Severe Storm Outlook Narrative (AC)
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508
ACUS01 KWNS 201629
SWODY1
SPC AC 201628

Day 1 Convective Outlook
NWS Storm Prediction Center Norman OK
1128 AM CDT Sun May 20 2018

Valid 201630Z - 211200Z

...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS FROM THE
MID-MISSISSIPPI VALLEY INTO THE MID-SOUTH...

...SUMMARY...
Strong to severe storms are possible across a large area from parts
of southern and eastern Texas into the Ozarks and eastward into the
Ohio Valley.  A more focused area of severe threat is expected from
southern Illinois and southeast Missouri into northeast Arkansas and
west Tennessee.

...Mid Mississippi Valley into the Mid-South...
Several low-level convective boundaries are analyzed extending from
southwest Indiana across far southeast Missouri, with an additional
boundary arcing southward from ongoing convection over east central
Missouri then extending westward across northern Arkansas.  The
latter boundary is moving southeastward across southeast Missouri
into a very moist low-level air mass where surface dewpoints are in
the upper 60s to lower 70s.  Moderately steep mid-level lapse rates
sampled by the 12Z LZK sounding extend into the Mid-South and are
contributing to MLCAPE of 1500-2500 J/kg.  The area is along the
southern edge of stronger westerly mid-level winds which may aid in
storm organization with primarily multicell storms likely this
afternoon.

Model guidance including 12Z CAMs/HREF indicate storms will increase
in coverage this afternoon as continued heating weakens the cap and
convergence is focused along the primary outflow boundary.  The
strong thermodynamic support suggest a few storms will become severe
capable of producing large hail and strong wind gusts.  Activity
will continue into the evening before storms begin to diminish by
01-03z.

...Parts of eastern Oklahoma into western Arkansas and southwest
Missouri...
Visible imagery indicates clouds have thinned over this area in
advance of a mid-level short-wave trough lifting northeastward from
the Plains toward the lower Missouri Valley.  This will promote air
mass destabilization in the wake of overnight convection.  Model
guidance indicates a few storms will redevelop this afternoon from
eastern Oklahoma into southwest Missouri along/immediately ahead of
an advancing cold front and/or convective outflow.  Although winds
aloft may slowly weaken into tonight, sufficient vertical shear may
persist to aid development of a few strong to severe storms this
afternoon and evening.  A Marginal Risk has been added across parts
of this area.

...Southern into Eastern Texas...
An extensive band of convection is moving southward/southeastward
extending from east of the DFW Metroplex into central Texas.  The
air mass across the region in advance of the convection remains
moist and unstable with MLCAPE of 1500-2500 J/kg expected this
afternoon.  The convection is likely to continue this afternoon with
a few strong to severe storms possible.  Overall severe threat will
be limited by marginal vertical shear, but stronger cells will be
capable of producing hail and strong wind gusts.

Other strong to isolated severe storms are expected to develop this
afternoon within the moist upslope low-level flow over southwest
Texas.

...Ohio Valley...
A few strong storms will be possible this afternoon and evening over
parts of the Ohio Valley near and south of a frontal boundary
extending from southern Indiana into southern Ohio.  Forcing for
large scale ascent appears to be weak which may limit the coverage
of storms.

..Weiss/Leitman.. 05/20/2018

$$



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