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SPC AC 141257

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0757 AM CDT Thu Mar 14 2024

   Valid 141300Z - 151200Z


   Tornadoes, large to very large hail, and strong to locally severe
   thunderstorm gusts are expected from parts of north-central Texas
   into the Midwest and Mid-South.  The greatest overall severe threat
   still appears to be across eastern Oklahoma into Arkansas and
   southern Missouri.

   In mid/upper levels, the pattern will slow considerably and become
   blocky over the western CONUS, as a closed cyclone continues to
   retrograde south-southwestward across the southern Great Basin and
   southern CA.  The associated/cut-off 500-mb low should stall near
   the Imperial Valley by 12Z tomorrow, with troughing southward over
   Baja and northeastward over southwestern WY.  A broad swath of
   southwest flow aloft will extend downstream of the cyclone, across
   the southern Plains, lower/mid Mississippi Valley and lower Ohio
   Valley.  Weak synoptic-scale height rises are expected over much of
   OK, AR, east/north TX, and LA, in response to the cyclonic
   retrogression farther west.

   At the surface, 11Z analysis showed a low near TOP with cold front
   southwestward across northwestern OK to the TX Panhandle, and warm
   front over northern portions of MO/IL/IN to central OH.  A dryline
   was drawn from central OK across west-central TX.  The dryline is
   expected to move only slightly eastward through the day, being
   positioned over central/east-central to south-central OK and north-
   central/central TX during mid/late afternoon.  A combination of the
   cold front and convective outflow will overtake the dryline over MO
   and OK by late afternoon and evening, and the front should catch the
   dryline over north TX overnight.  Meanwhile, a sequence of
   convective outflows -- already underway across parts of the Midwest
   and forecast to continue through the day -- will shunt the effective
   baroclinic zone and best regional convective foci south of the
   synoptic warm-frontal position.

   ...Ozarks to north TX and Mid-South...
   Scattered thunderstorms should develop as early as midday along and
   east of the OK/TX dryline, and increase in coverage/intensity
   through the evening, while shifting eastward into AR, perhaps
   reaching parts of the Mid-South and Tennessee Valley regions
   overnight.  Supercells with tornadoes (some strong), locally very
   large/destructive hail, and severe gusts will be possible this
   afternoon into early evening.  Activity should evolve into one or
   two dominant quasi-linear segments with the threat trending more
   toward wind and sporadic QLCS/embedded tornadoes with time this
   evening and overnight.

   Continued theta-e advection east of the dryline, amidst strong
   diurnal heating, will erode MLCINH, which already did not appear
   particularly strong in the 12Z OUN sounding.  Modified RAOBs and
   forecast soundings over eastern OK this afternoon suggest peak/
   preconvective MLCAPE reaching the 2500-3000 J/kg range.  Increasing
   inflow-layer moisture beneath favorable deep shear (effective-shear
   magnitudes 45-55 kt) yield hail over 3 inches in diameter when 2D
   hail models and historical analogs are applied.  The duration of the
   threat, as well as the potential for tornadoes once hodographs
   enlarge greatly in late afternoon/evening, will depend on the number
   of relatively discrete supercells remaining.  That is uncertain,
   given a substantial component of deep-layer flow parallel to the
   likely corridor of greatest forcing, which suggest a transition to
   messier convective modes.  However, at least a few tornadoes are
   possible, including those with strong damage potential.  Upscale
   evolution to an MCS appears possible this evening into tonight,
   spreading toward the Mid-South/Tennessee Valley regions.

   A separate area of convection also may develop this afternoon into
   early evening east of the modest EML plume, over parts of the Mid-
   South/Delta region.  The environment will be characterized by rich
   low-level moisture (dewpoints increasing into the upper 60s to low
   70s F), subtle mass confluence/convergence, and weak CINH.  While
   low-level flow/shear will be modest, enough mid/upper-level flow
   will spread over the area to support a mix of multicell and
   supercell modes, with damaging gusts, large hail possible, and a
   marginal tornado threat.  Forecast soundings reasonably depict a
   deep troposphere occupied by MLCAPE increasing into the 2000-3000
   J/kg range, and around 30-40 kt effective-shear magnitudes. 
   Ultimately, the main area of convection from the west and northwest
   also may shift into this area overnight, overtaking any remaining
   warm-sector activity and bringing severe probabilities as well.

   ...Midwest/Ohio Valley region...
   Bands of thunderstorms with trailing, progressive outflows are
   ongoing across portions of IL/MO.  This activity should continue
   offering sporadic severe gusts and isolated potential for an
   embedded tornado or two, while moving rapidly east-northeastward
   this morning.  Severe-gust and tornado threats may focus best along
   the warm front and leading outflow boundary.  Those boundaries are
   where low-level convergence/vorticity will be maximized.  However,
   severe potential also may be tempered regionally by the early-
   morning instability minimum in the boundary layer.  See SPC severe
   thunderstorm watches 39-40 and related mesoscale discussions for
   near-term details.  Some re-intensification of the combined complex
   may occur in parts of IN and perhaps KY today, as it encounters a
   plume of diurnal and warm-advection-related destabilization, before
   outrunning that plume.

   Farther west and southwest across portions of eastern/southeastern
   MO and up the lower Ohio Valley, a conditionally concerning threat
   is apparent for tornadoes, large to very large hail and severe wind
   from another potential round of convection this afternoon and
   evening.  Where the environment is undisturbed by (or can recover at
   least mostly from) morning convection, strong deep-layer shear and
   favorable moisture/destabilization will support potential for long-
   lasting supercells and bowing clusters.  Along and within an ill-
   defined distance north of the residual, diurnally destabilized
   outflow boundary, enlarged low-level hodographs and maximized
   boundary-layer shear/vorticity should concentrate tornado potential.
   At this time, however, with
   1.  Outflow still being newly produced, and
   2.  Its depth/character not well-resolved yet by higher-resolution
   convective guidance, mesoscale uncertainties over the position and
   character of the boundary remain too great to assign larger
   unconditional risk at this point.  A focused corridor of enhanced-
   level probabilities may need to be introduced once the timing and
   location of the near-boundary threat become clearer.

   ..Edwards/Dean.. 03/14/2024


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