SPC AC 061301

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0701 AM CST Mon Mar 06 2017

   Valid 061300Z - 071200Z

   ...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS OVER THE OZARKS
   REGION OF MISSOURI...OKLAHOMA AND ARKANSAS...

   ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS ELSEWHERE FROM
   EASTERN OKLAHOMA AND ARKANSAS TO CENTRAL MINNESOTA AND WESTERN
   ILLINOIS...

   ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS SURROUNDING THE
   SLIGHT RISK FROM THE ARKLATEX TO NORTHERN/WESTERN MINNESOTA...

   ...SUMMARY...
   Severe thunderstorms are expected starting late this afternoon and
   expanding across the upper and middle Mississippi Valley to the
   Ozarks and Mid-South.  Damaging wind gusts, large hail and a few
   tornadoes are possible.

   ...Synopsis...
   The upper-air pattern begins with synoptic-scale troughing from the
   AK Panhandle to the northern Rockies, Four Corners and northwestern
   MX.  A strong/basal shortwave trough now located over UT will phase
   with another currently over portions of eastern MT, WY and CO, as
   they collectively shift eastward across the central/northern Plains
   through the period.  By 00z, the northern perturbation should evolve
   into a closed 500-mb cyclone over central/eastern ND, with the
   combined shortwave trough arching southeastward over the FSD area
   then southwestward across western KS.  By 12Z, the cyclone should
   eject northeastward to northwestern ON near the MB border, with
   associated vorticity lobe arching across Lake Superior, eastern WI,
   northern MO, and eastern KS.  The height trough will extend
   southwestward over west TX by that time. 

   At the surface, the main low was analyzed at 11Z between PIR and
   HEI, with a Pacific cold front southward across the NE Sandhills
   then southwestward over central NM.  An arctic front extended
   southwestward over western WY.  A dryline was drawn from
   east-central SD to west-central KS, the eastern TX Panhandle, the
   southeastern corner of NM, and northern Coahuila.  By 00Z, the
   surface low should deepen to around 978-980 mb and move to near the
   northern terminus of I-29 where MB, MN and ND join.  The arctic and
   Pacific fronts will merge from north to south, with arching
   southeastward from the low over MN and northern IA, then
   southwestward over eastern KS, northwestern OK and the TX Panhandle,
   having overtaken some of the dryline into eastern KS.  The dryline
   should extend from there south-southwestward across central OK to
   the Edwards Plateau of TX.  By 12Z, the surface cyclone will be
   stacked with its manifestation aloft, and the cold front should
   extend from lower MI across IN, southeastern MO, central AR, and
   central TX. 

   ....Ozarks, mid/upper Mississippi Valley to Mid South...
   Scattered thunderstorms should develop along and ahead of the cold
   front late this afternoon between eastern KS and central/western MN,
   increasing in coverage and backbuilding southward into eastern OK
   this evening, while an initially separate area of convection
   develops in a low-level warm-advection/confluence zone from eastern
   OK/western AR at least into northern MO.  Each will shift eastward
   with time, offering the potential for all severe modes. 

   The CAPE/shear parameter space should become favorable this
   afternoon and evening in a narrow, triangular corridor of warm
   sector, tapering with northward extend from eastern OK and AR
   northward to southern MN and western WI.  Surface dew points are
   expected to reach the 50s F over the upper Mississippi Valley area,
   with 60s from the lower Missouri Valley region southward.  This, in
   conjunction with diurnal heating, boundary-layer warm advection, and
   cooling aloft preceding the shortwave trough, should support MLCAPE
   from around 2000 J/kg in the western Ozarks region to around 500
   J/kg in north-central MN.  Forecast soundings reasonably suggest
   that, amidst strengthening deep-layer winds with time, 55-65-kt
   effective-shear magnitudes should become common along/ahead of the
   cold front, with effective SRH commonly in the 250-400 J/kg range.

   Within that corridor, the two regimes discussed above should support
   at least a short temporal window of discrete-supercell potential:
   1.  Along the front for a brief maturation period before the frontal
   convection evolves to more of a QLCS configuration, or (on the
   northern/MN end) the mean-wind vector is less parallel to the front,
   but activity moves quickly across the very narrow warm sector into
   more stable air.  The steepest lapse rates and largest buoyancy will
   be available to this process, with any sustained/discrete cells
   offering the most large-hail and tornado risk. 
   2.  The prefrontal warm-advection plume, which will develop near the
   eastern rim of the surface-based effective-inflow parcels in weaker
   but still sufficient surface-based instability. This activity also
   will exit the favorable buoyancy sooner than in the frontal regime. 

   Collectively, the greatest concentration of severe should be in or
   near the enhanced risk, corresponding to the most probable area
   affected by both regimes over the longest period of time. 

   Overnight, as the main band of thunderstorms moves eastward past the
   Mississippi Valley, it should overtake the northern part of the
   warm-advection band and weaken on the northeastern end; then the
   zone of weakening zippers southward with time.  This will take place
   as the eastern theta-e/CAPE gradient effectively collapses southward
   down the Mississippi Valley, on the western fringes of more poorly
   modified continental/polar trajectories, and convection outruns
   favorable surface-based buoyancy.  With the northern warm sector
   getting "pinched off" in such a manner late tonight and early
   tomorrow morning, the zone of most favorable severe potential will
   shift into southeastern OK, southeastern MO, AR, the Arklatex
   region, and the Mid South.  Damaging wind will become the main
   threat with a mostly linear mode, though
   embedded/short-lived/small-scale circulations may be capable of a
   brief tornado or two.

   ..Edwards/Peters.. 03/06/2017

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