SPC AC 211251

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0751 AM CDT Fri Jun 21 2019

   Valid 211300Z - 221200Z

   ...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS ACROSS THE
   LOWER MISSOURI AND MID MISSISSIPPI VALLEY REGIONS...

   ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS OVER PARTS OF THE
   CENTRAL HIGH PLAINS AND EASTERN DAKOTAS...

   ...SUMMARY...
   The greatest potential for severe weather today will be with
   thunderstorm complexes moving across the lower Missouri and mid
   Mississippi Valley regions, with damaging wind as the main threat.

   ...Synopsis...
   On the large scale, the North American segment of the hemispheric
   upper-air pattern will be characterized by mean troughs over western
   Canada and the U.S. Western States, a ridge from the Gulf to Hudson
   Bay, and slowly progressive troughing from QC across the U.S.
   Atlantic Seaboard.  A substantial cyclone embedded within the
   western trough was centered over central MT initially, and is
   forecast to pivot eastward then northeastward across eastern MT to
   southern SK through the period.  A broad fetch of southwesterly flow
   aloft will spread across the central/southern Rockies and Great
   Plains, between that cyclone and the aforementioned ridge.

   At the surface, 11Z analysis showed an occluded surface low near the
   MT/SD/ND border confluence, with weak front and surface trough
   southeastward to lows near BBW and near the CO/KS line northeast of
   SPD.  A cold front connected the latter lows and then arched
   westward over southern/western CO.  A warm front was drawn from
   northern MO southeastward across the Mid-South to northern AL.  The
   northern surface low will move slowly northward, gradually becoming
   vertically stacked with the mid/upper-level cyclone tonight.  The
   southern surface low should migrate generally eastward today near
   the outflow boundary from ongoing convection over northern MO and
   the KS/NE border region.

   The surface trough/occluded front should arc across the Dakotas and
   NE, shifting generally eastward to east-northeastward across those
   states through the period.  The warm front should shift northward
   across the Tennessee and lower Ohio Valleys, modulated to some
   extent by MCS activity.  The cold front should move southward to
   southeastern CO by 00Z, and from central KS to east-central/
   northeastern NM by 12Z.  A dryline -- initially analyzed from the
   Trans-Pecos region of far west TX across southeastern NM to the
   northeastern TX Panhandle -- should mix eastward this afternoon to
   southwestern OK, northwest TX, the southern Permian Basin region,
   and northwestern Coahuila.

   ...Lower Missouri/mid Mississippi/lower Ohio Valley regions...
   Two strong-severe bands of convection have developed -- the first a
   longer-lasting MCS now located across parts of southern IA,
   northeastern MO and extreme eastern KS, the second an increasingly 
   organized, expanding complex over southeastern NE and north-central/
   northeastern KS.  For near-term guidance on severe potential with
   these convective complexes, refer to SPC severe thunderstorm watch
   414 and related mesoscale discussions.

   The trailing complex will be mostly atop the cold pool from the
   leading one for a few more hours.  Still, given the more-shallow
   nature of the leading outflow with southern extent, and the strong
   pressure perturbation accompanying the second complex, it has the
   potential to punch at least isolated damaging to severe gusts to the
   surface.  Full merger of the cold pools' leading sectors is probable
   at some point through midday.  Severe wind potential will be
   maintained or increase as the combined cold pool and isallobaric/
   theta-e anomaly impinge on a destabilizing, surface-based inflow
   sector to its east and southeast, along and south of the warm front
   zone.  Rich low-level moisture, diurnal heating, and favorably steep
   low/middle-level lapse rates will contribute to the development of
   3000-4500 J/kg warm-sector MLCAPE, decreasing northward through the
   warm-frontal zone.

   ...Central High Plains, post-frontal...
   Widely scattered to scattered thunderstorms, including a few
   supercells, are possible as upslope component of post-frontal
   low-level flow increases into the higher terrain of the Foothills
   and Palmer Divide this afternoon.  Convection then should move
   generally eastward across parts of eastern CO and the adjoining
   areas of northwestern KS/southwestern NE.  Although buoyancy will be
   modest, related to lack of more-robust boundary-layer theta-e, areas
   of 500-800 J/kg MLCAPE are expected (locally higher).  Strong
   veering of winds with height will contribute to enlarged low-level
   hodographs/SRH for eastward-moving supercells, and effective-shear
   magnitudes in the 50-60-kt range.

   ...Northern Plains...
   Scattered thunderstorms are possible by late afternoon, in a narrow
   band or arc corresponding closely to the location of the occluded
   front/surface trough, where low-level convergence will be maximized.
    Large hail and strong-severe gusts are possible, and at least a
   marginal tornado threat may be realized.

   Though some diurnal/diabatic destabilization is likely in a corridor
   immediately preceding the boundary, CAPE over much of central/
   eastern NE and portions of SD will be substantially limited by the
   presence of the extensive, doubly developed cold pool to its south
   and southeast (upstream in low levels).  Remaining portions of the
   eastern Dakotas, under colder air aloft and farthest from the
   central Plains/Missouri Valley air mass stabilized by the MCS
   activity, will have the greatest magnitude and width of favorable
   instability, with MLCAPE reaching to near 1500 J/kg.  Backed
   near-surface winds will contribute to 35-45-kt effective-shear
   magnitudes and 150-250 J/kg effective SRH, supporting some supercell
   potential.

   ...Southern Plains dryline...
   Widely scattered to isolated, slow-moving thunderstorms are possible
   for a few hours late this afternoon into early evening, offering
   localized severe gusts and hail.  The threat should diminish
   markedly from sunset onward.

   Strong boundary-layer heating and mixing are expected this afternoon
   as sustained near-solstice insolation drives surface temperatures
   into the upper 90s to lower 100s F, enough to offset basal EML
   capping briefly and reach convective temperature.  Lift along the
   dryline will support high-based convective towers, some of which may
   mature into cumulonimbi long enough to produce strong-severe
   downbursts or marginal hail amidst 3000-4500 J/kg MLCAPE.  Weak
   deep-tropospheric flow fields and related lack of shear will limit
   storm organization.  Diabatic cooling near sunset should boost
   MLCINH quickly and weaken dryline lift, contributing to diminishing
   convective coverage/intensity.

   ..Edwards/Mosier.. 06/21/2019

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