Severe Storm Outlook Narrative (AC)
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ACUS01 KWNS 211252
SPC AC 211251

Day 1 Convective Outlook
NWS Storm Prediction Center Norman OK
0751 AM CDT Sat Apr 21 2018

Valid 211300Z - 221200Z


Isolated severe thunderstorms are possible from
northwest/north-central Texas this afternoon to southeast Texas and
southwest Louisiana tonight.

A progressive, split-flow pattern will persist in mid/upper levels
this period.  The main feature for this forecast is a synoptic-scale
cyclone initially centered over south-central/southeastern CO, with
trough south-southwestward over far west TX to Chihuahua.  By 00Z,
the associated 500-mb low should move to the OK/KS border region
between ICT-GAG, with trough southward across west-central/southwest
TX.  The mid/upper low should reach northeastern OK overnight, with
the trough becoming somewhat negatively tilted, extending across the
Arklatex and TX/LA border area by 12Z.

At the surface, 11Z analysis showed a frontal-wave low near PVW,
with cold front arching southwestward across the Llano Estacado and
over extreme southeastern NM.  A warm front was drawn southeastward
across the SAT area then eastward over the upper TX coast and Gulf
waters south of LA.  The low should move east-southeastward across
northwest/north-central TX today as the warm front shifts/redevelops
northward across the Hill Country and into parts of north-central
TX.  By 00Z, the cold front should extend from the low over north-
central TX southwestward across parts of the Hill country to the
lower Pecos Valley.  By 12Z, the cold front will have crossed most
of southeast and south-central TX, extending southwestward from the
low near the LA/AR state line.

Isolated to widely scattered strong/severe thunderstorms are
expected to develop this afternoon over portions of northwest/
north-central TX, in a zone of maximized low-level convergence/lift,
close to the surface low.  This activity should move east-
southeastward to eastward across parts of north-central or central
TX through the rest of the afternoon into the evening, offering the
potential for hail and isolated severe gusts.  The initial activity
will be supported by a pocket of diurnal/warm-sector diabatic
heating and related boundary-layer destabilization, behind an area
of clouds, precip and non-severe thunderstorms now moving across
parts of OK and north TX.  That increasing low-level instability,
along with surface dew points generally upper 50s to mid 60s F, will
be juxtaposed with cooling temperatures aloft as areas of DCVA pass
through the southeastern quadrant of the mid/upper cyclone.
Forecast soundings accordingly depict MLCAPE 1000-1500 J/kg over
parts of north-central TX in the 21Z-00Z time frame.  Effective-
shear magnitudes around 45-55 kt and curved low-level hodographs
suggest potential for a supercell or two in the first few hours,
followed by upscale development of a near-frontal convective band
that would cross parts of north-central and east TX, reaching
western LA overnight.  Isolated severe hail and gusts will be
possible in that convective band as well.

Coverage and boundary timing/placement uncertainties with respect to
the convection are the main concerns precluding greater
unconditional severe probabilities at this time, but a small area of
greater risk may be added as the event gets closer and presumably
better-focused on the mesoscale.

..Edwards/Peters.. 04/21/2018

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