Severe Storm Outlook Narrative (AC)
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ACUS02 KWNS 270658
SWODY2
SPC AC 270657

Day 2 Convective Outlook
NWS Storm Prediction Center Norman OK
1257 AM CST Mon Feb 27 2017

Valid 281200Z - 011200Z

...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS FROM CENTRAL
AND NORTHERN ARKANSAS INTO SOUTHEASTERN MISSOURI AND ADJACENT
PORTIONS OF SOUTHERN ILLINOIS...WESTERN KENTUCKY...AND WESTERN
TENNESSEE...

...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS FROM THE ARKLATEX
NORTHEASTWARD THROUGH THE LOWER AND MIDDLE MISSISSIPPI VALLEY AND
INTO THE OHIO VALLEY...

...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS SURROUNDING THE
SLIGHT RISK...

...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 is in good agreement in showing a larger-scale
mid-level trough axis moving from the Great Basin/southern CA region
eastward to the Great Plains late Tuesday night.  A very strong
500-mb speed max over the Chihuahuan Desert will move downstream
within a belt of southwesterly flow and be located over the middle
MS and lower OH Valleys and intensify to 100+ kt by early Wednesday
morning.  In the low levels, a broad and strengthening southwesterly
low-level jet will extend from the northwest Gulf Coast
northeastward into the lower Great Lakes.  The primary surface low
will develop eastward from NE/IA vicinity eastward to the lower
Great Lakes during the period.  Concurrently, a warm front will
advance northward through the OH Valley as a cold front pushes
southeastward through the lower MO Valley and through eastern OK
overnight.

...ArkLaTex and eastern OK northeastward into the MS Valley and OH
Valley...
This remains a complicated forecast and there remains considerable
uncertainty regarding several factors described below, some of which
are conditional, that will have large influence on the unconditional
severe probabilities.

Low-level moisture will continue to stream poleward within a ribbon
of strong south-southwesterly flow as a warm front advances
northward from the lower MO and lower OH Valleys to the southern
Great Lakes.  Lowest 100-mb mean mixing ratios of 10-13 g/kg will
translate to 58-62 degrees F dewpoints over the OH Valley and the
middle 60s reaching as far northeast as the MO Bootheel and
encompassing areas farther southwest over the ArkLaTex.  A cirrus
canopy is likely to overspread a large portion of the enlarging
warm/moist sector and partially modulate heating.  A capping
inversion located in the 850-700 mb layer will likely delay storm
development until late in the day across AR and areas downstream
over the MS/OH River confluence.  By late afternoon/early evening,
there is uncertainty whether storms will initiate along a
pre-frontal confluence/850-mb moisture axis bisecting AR from
southwest to northeast and into southeastern MO.

Models show only weak convective inhibition but this area will
largely be on the eastern edge of 500-mb height falls beginning to
tease the area.  In other words, only weak forcing for ascent
primarily in the form of isentropic lift/warm-moist advection, is
expected with any possible diurnally-driven development.
Deterministic models including the ECMWF, UKMET, GFS, and to a
certain extent the ARWKF, show warm-sector activity developing over
AR and moving downstream towards the MS/OH River confluence.  The
NAM and NMMB models are largely void of storm development with this
scenario and are uncharacteristically "dry".  It appears
increasingly probable that a cluster of storms develops within this
corridor.  The supercell-wind profile, moderate buoyancy (1000-1750
J/kg MLCAPE), and 300-400 m2/s2 effective SRH would favor
surface-based supercells with all hazards possible, including the
conditional possibility for a strong tornado(s).

As stronger DCVA/500-mb height falls overspread the western
periphery of the warm sector as the cold front begins to accelerate
southeastward, scattered to numerous storms are forecast to develop
across the middle MS Valley and farther south over eastern OK and
northeast TX.  Strong deep-layer shear will aid in storm
organization and upscale growth into one or more bands of storms
potentially capable of damaging winds/hail with the stronger storms.
The strong background low-level shear may support some tornado risk
into the overnight associated with stronger supercells and/or QLCS
mesovortices.  The frontal convection will probably reach the MS
River and lower portion of the OH River by Wednesday morning.

..Smith.. 02/27/2017

$$



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