Severe Storm Outlook Narrative (AC)
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ACUS02 KWNS 271756
SWODY2
SPC AC 271755

Day 2 Convective Outlook
NWS Storm Prediction Center Norman OK
1155 AM CST Mon Feb 27 2017

Valid 281200Z - 011200Z

...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS EXTENDING FROM
CENTRAL ILLINOIS AND ADJACENT WEST-CENTRAL AND SOUTHWEST INDIANA
THROUGH THE SOUTHEAST HALF OF MISSOURI...EXTREME WESTERN KENTUCKY
INTO NORTHERN AND CENTRAL ARKANSAS...AND A SMALL PART OF EASTERN
OKLAHOMA...

...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS SURROUNDING THE
ENHANCED RISK FROM EASTERN OKLAHOMA AND NORTHEAST TEXAS INTO
NORTHERN MISSOURI...AND THE LOWER OHIO VALLEY AND SOUTHERN GREAT
LAKES REGION...

...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS SURROUNDING THE
SLIGHT RISK FROM EASTERN PORTIONS OF THE SOUTHERN PLAINS TO THE
UPPER OHIO VALLEY AND SOUTHWEST GREAT LAKES...

...SUMMARY...
Severe thunderstorms will be possible on Tuesday and Tuesday night
from eastern portions of Oklahoma and Texas northeastward into parts
of the lower and middle Mississippi Valley and Ohio Valley.

...Synopsis...
Model guidance remains in good agreement in showing a larger-scale
positively tilted mid-level trough moving from the Great
Basin/southern CA region eastward to the Great Plains by late
Tuesday night.  As this trough move into the Plains states, it is
expected to take on a more neutral orientation, resulting in
strengthening deep-layer wind fields.  A very strong 500-mb speed
max over northwest Mexico will move downstream and strengthen from
the southern Plains to the mid-MS and lower OH Valleys, likely
exceeding 100+ kt by Tuesday evening.  Subsequent strengthening of a
broad southwesterly low-level jet will allow moisture to spread
poleward across the warm sector, with a large area of precipitable
water values exceeding 1-1.25 inches spreading across much of the
eastern half of the United States this forecast period.

At the surface, an area of low pressure should track from the
vicinity of the NE/IA border to the IL/WI border by 01/00Z, and then
across Lower MI to southwest Ontario.  A trailing cold front will
advance east from the central Plains into the middle MS and lower OH
Valleys, while a second deepening area of low pressure tracks along
this boundary from OK Tuesday afternoon to central IL and Lower MI
Tuesday night.  Concurrently, a warm front will advance northward
through the OH and upper TN Valleys.

...Ohio and Tennessee Valleys Tuesday morning and afternoon...
Despite the lack of height falls during the first half of the
forecast period, forcing for ascent attendant to a weak transient
midlevel impulse tracking across this region and low-level warm air
advection suggests an ongoing cluster and/or new thunderstorm
development will spread from west to east.  The eastern extent of a
plume of steep midlevel lapse rates spreading east atop low-level
moistening should prove favorable for mainly elevated storms with
hail being the primary threat.  However, forecast soundings suggest
some potential for a few surface-based storms, especially by Tuesday
afternoon across parts of northern AL through middle TN and eastern
KY.  Vertically veering winds suggest a tornado and locally strong
wind gusts cannot be ruled out with these afternoon storms.

..ArkLaTex/eastern OK northeastward into the mid MS and OH Valleys
Tuesday afternoon through Tuesday night...

Consensus of deterministic and convective allowing models suggest
there will be the potential for several zones of storm development
late Tuesday afternoon through Tuesday evening across the full
extent of the severe risk areas from along the cold front with
northward extent to along low-level confluence zones farther south.
Given the 12Z ESRL HRRR and 12Z NAM 4km suggesting a broken band of
discrete storms developing from east-central MO into central and
northeast IL, the enhanced severe risk area has been expanded north
across to now include more of eastern MO and into central IL, with a
northward expansion of the slight and marginal risks as well.
Moderate instability and strengthening deep-layer and low-level
shear will favor all severe hazards, with some indication for a
strong tornado threat from parts of AR to southern IL along a
strengthening low-level jet enhancing hodograph curvature.  The
potential exists for storms to evolve into a QLCS Tuesday evening
from Missouri into the lower OH Valley, with damaging winds and
meso-vorticity tornadoes being the primary threats.

Farther north, a marginal risk for severe storms may develop and
affect parts of east-central IA and southeast WI Tuesday afternoon
and evening, with the marginal risk expanded north to now include
these areas.

..Peters.. 02/27/2017

$$



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