Severe Storm Outlook Narrative (AC)
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ACUS01 KWNS 240551
SWODY1
SPC AC 240549

Day 1 Convective Outlook
NWS Storm Prediction Center Norman OK
1149 PM CST Thu Feb 23 2017

Valid 241200Z - 251200Z

...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS ACROSS PARTS OF
SOUTHERN LOWER MICHIGAN...CENTRAL AND EASTERN INDIANA...WESTERN AND
CENTRAL OHIO...AND PARTS OF NORTHERN KENTUCKY...

...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS SURROUNDING THE
ENHANCED RISK AREA...AND EXTENDING SOUTHWARD INTO PARTS OF
TENNESSEE...

...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS SURROUNDING THE
SLIGHT RISK AREA...EXTENDING FROM THE UPPER GREAT LAKES TO THE
SOUTHERN APPALACHIANS...

...SUMMARY...
Strong to severe thunderstorms are forecast to affect portions of
the southern Great Lakes, Ohio and Tennessee Valleys on Friday and
Friday night.  The primary threat appears to be damaging wind, but
some hail and a couple of tornadoes will also be possible.

...Synopsis...
An upper short-wave trough -- embedded within broader/longer-wave
troughing -- is forecast to shift out of the Plains and across the
mid and lower Missouri valley through the day.  By evening, the
trough is progged to continue eastward across the mid and upper
Mississippi Valleys, and eventually into the Midwest/Great Lakes
region as it acquires neutral to slightly negative tilt with time.

As the upper system advances/sharpens, a surface low in the vicinity
of western Illinois early in the day is forecast to move across
Illinois and northwestern Indiana through the afternoon, and then
across Michigan toward Lake Huron through the latter stages of the
period.  Ahead of the low, a warm front is forecast to advance
northward across the lower Great Lakes into southern Canada, while a
cold front sweeps eastward across the Ohio/Tennessee/Mississippi
valleys and reaches the Appalachian crest by the end of the period.

...Great Lakes region southward to the southern Appalachians...
Showers and scattered thunderstorms are forecast to be ongoing from
parts of northeast Illinois/southeast Wisconsin eastward into the
northeast U.S. at the start of the period, north of a warm front
extending eastward across the Midwest.  Modest low-level moisture
into the warm sector /50s surface dewpoints/ beneath an elevated
mixed layer advecting northeast across the region should keep the
warm sector capped -- likely until late afternoon.  Eventually
however, as the surface low moves northeast toward southern lower
Michigan and the trailing cold front advances into Indiana, limited
diurnal heating combined with persistent ascent is expected to
steadily/gradually weaken the cap, eventually allowing development
of isolated to scattered thunderstorms.  Initial storm mode may be
cellular, but upscale growth into a broken line of storms is
expected, which should expand southward along the advancing front
through the evening hours.

Though degree of shear will be sufficient for supercells, somewhat
veered low-level flow and tendency for more linear storm mode should
limit overall tornado potential.  Greatest risk for supercell
tornadoes should exist during the afternoon with initial convective
development, and possibly near the retreating warm front where more
backed low-level winds should exist.  With time, risk should evolve
toward primarily damaging winds, though an isolated QLCS tornado or
two, as well as marginally severe hail, will be possible through the
evening and into the overnight hours.  It appears at this time that
the greatest severe risk will exist from late afternoon/early
evening through midnight or an hour or two later, across southern
lower Michigan, eastern Indiana, western Ohio, and northern Kentucky
-- coincident with the Enhanced Risk area.  Storms should eventually
begin to diminish in intensity given weaker instability expected
into the upper Ohio valley/lower Great Lakes region, but risk may
linger across the southern Appalachians region through the end of
the period.

..Goss/Picca.. 02/24/2017

$$


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