Severe Storm Outlook Narrative (AC)
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ACUS01 KWNS 290601
SWODY1
SPC AC 290559

Day 1 Convective Outlook
NWS Storm Prediction Center Norman OK
1259 AM CDT Thu Jun 29 2017

Valid 291200Z - 301200Z

...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS FOR NORTHEAST
KANSAS...NORTHERN MISSOURI...AND FAR SOUTHEAST NEBRASKA...

...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS FROM THE CENTRAL
PLAINS INTO THE MIDDLE MISSISSIPPI VALLEY...

...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS ACROSS THE
CENTRAL HIGH PLAINS THROUGH THE MIDWEST AND INTO THE LOWER GREAT
LAKES...

...SUMMARY...
Scattered significant severe thunderstorms are possible late this
afternoon into this evening across the central Plains with hail and
wind the primary threats.  Isolated strong to severe storms are also
possible across the Midwest eastward into the Lower Great Lakes, and
across the central High Plains into Wyoming.

...Synopsis...
A series of central U.S. shortwave troughs will move through the
base of a larger-scale mid-level trough located over the
north-central states on Thursday.  A lead disturbance, perhaps
modulated by convective processes overnight Wednesday into this
morning, will move from the central Plains to eastern IA by late
afternoon.  An upstream disturbance will move east across the
Dakotas during the daylight hours and into the Upper Midwest late
Thursday night.  Another mid-level impulse will move southeast from
the upper Columbia Basin into western WY by early evening.  A belt
of strong 500-mb flow will encompass much of the area from the
north-central Rockies/Intermountain regions eastward through the
Plains and into the Great Lakes.  In the low levels, a surface low
will occlude over the upper Great Lakes with frontal segments,
influenced by convective outflow, extending southwestward into the
central Plains.  Farther west, a lee low will meander over the
south-central High Plains as a front accelerates south over the
central High Plains.  An inverted trough will probably serve as a
focus for isolated thunderstorm development during the late
afternoon from KS south into the TX Panhandle.

...Central Plains east into the middle MS Valley...
It is a complicated forecast for today with a considerable amount of
uncertainty due in part to perceived consequential influences of
overnight storms and boundary placement for late afternoon
development.  Several convective complexes will likely move east
across the central Plains/middle MS Valley during the early morning
on the nose of a LLJ situated over the central Plains.  Southerly
low-level flow will maintain a reservoir of 14-16 g/kg lowest 100-mb
mean mixing ratios (translating to upper 60s-lower 70s degrees F
dewpoints) from eastern KS into northern MO/southern IA to the south
of early-day outflow.  A moderately to very unstable boundary layer
with 2500-3500 J/kg MLCAPE is forecast by late afternoon with MLCINH
eroding and storm development focused along boundaries over
northeast KS eastward into northern MO.  Strong westerly mid-level
flow will favor storm organization and supercells capable of large
to very large hail early in the convective lifecycle.  It appears
the tornado risk is highest over northeast KS into parts of west and
northern MO where low-level shear and buoyancy may maximize.  The
transition to a wind threat and subsequent increase in damaging
gusts will probably occur during the evening over eastern KS east
towards the MS river.  Models vary farther west in central KS into
the central High Plains.  Several possible scenarios lend an
isolated risk for large hail/severe gusts beginning during the late
afternoon and persisting through the evening and possibly overnight
as they move east.

...Southern Great Lakes into the NY Adirondacks...
An upper disturbance over the Great Lakes is forecast to move into
southeast Canada and an associated strong belt of low- to mid-level
flow will overspread the Lower Great Lakes during the morning.
Models show surface dewpoints on southwesterly flow increasing into
the upper 50s (western NY) to upper 60s (southern Great Lakes)
during the day.  Destabilization of the boundary layer remains in
question across the Lower Great Lakes.  Storm intensity and a
resulting threat for wind damage appear too isolated for a 15% wind
area at this time.  Farther southwest, storm coverage concerns due
in part to prior convective activity renders confidence low
regarding the overall severe risk.  Isolated hail/wind appear to be
the primary threats before storms weaken during the evening.

..Smith/Jirak.. 06/29/2017

$$



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