Severe Storm Outlook Narrative (AC)
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ACUS01 KWNS 201950
SWODY1
SPC AC 201948

Day 1 Convective Outlook
NWS Storm Prediction Center Norman OK
0248 PM CDT Sun Aug 20 2017

Valid 202000Z - 211200Z

...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS ACROSS PARTS OF
FAR EASTERN NEBRASKA...IOWA...FAR SOUTHEAST SOUTH DAKOTA AND FAR
SOUTHERN MINNESOTA...

...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS ACROSS PARTS OF
THE CENTRAL PLAINS...MID MISSOURI VALLEY AND MID TO UPPER
MISSISSIPPI VALLEY...

...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS ACROSS PARTS OF
ILLINOIS...INDIANA AND FAR SOUTHWEST LOWER MICHIGAN...

...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS ACROSS PARTS OF
THE OZARKS AND MID MISSISSIPPI VALLEY...

...SUMMARY...
A swath of scattered severe storms with large hail and damaging
gusts are possible over parts of Iowa and vicinity late tonight.

The only change to the outlook for this issuance is to remove the
Marginal Risk from parts of central and southern Missouri.  The
airmass in south-central Missouri has become much less unstable due
to outflow associated with convection from earlier today. A marginal
severe threat will still be possible across parts of southwestern
Missouri where convection could develop later the evening in a
moderately to strongly unstable airmass.

..Broyles.. 08/20/2017

.PREV DISCUSSION... /ISSUED 1127 AM CDT Sun Aug 20 2017/

...IA and vicinity...
An MCV over east-central IA should drift east across northern IL/IN
through tonight. A few multicell clusters are ongoing to the south
and west of this MCV within weak low-level warm advection over parts
of the Lower MO Valley. Isolated damaging winds and severe hail are
possible with any of this activity, mainly near the IL/IN and MO/AR
border areas.

In the wake of this morning activity, gradual recovery of low-level
moisture is expected from KS to the Mid-MO Valley. A robust elevated
mixed layer should largely inhibit surface-based storm development
through this evening. A cluster of elevated storms is expected late
tonight from northeast NE into IA in a corridor of strengthening
warm advection to the north of the outflow-reinforced baroclinic
zone. Rather steep mid-level lapse rates of 7.5-8.5 C/km from
700-500 mb will yield large buoyancy with a broad plume of MUCAPE
above 2000 J/kg. On the periphery of 25-35 kt 500-mb westerlies,
effective shear of 30-40 kt should support a few embedded supercell
and/or bowing structures as an MCS likely matures over parts of IA.
This scenario appears sufficiently probable to warrant an upgrade to
Slight risk for severe hail/wind.

$$



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