SPC AC 241950
Day 1 Convective Outlook
NWS Storm Prediction Center Norman OK
0250 PM CDT Sun Oct 24 2021
Valid 242000Z - 251200Z
...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS FROM THE OZARKS
AREA INTO THE MIDDLE MISSISSIPPI VALLEY REGION...
Scattered severe thunderstorms with damaging wind gusts, large hail
and a couple of strong tornadoes are likely from parts of the Ozarks
into the middle Mississippi Valley the remainder of this afternoon
into tonight. Isolated damaging gusts and a tornado or two will also
be possible along the Oregon coast.
...Ozarks through the middle Mississippi Valley region...
Scattered thunderstorms including a few supercells are underway over
northwest MO within zone of deeper forcing ahead of the upper
vorticity maximum and surface low. Additional storms will soon
increase in coverage and intensity farther south along the cold
front. These storms will continue east through a corridor of
moderate surface-based instability with 1500-2000 J/kg MLCAPE, 40-50
kt effective bulk shear and 200-300 m2/s2 0-1 km storm relative
helicity. Mixed storm modes with both supercells and line segments
are likely into the evening. A few discrete pre-frontal storms are
also possible. All hazards are expected including the potential for
a few strong tornadoes into the evening.
.PREV DISCUSSION... /ISSUED 1152 AM CDT Sun Oct 24 2021/
...Ozarks to Mid MS Valley this afternoon into tonight...
A midlevel shortwave trough over central KS as of late morning will
progress to MO later this afternoon and IL/IN overnight, and will be
preceded by a surface cyclone across northern MO this afternoon into
northern IL/IN tonight. South of the surface cyclone, a cold front
will move eastward/southeastward from eastern KS/OK across MO/IL/AR
through tonight. The focus for severe storms will be along and just
ahead of the cold front, with the surface warm front delineating the
northern extent of the main tornado/wind threat.
Mid 60s boundary-layer dewpoints will continue to spread
north-northeastward in the warm sector from OK/AR to MO/IL through
this evening in advance of the cold front. The moistening will
occur beneath midlevel lapse rates near 8 C/km, and will combine
with daytime heating to support MLCAPE values around 2000 J/kg and
weakening convective inhibition by mid afternoon. Thunderstorm
development is anticipated by 19-21z in broken bands along and 1-2
counties ahead of the cold front (as evidenced by the weak
pre-frontal convection now forming in east/southeast KS), and storms
will move northeastward into the warm front zone in MO and into west
central IL later this afternoon/evening. Additional storm
development will occur southwestward into eastern OK/western AR.
Frontal forcing for ascent typically favors squall line development,
though there is some potential for semi-discrete supercells given
relatively fast storm motions compared to frontal motion, and
substantial cross-frontal deep-layer flow/shear. Long hodographs
(effective bulk shear of 40-50 kt) and some low-level hodograph
curvature (effective SRH 200-300 m2/s2, especially east of the cold
front in the open warm sector) will favor supercells initially with
the potential to produce tornadoes (a few of which could be strong),
in addition to damaging winds and large hail.
The severe threat will transition more to damaging winds by late
evening and early tonight, though supercells will still be possible
into AR, with QLCS mesovortices and some tornado potential farther
north in MO/IL.
...OR coast and vicinity this afternoon...
A deep, occluded cyclone near 46 N and 131 W will move toward
Vancouver, while the associated baroclinic zone moves inland across
the Pacific Northwest coast today. Very strong wind profile/long
hodographs and weak buoyancy close to the coast could sustain an
isolated damaging wind/tornado threat along the coast with
low-topped convection spreading inland, mainly this afternoon.
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