SPC AC 051338

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0838 AM CDT Sun May 05 2019

   Valid 051300Z - 061200Z

   ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS 1.  OVER THE
   NORTHEASTERN NORTH CAROLINA/SOUTHEASTERN VIRGINIA AREA...2.  FROM
   THE LOWER PECOS VALLEY TO THE EASTERN PANHANDLE OF TEXAS...AND 3.
   FROM PARTS OF SOUTHEASTERN NEBRASKA TO SOUTHWESTERN/CENTRAL IOWA...

   ...SUMMARY...
   Scattered severe storms capable of hail and wind are most probable
   from west Texas into eastern Nebraska and southwest Iowa, and over
   parts of southeastern Virginia and North Carolina.

   ...Synopsis...
   A fairly progressive, mostly low-amplitude upper-air pattern will
   prevail through the period over the CONUS, except for a well-defined
   cyclone now apparent in moisture-channel imagery west of the central
   CA coastline.  The associated 500-mb low should move slowly eastward
   across the Pacific today and onshore late overnight.  This will
   shift downstream ridging slowly eastward toward the central/southern
   Rockies, with mid/upper-level heights subtly rising over the
   central/southern Plains through most of the period.  Farther
   downstream, a shortwave trough was evident over eastern portions of
   OH/KY/TN and across AL to the Gulf Coast of southeastern LA/MS. 
   This feature should move east-northeastward today, reaching
   central/eastern PA, VA, the Carolinas and northern GA by 00Z.

   At the surface, an area of low pressure was evident on frontal zone
   roughly along the I-81 corridor between CHO-TRI, with cold front
   southwestward over southern AL to near the Mississippi River mouth. 
   The low should sharpen over central/southern VA later this morning
   and move eastward, crossing the Delmarva Peninsula late afternoon/
   early evening, before deepening offshore tonight.  The accompanying
   cold front is expected to move eastward across the Carolinas/VA
   through the period, reaching southern GA and the northeastern Gulf
   by 12Z.

   Another cold front was analyzed initially from a low near EAU
   southwestward across southeastern SD to near AIA, then across
   northeastern WY.  This front is forecast to move southeastward/
   southward by 00Z to eastern lake Superior, central/northern WI,
   western IA, south-central/southwestern NE, and southeastern WY.  By
   12Z the front should reach parts of Lower MI, southern IA, northern
   KS, and northeastern CO.  A dryline to its south will mix eastward
   across the central/southern High Plains today, before some segments
   of it are muddled by convective outflows, and other/unaffected
   segments retreat westward overnight.

   ...NC/southern VA...
   Widely scattered to scattered thunderstorms are expected this
   afternoon, with damaging gusts and sporadic hail possible.  The
   tornado threat appears more marginal/conditional, given the lack of
   enlarged low-level hodographs amidst nearly unidirectional wind
   profiles; however storm-scale/boundary interactions and slightly
   backed surface flow just ahead of the low may support one from a
   supercell taking advantage of favorable boundary interactions.

   The boundary layer will need several hours to destabilize behind
   morning clouds/precip across this region.  Lapse rates aloft will
   remain marginally favorable, recovering slightly behind the early
   activity with glancing/subtle vertical-motion influences from:
   1.  DCVA ahead of the shortwave trough, and
   2.  The right-entrance region of a cyclonically curved jet segment
   located over the northern Mid-Atlantic and New England.
   Still, during mid/late afternoon, sufficient strength/duration of
   surface heating should occur to virtually eliminate MLCINH,
   fostering steep boundary-layer lapse rates and around 1000-1500 J/kg
   MLCAPE.  40-50 kt effective-shear magnitudes should be common,
   supporting some supercell potential.

   ...NE/IA and vicinity...
   Widely scattered to scattered thunderstorms are expected along/ahead
   of the surface cold front this afternoon into early evening,
   offering damaging to severe gusts and sporadic large hail.  Surface
   heating, near a moist axis characterized by 50s F surface dew
   points, should remove MLCINH and boost buoyancy to values favorable
   for strong-severe convection.  Forecast soundings suggest peak
   preconvective MLCAPE ranging from 1500-2000 J/kg over southern NE to
   around 1000-1500 J/kg in central IA, with the corridor of supportive
   MLCAPE becoming weaker/narrower from there into WI.  Low-level winds
   are forecast to remain weak, but clusters or bands of convection may
   organize and shift southeastward away from the frontal zone with
   time.

   ...Southern/central High Plains...
   Isolated to scattered thunderstorms will develop near the dryline
   today and move eastward to southeastward into a moistening boundary
   layer. Some of this activity will evolve into supercells with large
   hail and damaging gusts possible.  Significant/2-inch+ hail may
   occur from the most intense cores, but the focus for that potential
   appears too nebulous over a broad swath of the region  to assign a
   targeted sig-hail area pending more precise mesoscale developments. 
   Only marginal tornado potential is apparent given the lack of
   stronger low-level moisture, and ridging aloft with related lack of
   stronger deep shear.  Still, a tornado cannot be ruled out,
   especially for any relatively discrete/sustained supercells lasting
   into somewhat larger hodographs and lower LCL this evening.

   The prospective inflow-layer air mass over the southern Plains has
   been modified heavily by several days of antecedent convection, but
   enough residual moisture exists to support severe potential, which
   should be most concentrated in the 15%/slight area.  An axis of 60s
   surface dew points was evident from the Permian Basin southward this
   morning, grading to 50s northward through the TX/OK Panhandles. 
   Moist advection east of the dryline, combined with diurnal heating
   and steep midlevel lapse rates, should contribute to preconvective
   MLCAPE in the 2000-3000 J/kg range, amidst 40-50 kt effective-shear
   magnitudes.  Hodographs are forecast to be well-curved but not
   particularly large, especially in the 2-3 km layer. Still, the CAPE/
   shear parameter space supports a few severe supercells between the
   Big Bend and the OK Panhandle, and a small MCS or two may evolve
   upscale from them into the evening and move east/southeast over the
   outlook area before weakening tonight.

   ...Elsewhere in the Southeast...
   Widely scattered to scattered thunderstorms are possible virtually
   anywhere along/ahead of the surface front over the rest of the
   Southeast, offering the potential for damaging gusts and marginally
   severe hail.  Rich boundary-layer moisture/theta-e and diabatic
   surface heating will offset weak midlevel lapse rates enough to
   produce 1000-1500 J/kg MLCAPE, locally/briefly near 2000 J/kg. 
   Boundary-layer flow will be veered enough to yield nearly
   unidirectional deep-layer vertical wind profiles, except near
   mesobeta-scale to localized boundaries (outflows and sea breezes
   mainly).  Still, mid/upper winds will be strong enough to yield
   effective-shear magnitudes 30-45 kt in support of some storm
   organization south of the 15% probabilities.

   Farther south along the FL sea breeze, especially across Atlantic
   coastal central/southeast FL, sufficient propagational component to
   storm motion may encourage cells to deviate sharply rightward along
   the boundary during maturity.  This potentially maximizes ingestion
   of associated low-level vorticity and support at least transient
   supercellular potential.  The scenario would be very dependent on
   storm-scale processes and relatively discrete storm mode, given the
   modest ambient hodographs, but marginal tornado probabilities are
   added to account for this conditionality.

   ..Edwards/Smith.. 05/05/2019

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