SPC AC 161252

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0752 AM CDT Tue May 16 2017

   Valid 161300Z - 171200Z

   ...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS FROM PARTS OF
   WEST TEXAS TO PARTS OF IOWA...

   ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS SURROUNDING THE
   ENHANCED RISK AND EXTENDING TO PARTS OF THE CENTRAL HIGH PLAINS AND
   UPPER MISSISSIPPI VALLEY...

   ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS SURROUNDING THE
   SLIGHT RISK...

   ...SUMMARY...
   Episodes of severe thunderstorms are expected today and tonight
   within a swath from west Texas across the central Plains to the
   western Corn Belt and mid Missouri Valley.  Tornadoes, severe gusts,
   and large, damaging hail are expected within this corridor.

   ...Synopsis...
   Progressive synoptic-scale troughing will dominate the upper-air
   pattern today over the western/central U.S., with two
   primary/embedded perturbations influencing low-level mass fields and
   convective potential:
   1.  A strong, compact, mid/upper-level cyclone -- now apparent in
   moisture-channel imagery over Vancouver Island, forecast to move
   southeastward to southern ID by the end of the period.  As heights
   fall across the northwestern U.S. and Great Basin ahead of that
   feature...
   2.  A strong shortwave trough, with embedded 500-mb low over
   southern NV at this time, will pivot eastward then
   east-northeastward across the Four Corners and northwestern MX.  By
   12Z, the trough should extend from eastern CO across eastern NM and
   far west TX.

   The 11Z surface analysis showed a surface low over northeastern SD,
   warm front from there across northeastern IA and southern Lake
   Michigan, and cold front southwestward across central NE to
   northeastern Co, and a reinforcing cold front over central SD and
   south-central WY.  A wavy dryline was drawn from near IML southward
   across the KS/CO border area to the western TX Panhandle,
   southeastern NM and far west TX.  The SD low should move
   northeastward across MN to northern ON through the period, while a
   second low deepens by 12Z along the combined frontal zone over
   southern NE.  The dryline will mix eastward across western KS, the
   TX/OK Panhandles and South Plains today, and off the Caprock by
   mid/late afternoon.  Cold frontogenesis is expected over eastern NM
   and west TX this evening and overnight as the shortwave trough
   ejects, with the resulting baroclinic zone overtaking the dryline
   from the west tonight.

   Though the combined "slight" and "enhanced" outlook area appears as
   one continuous swath on the map, it mostly represents the spatial
   overlap of three rather distinct severe-storm regimes.  We'll
   emphasize those below, in rough start-time order.

   ...Early afternoon through evening: northern TX/OK Panhandles,
   Western/central KS to IA...
   Widely scattered to scattered thunderstorms should form during
   early-mid afternoon near the dryline where the EML and MLCINH weaken
   soonest.  This activity initially will risk large/damaging hail,
   severe gusts and a few tornadoes.  Within the first few hours after
   initiation, however, a preponderance of guidance from synoptic down
   to high-res/convection-allowing progs indicate aggregation of
   convection into a forward-propagating MCS, which may sweep
   northeastward across KS and the Missouri Valley at least across
   parts of IA through this evening and tonight, before encountering
   enough stable air to weaken.  This convection-evolution scenario
   appears plausible given:
   1.  The geometry of the deep-layer flow,
   2.  Sufficiently weak afternoon/early-evening CINH to be prone to
   forced slab ascent by aggregating cold pools,
   3.  A moist axis already in place to support a corridor of maximized
   buoyancy later today from central KS to IA, southeast of residual
   outflow pools from prior overnight convection, and
   4.  Well-mixed subcloud layers in forecast soundings that support an
   increasing damaging-wind potential with northeastward extent, given
   such modal morphology.

   Activity will be supported by a southwest-northeast plume of
   preconvective MUCAPE ranging from 2000-3500 J/kg, weakening late
   tonight in the northeastern part as CINH increases, along with 35-45
   kt effective-shear magnitude.  Pockets of favorable low-level SRH
   may develop to support some tornado potential even in a QLCS mode,
   whether with embedded supercells or misovortices.

   ...Midafternoon into evening: Southern Plains dryline...
   In mid/late afternoon, several discrete to semi-discrete supercells
   are expected to develop and move northeastward at 25-35 kt off the
   dryline.  As this activity crosses the eastern Panhandle region,
   northwest TX, and parts of western OK, it will offer tornadoes, very
   large/damaging hail and sporadic severe gusts.  Some giant (3-inch
   or more) hail is possible, along with potential for a couple of
   strong, relatively long-lasting tornadoes, from the mature phases of
   at least one or two cyclic supercells within convective arc.  The
   all-hazards threat from these storms should last at least an hour or
   two into early evening, when the low-level jet increases and
   enlarges hodographs, and before the boundary layer stabilizes too
   much to permit sustained surface-based inflow.  Thereafter, these
   supercells should spin down over western OK and northwest TX, south
   of the relatively fresh/cold outflow from the KS complex.

   Te environment will be characterized by strong surface heating and
   mixing along the dryline, with forecast soundings suggesting:
   near-zero MLCINH as early as around 20Z, 60s F surface dew points
   beneath steep midlevel lapse rates, 2500-3500 J/kg MLCAPE in a
   narrow plume ahead of the dryline, 60-65 kt effective-shear
   magnitude, and afternoon effective SRH 150-250 J/kg increasing to
   near 400 J/kg along the western rim of the 40-55 kt evening LLJ. 
   Winds aloft should be oriented with enough of an orthogonal
   component to the dryline to support some discrete storms, though a
   few forecast hodographs show evidence of slight veer-back-veer
   weaknesses in the 2-4-km AGL layer.  CINH will strengthen
   considerably eastward toward the US-81 corridor ahead of this
   activity and ahead of the main plume of large-scale forcing arriving
   later overnight.  This should lead to late-evening demise of most of
   these afternoon/dryline supercells.

   ...Overnight:  Southern Plains front/dryline...
   A mixture of initially discrete supercells and a band of
   thunderstorms should develop along the dryline and the overtaking
   front this evening and tonight.  Any supercells will pose the risk
   of large, damaging hail, given the available moisture, large lapse
   rates/buoyancy, and favorable deep shear.  Frontogenetic forcing and
   large-scale DCVA/cooling aloft will impinge on the moist sector and
   lead to a mostly quasi-linear belt of convection overnight, with
   damaging wind becoming the dominant threat before the activity
   encounters increasing CINH and weakening LLJ-related storm-relative
   flow late overnight.  A tornado or two cannot be ruled out
   throughout either phase as well.

   ..Edwards/Dial.. 05/16/2017

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