Severe Storm Outlook Narrative (AC)
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ACUS01 KWNS 201251
SWODY1
SPC AC 201249

Day 1 Convective Outlook
NWS Storm Prediction Center Norman OK
0749 AM CDT Sun Aug 20 2017

Valid 201300Z - 211200Z

...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS THROUGH TONIGHT
FOR PARTS OF MO...KS...NE...AND IA...

...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS THIS AFTERNOON
FROM NORTHERN WI INTO UPPER MI...

...SUMMARY...
Isolated large hail and damaging gusts will be possible mainly this
afternoon into tonight across parts of Missouri, northern and
eastern Kansas, eastern Nebraska, and Iowa.  Isolated severe storms
will also be possible across northern Wisconsin into Upper Michigan
this afternoon.

...NE/IA/MO/KS through tonight...
A weakening MCS and associated MCV will move across southern
IA/northern MO this morning, and trailing convection over the
surface cold pool will slowly weaken by late morning.  The remnant
outflow boundary will likely stall from northern KS into
central/southern MO, and could provide a focus for additional
thunderstorm development this afternoon (generally south of the
stronger midlevel flow/shear).  Any storm clusters that form late
this afternoon across KS/MO could persist into tonight as the
low-level jet and associated warm advection both strengthen.

In the wake of the morning MCS, gradual recovery of low-level
moisture is expected eastern NE/IA, though the moistening will
probably not be quite as fast as suggested by the NAM/RAP, and a cap
(based on warm elevated mixed layer sampled in 12z LBF sounding)
should inhibit surface-based convection this afternoon.  Elevated
thunderstorm development will become more probable tonight in the
zone of stronger warm advection to the north of the
outflow-reinforced baroclinic zone, from northeastern NE into IA.
MUCAPE in the 2000-3000+ J/kg range rooted above the surface and
steep midlevel lapse rates will support a risk for large hail, which
could be augmented locally by some embedded supercell structures
given sufficient deep-layer shear and low-level hodograph curvature.
 However, given the substantial convective overturning with the MCS
this morning and a lack of any well-defined upstream shortwave
trough to help focus later storm development, will maintain only low
wind/hail probabilities over a broad area.

...Northern WI and Upper MI this afternoon...
A zone of low-midlevel frontogenesis and associated weak convection
over SD this morning will spread eastward to northern WI and Upper
MI by this afternoon.  This zone of ascent, along with a weak
surface cold front trailing southward from the primary cyclone in
MB/ON, should support thunderstorm development in a band this
afternoon from northern WI into Upper MI.  Surface heating and
boundary layer dewpoints in the 60s, beneath steep midlevel lapse
rates, will contribute to MLCAPE of 1500-2000 J/kg.  Vertical shear
will be sufficient for some organized multicell clusters and/or
marginal supercells, with an attendant threat for isolated damaging
gusts and marginally severe hail.

..Thompson/Dial.. 08/20/2017

$$



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