Severe Storm Outlook Narrative (AC)
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ACUS01 KWNS 210103
SWODY1
SPC AC 210101

Day 1 Convective Outlook
NWS Storm Prediction Center Norman OK
0801 PM CDT Sun Aug 20 2017

Valid 210100Z - 211200Z

...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS ACROSS FAR
NORTHEAST/EAST-CENTRAL NEBRASKA INTO MUCH OF NORTHERN AND CENTRAL
IOWA...

...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS SURROUNDING THE
SLIGHT RISK FROM EASTERN NEBRASKA TO EASTERN IOWA AND FAR SOUTHERN
MINNESOTA...

...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS ACROSS A SMALL
PART OF SOUTHWEST MISSOURI...

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

...Eastern NE and IA...
Last several runs of the operational HRRR, other CAMs, 12Z ECMWF,
and 18Z NAM/GFS continue to indicate thunderstorms will develop
across northeast and/or east-central NE into adjacent western IA
later this evening to the early overnight (03-06Z), with upscale
growth likely thereafter across northern and central IA.  Early
evening surface analysis showed a cold front extending from northern
WI through southeast MN, northeast NE to southwest NE.  Meanwhile, a
couple of convective outflow boundaries extended west-northwestward
from northern MO and from along the IA/MO border.  A strengthening
south-southwesterly low-level jet at 30-40 kt this forecast period
will result in an increase in warm-air advection across eastern NE
into western IA, providing the primary lift for the new thunderstorm
development. The eastern extent of steep mid-level lapse rates,
moderate elevated instability and sufficient bulk shear will support
organized strong to severe storms with large hail and damaging winds
the primary severe threats.

..Peters.. 08/21/2017

$$


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