SPC AC 301252

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0752 AM CDT Tue Apr 30 2019

   Valid 301300Z - 011200Z

   ...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS FROM NORTH
   TEXAS/SOUTHERN OKLAHOMA NEAR THE RED RIVER ACROSS EASTERN OKLAHOMA
   TO THE WESTERN OZARKS OF MISSOURI/ARKANSAS...

   ...SUMMARY...
   Severe thunderstorms are possible in a corridor across parts of the
   southern Plains northeastward toward Illinois/Indiana today and
   tonight.  This will include a risk for tornadoes, large hail, and
   locally damaging gusts.

   ...Synopsis...
   The mid/upper-level pattern will remain characterized by a mean
   trough over the western U.S., south of a retrograding cyclone over
   northwestern Canada.  The western troughing will be traversed by
   several shortwaves, the most important for convective purposes being
   located initially over eastern AZ and Sonora.  This feature should
   eject northeastward and lose amplitude through the day, reaching
   western KS and southeastern NM by 00Z, reaching eastern IA as a
   weak/small trough by 12Z.  Meanwhile, a strong shortwave trough --
   evident in moisture-channel imagery over northern/western OR and
   adjacent Pacific waters, will pivot southeastward then eastward
   across the northern Great Basin and southern ID into this evening. 
   By 12Z, a somewhat weaker, positively tilted version of this
   perturbation should extend from central/western WY to central/
   southwestern UT.  A related low-level cold front will move
   southeastward across the Great Basin and western CO through the
   period.

   At the surface, a wavy/quasistationary frontal zone was drawn from
   southeastern NM across the TX Permian Basin, northeastward over
   central OK and southwestern MO eastward and more diffusely to
   portions of WV.  The eastern segment of the front should become
   better-defined through the day over the northern VA/MD area, while
   the boundary moves northward as a warm front over portions of
   eastern MO, IL and IN, and mesoscale oscillations occur (with some
   convective modulation) along the OK section.  The west TX/NM section
   should erode and shift erratically northward through the day.  A
   frontal-wave low may form on the MO segment by 00Z, moving
   northeastward across western/northern IL overnight.  A dryline --
   initially over far west TX and southeastern NM near the front --
   will mix eastward/northeastward today, reaching the southern TX
   Panhandle, west-central TX and northern Coahuila by 00Z.

   ...Southern Plains to IL...
   A complex and somewhat conditional scenario is evident with all
   forms of severe possible, and a wide range of uncertainties and
   possible outcomes.  The most concentrated potential for severe
   weather (tornadoes, hail, gusts) still appears to be in and near the
   enhanced-risk area.

   Given the weak CINH expected and high warm-sector moisture content,
   multiple rounds of thunderstorms are expected to move northeastward
   across this corridor through the afternoon, especially from southern
   OK northward.  Locally dense concentrations of convection and mixed
   storm modes are possible.  Wherever relatively sustained/discrete
   convection can encounter high SRH, such as in the southern OK to
   Ozarks warm sector or near the front in the eastern MO/IL section of
   the outlook, supercellular tornadoes may occur.  Any well-organized
   QLCS encountering strong low-level shear also will offer a locally
   maximized tornado threat.  Isolated to scattered thunderstorms also
   may form on the dryline and/or front over west-central/northwest TX.
    Significant, damaging hail above 2 inches in diameter is most
   probable over southern parts of the outlook swath (southern OK,
   north TX) where low/middle-level lapse rates will be steepest and
   discrete storms most probable, with enough vertical shear for
   supercells.  Storms also may organize along and into the cool side
   of the front, then move across the boundary and ramp up severe
   potential upon encountering low-LCL/high-SRH warm-sector air with
   rich low-level moisture.

   The parameter space along/southeast of the front will support the
   potential for significant tornadoes, given sufficiently sustained/
   discrete supercell(s).  However, the convective scenario still
   appears messy, inconsistently progged, and rather ambiguously
   focused on the mesoscale, as are potential positions of influential
   boundaries such as the front and outflows from morning/midday
   storms.  Some weaknesses also are apparent in the forecast low-level
   hodographs above roughly the lowest km, likely related to the
   geometry of mass response to a positively tilted, weakening
   shortwave trough farther west.  As such, too much spatial
   uncertainty in the threat exists to draw such an area at this time,
   though one may be needed in an update as mesoscale trends warrant.

   ...Central/northern VA/MD region...
   Widely scattered to scattered thunderstorms should develop this
   afternoon along or south of the front, as CINH weakens in a
   diabatically destabilizing air mass.  Isolated damaging gusts, one
   or two of which may be severe, are possible, along with marginal
   hail.  Despite steadily rising heights through the day, this area
   will reside near the southern rim of substantial mid/upper-level
   westerlies, with weak low-level flow.  As such, vertical shear will
   be modest, with effective-shear magnitudes about 35 kt.  Favorable
   surface heating, combined with offsetting effects of moist advection
   and boundary-layer mixing, should result in around 1000 J/kg MLCAPE
   (locally near 1500 J/kg), atop a well-mixed subcloud layer.  The
   parameter space should support mainly multicells, perhaps a
   weak/short-lived supercell or two.

   ...South-central High Plains...
   Isolated to widely scattered thunderstorms should develop this
   afternoon on the higher terrain of northeastern NM and southeastern
   CO, moving across adjoining High Plains with the potential for
   sporadic hail and gusts near severe levels.  Diabatic heating and
   warm/moist advection will aid low-level air mass recovery behind
   morning clouds/convection over the Panhandles.  Large-scale ascent
   will increase with time through the afternoon in any given locale in
   this region ahead of the ejecting shortwave trough, despite the
   trough's Lagrangian weakening trend.  This will help to offset
   marginal low-level theta-e that will weaken with northwestward
   extent, enough to support surface-based effective-inflow parcels
   with MLCAPE 250-500 J/kg over southeastern CO and 1000-1500 J/kg
   over parts of the TX Panhandle.  A mixture of linear and discrete
   modes is possible, with favorable deep shear (effective-shear
   magnitudes 35-50 kt) for supercell organization in any discrete
   storms that do not become outflow-dominant quickly.

   ...Great Basin to western CO...
   Scattered-numerous showers and widely scattered embedded
   thunderstorms are forecast near a low-level cold front that will
   move southeastward across this region, ahead of the northwestern
   mid/upper-level perturbation.  Low-level heating/mixing in the
   prefrontal sector, related steepening of low/middle-level lapse
   rates beneath cooling temperatures aloft, and marginal residual
   moisture, should support the development of MLCAPE in the 300-700
   J/kg range.  This buoyancy will develop atop an "inverted-V"
   thermodynamic profile suitable for at least isolated strong-severe
   gusts as activity expands and shifts eastward across the outlook
   area this afternoon into early evening.  Thereafter, a stabilizing
   boundary layer (with time and eastward extent) will close any
   lingering severe threat.

   ..Edwards/Smith.. 04/30/2019

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