Severe Storm Outlook Narrative (AC)
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ACUS01 KWNS 161706
SWODY1
SPC AC 161705

Day 1 Convective Outlook CORR 1
NWS Storm Prediction Center Norman OK
1205 PM CDT Wed Aug 16 2017

Valid 161630Z - 171200Z

...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS FROM SOUTHEAST
KANSAS INTO WEST-CENTRAL MISSOURI...

...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS FROM OKLAHOMA
ACROSS THE LOWER MISSOURI VALLEY INTO IOWA...

...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS FROM THE
SOUTHERN PLAINS INTO THE UPPER MISSISSIPPI VALLEY...

CORRECTED FOR ENHANCED RISK GRAPHIC AREA

...SUMMARY...
Scattered to numerous severe storms are likely from Iowa to
Oklahoma, mainly later this afternoon and evening. Large hail,
damaging winds, and a couple of tornadic storms are expected.

...Upper Mississippi Valley into the Southern Plains...
Water vapor imagery shows a strong trough/vorticity maxima lifting
into southwest Nebraska this morning.  This system is forecast to
continue toward the upper Mississippi Valley tonight as a mid-upper
level jet streak moves northeast on the southern periphery of the
system.  An associated surface low over extreme eastern Nebraska is
expected to move into extreme southwest Minnesota by evening and
continue across southern Minnesota tonight.  A cold front trailing
southward from the low will progress eastward and southeastward
through the period.

A key mesoscale feature impacting thunderstorm evolution this
afternoon is an arcing band of convection from western Iowa
southward into extreme northwest Missouri and southeast Kansas.  The
intersection of the western extension of this boundary over Kansas
with the advancing cold front is expected to serve as a focus area
for new convective initiation this afternoon, with activity
expanding in coverage as storms move northeastward into the lower
Missouri Valley this evening.  In addition, there is potential for
ongoing storms in parts of Iowa and Missouri to intensify this
afternoon as they move into a warming and destabilizing environment.

The air mass from Oklahoma into central Missouri is undergoing
strong diabatic heating, and there is potential for some
thermodynamic recovery into parts of eastern Kansas in the wake of
the downstream convection as clouds diminish to the north of the
outflow boundary and ahead of the cold front.  Moderate to strong
instability is expected this afternoon with MLCAPE reaching
2000-3000 J/kg.  Winds aloft will be veering with height and
increasing in magnitude during the afternoon which will provide
sufficient vertical shear /35-40 kt in the lowest 6 km/ to support
organized convective structures including a few initial supercells.
It appears that convection will evolve into a series of linear
segments with time that may include a few bowing elements.  All
severe hazards including large hail, damaging winds, and several
tornadoes will be possible, especially during the earlier stages of
severe storm maturation when the likelihood of discrete cells will
be the greatest.  The threat is expected to evolve into primarily a
damaging wind threat during the evening with some severe potential
continuing into the early overnight hours.

Farther north, the slight risk has been extended northward into
parts of central/eastern Iowa where visible imagery shows some
breaks in clouds which will promote heating and air mass
destabilization.  Strong forcing for ascent along/in advance of the
ongoing convective band coupled with favorable shear profiles
suggest potential for a few severe storms to develop this afternoon
and early evening.

..Weiss.. 08/16/2017

$$



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