Severe Storm Outlook Narrative (AC)
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000
ACUS01 KWNS 271327
SWODY1
SPC AC 271326

Day 1 Convective Outlook CORR 1
NWS Storm Prediction Center Norman OK
0826 AM CDT Sat May 27 2017

Valid 271300Z - 281200Z

...THERE IS A MODERATE RISK OF SEVERE THUNDERSTORMS OVER PARTS OF
SOUTHEASTERN KANSAS...NORTHEASTERN OKLAHOMA...NORTHERN
ARKANSAS...CENTRAL/SOUTHERN MISSOURI...SOUTHERN
ILLINOIS...SOUTHWESTERN INDIANA...WESTERN KENTUCKY...AND WESTERN
TENNESSEE...

...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS ELSEWHERE FROM
EASTERN KANSAS AND CENTRAL OKLAHOMA TO EASTERN TENNESSEE/KENTUCKY...

...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS SURROUNDING THE
ENHANCED RISK AND EXTENDING INTO NORTH TEXAS AND PARTS OF VIRGINIA
AND NORTH CAROLINA...

...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS SURROUNDING THE
SLIGHT RISK AND EXTENDING TO PARTS OF SOUTHERN COLORADO AND
SOUTHWEST TEXAS...

CORRECTED FOR MDT ERROR IN KY/TN

...SUMMARY...
Widespread severe wind gusts are forecast from the Ozark region to
the lower Ohio Valley today.  Additionally, very large to giant hail
and tornadoes will be possible from the Red River Valley
northeastward to the Ozark Plateau. Damaging winds, large hail, and
a couple tornadoes will also be possible across the Tennessee
Valley, middle Ohio Valley, and portions of the Mid-Atlantic.

...Synopsis...
The upper-air pattern features a broad fetch of zonal to
west-southwesterly flow over much of the central/eastern U.S.,
downstream from a positively tilted trough.  That trough -- now
evident in moisture-channel imagery from a low over northern MB
across ND, WY, UT and southern NV -- should extend from MN to
northern NM by the end of the period.

At the surface, 11Z analysis showed a wavy, generally
quasistationary frontal zone from VA to OH then west-southwestward
across southern IL and central MO.  A cold front was evident from
northwestern IA across eastern/south-central NE to northeastern NM.
The front has been diffused temporarily behind an outflow boundary
related to ongoing convection over northeastern KS, but will catch
up to that boundary and thus the warm sector with time.  A wavy
dryline was drawn from the eastern TX Panhandle to the Big bend
region.  The dryline will move eastward today to central/southern OK
and west-central TX.  The cold front will overtake the dryline and
the outflow boundary from northwest-southeast through the overnight
hours, reaching eastern MO and northeast through southwest TX by
12Z.

...Lower Missouri Valley, Ozarks to Ohio Valley and Red River...
The ongoing convection over northeastern KS was forecast by most of
the synoptic and high-res progs initializing from 00-09Z to weaken
and shrink much sooner than evident in current coverage/intensity.
This casts some uncertainty on the northern and western parts of the
outlook area, in two ways:
1.  Whether this initial convection and related isallobaric
perturbation will evolve into or directly influence the
eventual/expected major wind-producing MCS as it encounters
diurnally destabilizing air over eastern KS and western MO, and/or
2.  The influence of the trailing outflow boundary on subsequent
thunderstorm potential over south-central/southeastern KS.
Thunderstorms either developing along this boundary, or crossing it
from behind, should gain access to surface-based inflow and
expand/intensify rapidly.

In either event, the downshear air mass will be very favorable for
upscale organization of a forward-propagating, potentially
derecho-producing MCS.  Significant-severe/hurricane-force
thunderstorm gusts and numerous damage reports are possible in the
moderate-risk area.  An upgrade may be warranted later today.
However, uncertainties related to influence of the ongoing activity
and timing of greatest storm organization preclude offering any
more-focused unconditional probabilities at this time.  In addition
to wind, line-embedded/tornadic circulations are possible.  Embedded
supercells may occur, especially in any frontal backbuilding that
may occur toward northeastern/central OK late afternoon and evening.


Very steep midlevel lapse rates will overlie a richly moist and
strongly heated boundary layer containing upper 60s to mid 70s F
surface dew points and 15-18 g/kg mean mixing ratios.  This yields
MLCAPE locally exceeding 6000 J/kg in forecast soundings southeast
of the cold front and outflows, and east of the dryline, from the
western Ozarks across eastern OK to the Red River, amidst
effective-shear magnitudes of 45-55 kt.  Such extreme instability is
near record levels, based on historic soundings.  The CAPE-shear
parameter space appears comparable to a fast-developing,
dryline-initiated supercell/tornado event on 26 May 1997 in eastern
OK, but with the added risk this time of a derecho on the northeast
side.  Such extreme instability will support explosive growth of
thunderstorms that do break the cap -- whether through forced ascent
on a cold pool or front, or in any discrete to semi-discrete cells
that can move off the dryline.  Dryline-storm risk is conditional
and more isolated due to the strong EML and related CINH, but also
potentially dangerous with giant hail of 3-5 inches in diameter and
significant tornadoes possible.  By late evening, a swath of
convection should extend from the Ohio Valley into southern OK and
perhaps north TX, with the overall severe threat diminishing
overnight.

...Mid South, Tennessee Valley to VA/NC...
Ongoing clusters of thunderstorms over the western parts of this
area will continue to pose a risk of severe gusts and large hail
through at least mid/late morning, with additional development
possible farther south over parts of northeastern AR and western TN.
Refer to WW 274 and related mesoscale discussions for latest
near-term guidance.

Embedded within the zonal-flow fetch, a southern-stream perturbation
is apparent over the eastern Ozarks region to southern IL,
associated with the ongoing convection.  This feature will proceed
eastward across the southern Appalachians today.  Associated
large-scale ascent/destabilization aloft, overlying diurnal
boundary-layer destabilization and related erosion of CINH, will
support of further convective development eastward toward the
southern Appalachians.  The ongoing complex, or a separate area of
convection progged by some models to develop over southern KY and
northern TN, would move east-southeastward toward the southern
Appalachians given an upscale-growing, forward-propagational forcing
from an aggregating cold pool.  Guidance is inconsistent in this
regard.  The potential for a more concentrated area of damaging wind
(separate from and ahead of the potential derecho discussed above)
currently is too conditional for a more concentrated zone of wind
probabilities, but one may need to be introduced in a later outlook
as mesoscale uncertainties are better-resolved.

The northern/glancing influence of the aforementioned perturbation
aloft also may support thunderstorm formation this afternoon into
early evening over VA near a surface frontal zone.  South and
southwest of the front, seasonally steep lapse rates aloft,
overlying surface dew points ranging from the upper 50s to mid 60s
F, will support areas of 1500-2500 J/kg MLCAPE.  Supercell potential
may be greatest near the front, which will serve as a source of
low-level vorticity and backed flow; however, at least transient
storm rotation will be possible anywhere within this swath, given
favorable deep shear.

...South-central High Plains...
Isolated to widely scattered thunderstorms should form behind the
surface cold front this afternoon, as surface flow veers to more of
an easterly upslope component and related forcing overcomes CINH.
Isolated severe hail or gusts are possible, and a supercell or two
cannot be ruled out.  Deep shear will be favorable for supercells,
but modest low-level moisture and theta-e should preclude a greater
severe threat.

..Hart.. 05/27/2017

$$


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