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ACUS01 KWNS 102246
SWODY1
SPC AC 102244

Day 1 Convective Outlook AMEND 1
NWS Storm Prediction Center Norman OK
0444 PM CST Fri Dec 10 2021

Valid 102000Z - 111200Z

...THERE IS A MODERATE RISK OF SEVERE THUNDERSTORMS THIS EVENING
INTO TONIGHT FROM NORTHEAST AR/SOUTHEAST MO INTO WESTERN TN/KY AND
SOUTHERN IL...

AMENDED TO INCREASE SEVERE PROBABILITIES IN NORTHEAST TEXAS AND
SOUTHEAST OKLAHOMA

...SUMMARY...
A few strong tornadoes, damaging gusts, and large hail are all
expected beginning this evening across Arkansas and Missouri, with
the greatest tornado threat close to the confluence of the
Mississippi and Ohio Rivers. The damaging wind and tornado threat
will persist overnight while spreading eastward into the Tennessee
Valley and northeastward across the Ohio Valley overnight.

Northeast Texas: Isolated thunderstorms are developing a bit farther
west across northeast Texas than earlier anticipated. With ample moisture/instability across this region ahead of the front will
increase severe probabilities across this region.

The previous forecast appears to be on track, and little change is
needed at 20Z.

At 19Z, a dryline was located near the I-35 corridor from north TX
into OK, with gradual warming and destabilization noted eastward
into AR. Upwards of 1500 J/kg MLCAPE is already in place there, with
further warming along with dewpoints holding in the upper 60s F.
While capping currently exists over most areas, the deepening moist
boundary layer is forecast become uncapped prior to 00Z over AR, and
this will support possible supercell development with tornado threat
evolving east-northeastward into western KY/TN through the evening.
Otherwise, expanding storm coverage is likely around 03Z along/ahead
of the cold front as large scale lift increases across the MS and OH
Valleys, with very strong shear favoring tornadoes, possibly strong.


For more information on the AR and southeast MO area, see mesoscale
discussion 1978.

..Darrow.. 12/10/2021

.PREV DISCUSSION... /ISSUED 1018 AM CST Fri Dec 10 2021/

...AR/MO this evening to TN/OH Valleys overnight...
A broad midlevel trough approaching the High Plains this morning
will continue eastward to the mid MS Valley overnight. The midlevel
trough is preceded by a remnant lee cyclone that will develop from
KS today to northern MO this evening, with more substantial
deepening overnight toward Lower MI. The surface warm sector will
spread northeastward from I-40 in eastern OK/AR to I-70 in MO/IL by
this evening, with additional expansion of the warm sector into
TN/KY/IN/OH through late tonight. Regional 12z soundings revealed
65-70 F boundary-layer dewpoints in the warm sector this morning,
beneath midlevel lapse rates close to 7.5 C/km. Surface heating in
cloud breaks and continued warm/moist advection through the day will
result in destabilization and weakening of convective inhibition in
the open warm sector and along the cold front by this evening, when
storm initiation is expected.

Deep-layer southwesterly winds will be strong with long hodographs
and some low-level, clockwise curvature in the warm sector
(effective bulk shear in excess of 70 kt and effective SRH in excess
of 300 m2/s2). MLCAPE of 1000-2000 J/kg, combined with the strong
vertical shear, will favor supercells capable of producing a few
strong tornadoes and large hail initially, with increasing storm
coverage and a greater threat for damaging winds overnight (in
addition to a continued tornado threat). A lead shortwave trough
now over the TX South Plains will likely provide the impetus for at
least widely scattered storm development in the open warm sector
from AR into MO by 21-00z, with storms expected increase in coverage
(both along and ahead of the cold front) and move rapidly
northeastward to the OH and TN Valleys overnight.

$$


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ACUS01 KWNS 152259
SWODY1
SPC AC 152257

Day 1 Convective Outlook AMEND 1
NWS Storm Prediction Center Norman OK
0457 PM CST Wed Dec 15 2021

Valid 152245Z - 161200Z

...THERE IS A MODERATE RISK OF SEVERE THUNDERSTORMS FROM IOWA TO
SOUTHEASTERN MINNESOTA AND A PORTION OF WESTERN WISCONSIN...

AMENDED FOR SOUTHWARD EXPANSION OF SLIGHT AND ENHANCED WIND
PROBABILITIES.

...SUMMARY...
Widespread severe wind gusts of 60-75 mph along with at least a few
tornadoes are likely from mid afternoon through early tonight across
the Mid-Missouri Valley to the Upper Mississippi Valley. Embedded
gusts of 75-100 mph and a strong tornado or two are also possible,
particularly from extreme eastern Nebraska across western to
northern Iowa and southeastern Minnesota.

...Discussion...
Moved slight and enhanced wind probabilities southward across
eastern Kansas and Western Missouri with 15 percent wind
probabilities into far northeast Oklahoma. Given the very strong
wind field, anywhere along the convective line has a history of
producing severe wind and therefore, have expanded this to the
southern extent of where both storms and a strong synoptic wind
field are expected.

..Bentley.. 12/15/2021

.PREV DISCUSSION... /ISSUED 1025 AM CST Wed Dec 15 2021/

...A rapidly evolving outbreak of severe storms with widespread
damaging winds and a few tornadoes is expected to begin by mid
afternoon across eastern NE and continue northeastward into the
upper MS Valley by early tonight...

...Mid MO and upper MS Valleys this afternoon into early tonight...
An intense midlevel shortwave trough and associated deepening
surface cyclone will eject rapidly northeastward today from the
central High Plains to the upper MS Valley by tonight. A developing
dryline will surge east-northeastward across KS/NE into western
IA/northwestern MO to the south of the cyclone, prior to being
overtaken by a cold front tonight. An unseasonably moist air mass
is spreading northward from the western Gulf of Mexico, with
boundary-layer dewpoints near or above 60 F already into central IA.
The moistening is occurring beneath a plume of steep midlevel lapse
rates near 8 C/km per regional 12z soundings. Destabilization will
continue northward through the day across eastern NE/IA into
southern MN in advance of the deepening cyclone, with MLCAPE
expected to reach or exceed 1000 J/kg.

Though the elevated mixed layer plume will act to cap the moistening
boundary layer, strong forcing for ascent with the surging dryline
and within the left-exit region of the intense mid-upper jet streak
will quickly remove convective inhibition in a narrow zone along the
dryline by mid afternoon (20-21z), when convective initiation is
likely across east central/southeast NE and northeast KS. Extremely
strong wind profiles will accompany the ejecting midlevel trough,
with speeds approaching 125 kt at 500 mb and 70-80 kt not far above
the ground. Effective bulk shear will also be quite strong at 70-80
kt. The strong linear forcing for ascent and cap will support a
narrow line of storms, though the very strong deep-layer shear
vectors/long hodographs oriented across the boundary, and increasing
low-level buoyancy by mid-late afternoon, should allow some
supercell structures within the band of storms. Downward momentum
transport by the convection will result in the potential for
widespread wind damage with peak gusts potentially in the 75-100 mph
range, and embedded circulations will be capable of producing a few
fast-moving tornadoes (one or two of which could be strong) with
both right-moving supercells and embedded QLCS mesovortices.

Gradually weakening buoyancy with east/northeastward extent should
result in a diminishing severe threat by 06z across WI. The severe
threat will also diminish with southward extent, where forcing for
ascent will be weaker and the orientation of the flow will be much
more parallel to the cold front tonight.

$$


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