ARZ000-280915-

Mesoscale Discussion 0893
NWS Storm Prediction Center Norman OK
0211 AM CDT Sun May 28 2017

Areas affected...portions of northwestern and west-central
GA...central/northern AL...northern MS...central/southeastern AR.

Concerning...Severe Thunderstorm Watch 285...290...

Valid 280711Z - 280915Z

The severe weather threat for Severe Thunderstorm Watch 285, 290
continues.

SUMMARY...Two primary bows or arcs of severe storms will pose the
greatest threat across this corridor the next few hours:  one moving
southeastward across northern AL, and another over western AR.  A
new watch may be needed for parts of AL and perhaps western GA as
convective and instability trends warrant.

DISCUSSION...The leading MCS for this discussion area is evident as
of 645Z over north-central/northwestern AL, from Limestone to Morgan
Counties.  This activity should continue to move southeastward
astride a convectively reinforced outflow boundary from earlier
convection in eastern TN and GA.  That outflow boundary was analyzed
from southern fringes of the ATL metro area to just south of CTJ and
GAD, into Cullman County, moving slowly southward.  Convection
developing along and north of this boundary over north-central/
northeastern AL will continue to reinforce its baroclinicity, while
some of the activity merges into the northern parts of the
progressive MCS.  The preconvective warm sector south of the
boundary is characterized by rich boundary-layer moisture with
surface dew points low-mid 70s F and PW in the 1.25-1.5-inch range,
contributing to MLCAPE around 1500 J/kg.  Although MLCINH is
gradually strengthening due to diabatic surface cooling, two factors
may permit maintenance of convective-wind potential near severe
limits past the current watch:
1.  Forced ascent of near-surface air into the MCS along the leading
part of the accompanying density current, aided by lift along the
foregoing outflow boundary, and
2.  Nearly surface-based effective-inflow parcels being maintained
for a few more hours, given the slow pace of surface cooling amidst
such rich moisture.
Reinforcing downward momentum transfer also is possible within the
complex, amidst roughly 50-kt west-northwesterly effective-shear
vectors with a substantial component orthogonal to the convective
alignment.

Farther west, the complex over west-central/southwestern AR may
continue to pose an episodic damaging-wind threat as it moves
eastward across southern AR and into MS over the next few hours, on
either side of the outflow boundary from the convective corridor
over AL, northern MS and eastern AR.  That boundary is stronger and
faster-moving than the one in eastern AL, however similarly
favorable warm-sector air is apparent to its south as well.  A
corridor of damaging-wind potential appears across the middle and
southern parts of watch 290, which may require local spatial
extensions as convective trends warrant.

..Edwards.. 05/28/2017

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

ATTN...WFO...FFC...OHX...BMX...HUN...MEG...JAN...LZK...

LAT...LON   35579218 34919071 34678916 34908832 35088766 34808621
            34238509 33588481 33088520 32858845 33609154 34139354
            35579218
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