SPC AC 101248

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0748 AM CDT Sun Oct 10 2021

   Valid 101300Z - 111200Z

   ...THERE IS A MODERATE RISK OF SEVERE THUNDERSTORMS OVER PORTIONS OF
   CENTRAL/EASTERN OKLAHOMA...

   ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS OVER PARTS OF
   NORTHERN MINNESOTA...AND SURROUNDING THE NESTED MODERATE AND
   ENHANCED AREAS OVER THE SOUTHERN PLAINS...

   ...SUMMARY...
   The greatest threat for severe weather will be over parts of
   Oklahoma and north Texas this afternoon and evening.  Tornadoes
   (some capable of significant/EF2+ damage), destructive hail, and
   severe gusts all are possible.

   ...Synopsis...
   A mid/upper-level mean trough will remain over western North America
   through the period, with an active, progressive and intense
   synoptic- to subsynoptic-scale wave train fostering severe-storm
   potential.  An initially low-amplitude shortwave trough -- evident
   in moisture-channel imagery over the northeastern Pacific southwest
   of the AK Panhandle -- will amplify considerably through the period
   as it moves rapidly southeastward to northern CA and OR.  A large,
   related area of height falls will spread across the Great Basin and
   Intermountain West late in the period, portending the major
   mid/upper cyclogenesis discussed in the day-2 and day-3 outlooks.

   As that occurs, two strong mid/upper lows/troughs will eject
   northeastward away from the mean-trough position and directly
   influence today's and tonight's severe-weather episodes:
   1.  A leading, compact cyclone -- initially located over the eastern
   Dakotas and MN, and centered near the three states' common point. 
   The associated 500-mb low should move to near HIB by 00Z, then eject
   northeastward over northwestern ON.
   2.  A well-defined, intense trough now apparent from west-central CO
   across the Four Corners to south-central/southeastern AZ.  This
   feature will pivot eastward to the southern High Plains through 00Z,
   becoming less positively tilted.  By 12Z, the trough should arc from
   an attached 500-mb low near MKC northwestward toward FSD, and
   southwestward to near DFW.

   At the surface, 11Z analysis showed a low over eastern ND, with
   occluded front southeastward over western/southern MN to a triple
   point near RST.  The warm front extended from there east-
   southeastward toward TOL.  The cold front was drawn across
   central/southwestern IA to south-central KS and northwestern OK, to
   a low over the eastern TX Panhandle, and further, to eastern NM.

   The northern low will remain nearly vertically stacked with respect
   to the mid/upper cyclone center, and move to northwestern ON through
   tonight.  By 00Z, the occluded/cold front will advance to western
   Lake Superior, central IA, east-central KS, central/southwestern OK,
   and the TX Permian Basin, overtaking the dryline from north to south
   from then onward into tomorrow morning.  Related to the southern
   mid/upper-level trough, the southern low will become better-defined
   this afternoon over northwest TX, moving northeastward across
   central/northeastern OK this evening and tonight while
   strengthening, then reaching west-central MO by 12Z tomorrow.  By
   that time, the cold front should be near a line from GRB-MLI-SZL-
   FSM-CRS-SAT, then southwestward over northern Coahuila.

   ...Southern Plains...
   A classic "second season" severe event appears to be in order for
   parts of the Southern Plains this afternoon and evening, featuring
   tornadoes, large hail and severe gusts.  The greatest threat will be
   bound on the north and west by the cold front and dryline, covering
   parts of OK and north TX.  Severe potential should persist into
   tonight toward the Ozarks/Ouachitas, Arklatex region and parts of
   north and central TX.

   For much of the day, the boundary layer across the southern Plains
   near the front and dryline will remain capped, restricting
   substantial vertical mixing to the boundary layer, and permitting
   moist advection amidst heating/destabilization.  This capping will
   be maintained into early/mid afternoon as a modest EML and some
   relatively warm midlevel temperatures move over the area, downstream
   from -2 to -4 deg C 500-mb temperatures analyzed over northwest
   through southwest TX and northern MX at 00Z last night.  However,
   strong height falls and cooling aloft, along with strengthening
   mid/upper flow and deep shear, will occur mid/late afternoon ahead
   of the strong mid/upper trough.  A north-south corridor of peak/
   preconvective MLCAPE around 1500-2000 J/kg should develop from
   central TX northward to southwestern/central OK.  Meanwhile frontal
   convergence/lift near and northeast of the dryline should act on
   diurnally weakening MLCINH.

   By the time of deep convective development (likely in the 22-00Z
   window), the parameter space over portions of southwestern/central
   and south-central OK ahead of the front and dryline will support
   supercells, with surface dew points into the mid 60s F, 50-60-kt
   southwesterly effective-shear vectors, and a strengthening LLJ that
   will enlarge hodographs.  Effective SRH should be favorable from the
   outset of first storms' maturity, increasing into the 300-450 J/kg
   range this evening.  This will support a cyclic tornado threat from
   any sustained, discrete supercells, which also will offer a
   damaging-hail and severe-gust hazard.  The big hodographs also will
   persist eastward after activity evolves to more quasi-linear modes
   and backbuilds into parts of north TX, supporting continued tornado
   potential.  Damaging-gust threats should expand eastward and
   southward as the convection does likewise this evening.

   ...Upper Great Lakes, Upper Midwest...
   The low- and middle-level air masses are expected to destabilize
   across extreme northern MN and parts of the Arrowhead behind earlier
   clouds/convection, as the mid/upper-level circulation approaches.  A
   combination of DCVA/cooling aloft, and pockets or narrow ribbons of
   at least some diurnal heating near the occluded front, will steepen
   lapse rates.  With minimal MLCINH anticipated, one or more arcs of
   convection should develop near and northeast through east of the
   surface low.  This activity may pose a threat for a few tornadoes,
   as well as large hail and damaging gusts, before crossing the
   Boundary Waters region and entering ON.  At this time, development
   appears probable south of the international border, potentially
   becoming supercellular as the convection interacts with backed
   low-level winds, related locally enlarged hodographs, and relatively
   maximized boundary-layer vorticity near the boundary.  A narrow,
   generally front-parallel plume of 500-1200 J/kg MLCAPE may be
   available, with effective SRH in the 150-300 J/kg range.

   The environment farther southeast across parts of WI and upper MI
   will be at least marginally favorable from a parameter-space
   perspective.  However, low-level lift will be weaker, and convective
   coverage uncertain but likely much less -- rendering the severe
   potential very conditional.  Any sustained convection that does form
   in those areas will be capable of all severe modes, especially late
   afternoon into early evening.

   ..Edwards/Gleason.. 10/10/2021

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