Severe Storm Outlook Narrative (AC)
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000
ACUS02 KWNS 261737
SWODY2
SPC AC 261736

Day 2 Convective Outlook
NWS Storm Prediction Center Norman OK
1236 PM CDT Fri May 26 2017

Valid 271200Z - 281200Z

...THERE IS A MODERATE RISK OF SEVERE THUNDERSTORMS FOR THE OZARK
PLATEAU INTO THE LOWER OHIO VALLEY...

...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS FROM EASTERN
OKLAHOMA AND KANSAS EASTWARD INTO MUCH OF KENTUCKY...

...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS FROM PARTS OF THE
CENTRAL-SOUTHERN GREAT PLAINS INTO THE SOUTHERN APPALACHIAN
STATES...

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

...SUMMARY...
Widespread severe gusts are likely from the Ozark Plateau into the
lower Ohio Valley.  Large to giant hail, tornadoes, and wind damage
are possible for parts of Oklahoma, Kansas, and eastward into
portions of the Ohio Valley.

...Synopsis...
A closed 500-mb low will meander southeast across Manitoba while a
mid-level trough will move from WY/UT east into the central High
Plains by Saturday evening and into the Upper Midwest/central Plains
by early Sunday.  A belt of strong west-southwesterly mid- to
high-level flow will extend from OK/TX eastward through the Ozarks
and into the OH Valley/southern Appalachians.  At the surface, a low
near the OK/KS border will develop east into the lower MO Valley
during the afternoon with a cold front extending from eastern KS
southwest into northern OK and arcing west into the TX Panhandle.  A
triple point near north-central/northeast OK is forecast with a
dryline extending south near I-35 in OK/TX during the late
afternoon.

...Ozark Plateau into the lower OH Valley...
Very rich low-level moisture will advect northward into the region
south of a west-east frontal zone.  A very unstable to extremely
unstable airmass will likely develop across at least the southern
portion of this region.  Several scenarios are possible regarding
the severe threat for this region, especially over MO.  Model
guidance appears to be converging towards the development of a
severe MCS moving east from MO into the lower OH Valley.  Details
regarding timing and evolution are still somewhat uncertain at this
time and possibilities include the following:
1) A remnant MCV from overnight Friday storm activity may serve as
an initiating feature for a thunderstorm cluster to grow upscale.
2) Supercells with all severe hazards (including tornadoes)
transitioning to a severe convective windstorm across the Ozarks.
Given the overall environment, it appears significant severe with
all hazards is possible.  The veering of a southwesterly LLJ into
the OH Valley during the evening will likely aid in maintaining a
severe threat into the evening.

...OK-TX dryline...
An initial strong capping inversion will lead to extreme instability
developing by mid afternoon to the east of the dryline over eastern
OK into northeast TX.  Rich low-level moisture characterized by
lowest 100-mb mean mixing ratios 17-19 g/kg beneath very steep
700-500 mb lapse rates (8-9 degrees C/km) will result in (4500-6500
J/kg MLCAPE per forecast soundings.  Despite generally weak forcing
for ascent, the combination of eroding MLCINH due to strong heating
and perhaps some contribution due to a 55-kt 500 mb speed max moving
over OK during the late afternoon will contribute to the cap locally
eroding and isolated thunderstorm development from northeast OK
south into north-central TX between 21-02Z.  Strong effective shear
(50 kt) and extreme buoyancy will rapidly favor supercell
development early in the convective life cycle.  Giant hail (3.0-4.5
inches in diameter) is possible with any established supercell.
NAM/GFS forecast soundings show gradual veering and strengthening of
the wind profile in the lowest 2-3 km --- yielding 100-250 m2/s2 0-1
km SRH from south to north across this region.  The possibility
exists for significant tornadoes with supercells, especially across
the northeastern quarter of OK into southeastern KS and into MO.  A
cluster of storms will probably evolve during the evening with
hail/wind becoming the predominant risk late Saturday night as
activity moves east into AR.

...Southern Appalachians...
Models show moderate buoyancy developing during the day with
deep-layer shear supportive of organized thunderstorms.  Hail/wind
will be the primary hazards with weakening of the storms and a
corresponding decrease in severe risk associated with cooling during
of the boundary layer during the evening.

..Smith.. 05/26/2017

$$



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