Severe Storm Outlook Narrative (AC)
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000
ACUS02 KWNS 281755
SWODY2
SPC AC 281754

Day 2 Convective Outlook
NWS Storm Prediction Center Norman OK
1254 PM CDT Wed Jun 28 2017

Valid 291200Z - 301200Z

...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS ACROSS EASTERN
NEBRASKA AND KANSAS INTO FAR NORTHWEST MISSOURI AND SOUTHWEST
IOWA...

...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS SURROUNDING THE
ENHANCED RISK AREA ACROSS MUCH OF NEBRASKA AND KANSAS TO SOUTHERN
LOWER MICHIGAN AND NORTHERN OHIO...

...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS SURROUNDING THE
SLIGHT RISK AREA FROM WY AND WESTERN SD AND THE CENTRAL HIGH PLAINS
TO WESTERN AND NORTHERN PENNSYLVANIA INTO NEW YORK...

...SUMMARY...
Scattered significant severe thunderstorms are most likely Thursday
into Thursday evening across the central Plains, with areas of large
hail and damaging winds. Isolated strong to severe storms are also
possible across the Midwest eastward into western and northern
Pennsylvania and New York, and across the central high plains to
Wyoming, South Dakota to southwest Minnesota.

...Synopsis...
Low-amplitude midlevel shortwave troughs will track east-northeast
across the Dakotas and mid MO Valley through the Great Lakes,
Midwest to the Northeast States Thursday and Thursday night.
Meanwhile, a large-scale midlevel trough, currently tracking
equatorward through southwest Canada per water vapor imagery, is
expected to move southeast reaching the central Rockies/High Plains
by 12Z Friday.  A warm front attendant to a midlevel trough shifting
east from the upper Great Lakes region should extend from northern
Lower MI into southern Ontario.  Meanwhile, the IA to the NE and
northern KS portion of a trailing cold front from the upper Great
Lakes should be the focus for thunderstorm development this forecast
period.  Additional storms will be possible attendant to surface
trough moving east across the Dakotas and a separate cold front
advancing south through the northern High Plains.

...Central Plains region...
Models continue to suggest the greatest coverage of severe storms
Thursday afternoon through Thursday evening should occur across the
initial Day 2 Enhanced risk area from eastern portions of NE/KS into
southwest IA and far northwest MO.  This region will have the
greatest juxtaposition of high surface moisture (dew points in the
upper 60s to around 70 F) beneath steep midlevel lapse rates
(exceeding 8 C/km) and diabatic heating, resulting in moderate
instability.

Despite some height rises in the wake of an early period midlevel
impulse across eastern NE/northeast KS, sufficient deepening of
boundary layer moisture and strong surface heating will result in a
reduction of MLCINH for thunderstorm development near the
aforementioned cold front and any differential heating boundaries
from early morning ongoing showers and storms.  Height falls ahead
of the approaching upstream larger scale northwest U.S. trough will
support thunderstorm development Thursday afternoon and evening.
Strengthening westerly midlevel winds across the central Plains and
south-southwesterly low-level winds into eastern KS/northwest MO
suggest strong effective bulk shear will support organized storms
including supercells.  Large hail, some very large, and a potential
tornado threat into southeast NE/northeast KS to southwest IA and
northwest MO will be the initial severe hazards.

Meanwhile, model-to-model and run-to-run consistencies in the
NAM/GFS/ECMWF suggest storms in the central High Plains could evolve
into a forward-propagating MCS Thursday night across mainly central
and eastern KS, with damaging winds being the primary severe threat,
as heights falls and further strengthening of deep-layer winds ---
with the approach of the upstream large-scale trough.

...Midwest into western PA and NY...
Although a synoptic warm front should be located farther north into
northern Lower MI into southern Ontario, a differential heating
boundary extending from southern Lower MI through northern IL should
be the primary focus for new thunderstorm development Thursday
afternoon and evening.  The 15% (Slight) and 5% (Marginal)
severe-risk areas have been expanded a little northward to include
more of northern IL and southern Lower MI, with this change based on
model consensus (12Z NAM/GFS and 00Z ECMWF) for the placement of
showers and thunderstorms in vicinity of the aforementioned surface
boundary.  Moderate instability and sufficient bulk shear oriented
parallel to the convective initiating boundary suggest line segments
with locally strong/damaging winds being the primary threat.

Farther northeast, stronger west-southwesterly low-level winds
(40-50 kt) from northern PA into NY should result in a threat for
damaging winds as storms develop across a destabilizing warm sector
Thursday afternoon.

...WY/western SD...
Marginal instability combined with forcing for ascent attendant to
the larger scale trough moving toward the central Rockies and
northern High Plains, and in vicinity of a cold front should allow
for afternoon to early evening thunderstorm development.  Weak to
modest bulk shear should limit the overall severe weather threat,
though steep low-level lapse rates could enhance locally
strong/damaging winds gusts.

...Eastern SD/southwest MN...
Effective bulk shear and forcing for ascent attendant to a midevel
shortwave trough moving east across ND/SD during this forecast
period should support storm development, with some organized.
Greatest severe threat, though a Marginal risk due to weak
instability, is possible across eastern SD and southwest MN.

..Peters.. 06/28/2017

$$



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