SPC AC 301249

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0749 AM CDT Tue Jun 30 2020

   Valid 301300Z - 011200Z

   ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS FROM
   CENTRAL/EASTERN NEBRASKA AND WESTERN IOWA TO EASTERN/CENTRAL NORTH
   DAKOTA...

   ...SUMMARY...
   Thunderstorms with large hail and damaging wind are possible this
   afternoon through tonight across parts of eastern Nebraska, the
   Dakotas and western Minnesota/Iowa.

   ...Synopsis...
   The upper-air pattern over the CONUS will be dominated by slow-
   moving cyclones over New England and the northern Rockies region. 
   The latter, substantially larger circulation, will evolve through
   the period from a merger of two initially separate vorticity
   lobes/circulation centers:  the first now over western MT, and the
   second over central BC.  As the southern lobe moves northward to
   southern AB, a basal shortwave trough -- now apparent in moisture-
   channel imagery from southeastern ID to northern AZ -- will eject
   northeastward and assume negative tilt.  By 00Z, this trough should
   extend across central MT, eastern WY and eastern CO, with meridional
   500-mb flow downstream across much of the Dakotas and western/
   central NE.  By 12Z, the trough should weaken and maintain negative
   tilt, reaching western ND, central/eastern SD and southwestern MN. 
   Meanwhile, a small shortwave trough and embedded MCV should stay cut
   off from the prevailing westerlies aloft, and as such, stay
   quasistationary over western IL.

   At the surface, 11Z analysis showed a series of weak lows along an
   occluded/cold front (reinforced on the mesoscale by antecedent
   outflow) extending from northwestern ND across western SD, western
   NE, and east-central/central CO.  With additional convective
   reinforcement of its baroclinicity, the cold-frontal zone will shift
   slowly eastward in the net across the Dakotas and central High
   Plains through the period.  By 12Z the frontal zone should reach
   east-central portions of the Dakotas, NE and KS, to the TX Panhandle
   and northeastern NM.  A warm to stationary frontal zone was drawn
   across west-central to northeastern SD, western IA, northern MO,
   southern IL, and middle TN, becoming a cold front across extreme
   northern GA and northern NC to a low over Atlantic waters near ACK. 
   A frontal-wave low may develop through the period under the low/
   middle-level MCV over western IL, while the accompanying boundary
   only experiences slight mesoscale perturbations either way, under
   weak mid/upper influences.

   ...Central/northern Plains region...
   Scattered thunderstorms are forecast to develop near the cold front
   this afternoon, increasing rapidly in coverage into the evening. 
   Initial, discrete to semi-discrete convection will offer the
   greatest potential for large hail.  With flow aloft being nearly
   parallel to the formative zone of low-level ascent, convection
   should aggregate rather readily into quasi-linear mode, with the
   predominant severe threat becoming damaging wind.  A tornado or two
   cannot be ruled out, though with the strongest flow aloft being
   displaced behind the cold front, ambient deep shear will be modest,
   indicating supercellular modes should be transient and likely
   heavy-precip in character.

   The warm sector will remain very moist, supplemented near the
   surface this afternoon by evapotranspiration.  The result should be
   surface dew points commonly in the 70s F, beneath steep midlevel
   lapse rates of 8-9 deg C/km.  In tandem with diurnal heating, this
   will yield strong buoyancy, with preconvective MLCAPE values
   commonly peaking in the 3500-5000 J/kg range from NE northward
   across the eastern Dakotas.  This, combined with a deep mid/upper-
   level layer of modest winds (e.g., 700-250-mb mean flow commonly in
   the 20-25-kt range east of the front) will support deep/intense
   updrafts, substantial precip loading of downdrafts, and rapid
   convective growth after initiation.  Somewhat stronger mid/upper
   winds over the south-central/southwestern NE area -- closer to the
   base of the ejecting shortwave trough and in stronger capping -- may
   support somewhat longer-lived, tail-end supercell characteristics
   early in the convective cycle.

   Some potential exists for activity in the middle/southern part of
   the convective swath to persist overnight at or near severe levels
   across the lower Missouri Valley into parts of IA.  This activity
   would be supported by lift near a low-level warm front and enhanced
   storm-relative flow near the nose of a south-southwesterly to
   southwesterly LLJ branch.  The balance between convective-scale
   forced ascent and stabilizing effects relating to a decoupling of
   the foregoing boundary layer will determine severe-wind potential;
   however, enough unconditional potential is apparent in convective
   guidance across multiple synoptic and convection-allowing progs to
   extend that portion of the outlook area southeastward somewhat.

   ...Lower Ohio Valley to Carolinas...
   Widely scattered to scattered thunderstorms -- comprised of discrete
   multicell/pulse storms and episodic larger clusters -- are expected
   to form across this corridor near and south of the warm/stationary
   front.  Rich low-level moisture, strong diurnal heating, and lift
   along the front and outflow/differential-heating boundaries, will
   weaken MLCINH substantially.  Sea-breeze boundaries will be a factor
   near the coastal sections of the Carolinas as well.  The greatest
   convective coverage is likely from mid-afternoon into early evening.
   Isolated large hail and severe gusts may occur in the most intense
   cores, with occasional subsevere gusts also capable of minor damage.
   With weakness of both flow aloft and vertical shear, the threat
   appears too disorganized and conditional on storm- to mesobeta-scale
   processes for more than a broad-brushed marginal outlook to be
   maintained at this juncture.

   ..Edwards/Gleason.. 06/30/2020

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