SPC AC 301251

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0751 AM CDT Sun Aug 30 2020

   Valid 301300Z - 311200Z

   ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS OVER PARTS OF THE
   NORTH-CENTRAL PLAINS THIS AFTERNOON AND EVENING...

   ...SUMMARY...
   Severe hail and gusts may occur from thunderstorms over parts of the
   north-central Plains this afternoon and evening.

   ...Synopsis...
   A progressive, increasingly amplified northern-stream pattern has
   evolved in mid/upper levels, characterized by:  1) more-intense flow
   than in previous weeks analyzed at 250 and 500 mb over the northern
   tier or two of states, and 2) the longstanding southwestern CONUS
   high/ridge now shunted well southward across northern MX, south TX
   and the Gulf.  A strong trough -- now located from southern QC
   across western New England -- will pivot northeastward across the
   Canadian Maritimes through the period.  Synoptic ridging in its wake
   will move eastward across the Great Lakes, ON and Hudson Bay. 
   Upstream, the southern portion of a strong shortwave trough,
   initially over MT, will shift eastward to western ND and
   northeastern WY around 00Z, reaching northern/central MN and
   southeastern SD by the end of the period.

   An "undercutting" current of upper westerlies will stream from the
   southern Great Basin across the mid Mississippi Valley to the
   central Appalachians.  An associated shortwave trough -- now evident
   in moisture-channel imagery over portions of KS/NE -- will move
   eastward over a lower-theta-e air mass in the mid Mississippi
   Valley.  Meanwhile, a series of low-amplitude shortwaves --
   containing convectively induced/enhanced vorticity maxima -- will
   move eastward across the Arklatex, Mid-South and Tennessee Valley
   regions, above an outflow-reinforced, low-level baroclinic zone.

   At the surface, 11Z analysis showed a cold front from northeastern
   NC northeastward over the Atlantic, becoming wavy and
   quasistationary westward across TN, where it became muddled by
   convective outflow.  The outflows, causing an effective southern
   displacement of the synoptic front, covered parts of AR, north and
   northwest TX, and east-central/northeastern NM.  A composite
   boundary should settle over the Permian Basin, central/northeast TX
   and the Mid-South over the next several hours.  Another cold front
   was drawn from a low between BIS-DIK southwestward over west-central
   WY.  This front should move southeastward to northwestern MN, east-
   central SD, central NE, and southern WY by 00Z.  By 12Z, the front
   should reach eastern MN, central IA, central KS, and southeastern
   CO/northeastern NM.

   ...North-central Plains...
   Isolated to scattered thunderstorms are forecast this afternoon into
   early evening over northern/western parts of the outlook area, as
   the cold front impinges on a favorably unstable boundary layer and
   overcomes previously prohibitive MLCINH at the base of the EML. 
   Severe hail will be possible, and isolated significant/2+ inch
   hailstones cannot be ruled out from any sustained supercell(s) that
   may develop.  The significant-hail threat still appears too
   conditional and isolated for an explicit area, but one may be needed
   as progs and mesoscale trends get better-focused today.  Damaging
   wind also is expected.

   A narrow south/north corridor of favorable moisture and buoyancy is
   expected to develop in the warm sector this afternoon by around 21Z,
   within 50-80 nm either side of an axis roughly from HDE-HON-BAC. 
   Surface dew points in the mid 50s to low 60s F should recover over
   the region, beneath steep midlevel lapse rates related to the EML. 
   This will foster peak MLCAPE in the 1500-2500 J/kg range near that
   axis, decreasing eastward and westward into lower theta-e.  Though
   the strongest mid/upper ascent/support associated with the
   positively tilted shortwave trough will remain behind the front,
   trough-related tightening of gradient winds aloft will contribute to
   favorable and increasing deep shear.  Forecast soundings depict
   well-curved low-level hodographs in the presence of 45-55 kt
   effective-shear magnitudes.  Both deep shear and convective coverage
   (due to capping) are forecast to decrease with southward extent
   through NE, though the more-isolated development should include
   supercell(s).  Activity overall should weaken as it shifts east of
   the theta-e axis this evening.

   ...Southern Plains to southern Appalachians...
   Widely scattered to scattered thunderstorms in clusters are possible
   through evening from portions of southeastern OK/northeast TX
   eastward across the Tennessee Valley region toward the southern
   Appalachians.  Locally/briefly severe hail and sporadic damaging to
   marginally severe gusts appear to be the main concern with
   multicellular convection.  The threat appears too poorly focused,
   unorganized and uncertain spatially for more than an unconditional
   marginal area to be maintained at this time.

   One prior/overnight MCS essentially has dissipated over the Arklatex
   region, while a second has moved along and either side of the Red
   River from the TX Panhandle into southeastern OK and northeast TX. 
   Boundaries from this activity still are progressive over much of the
   swath from north TX to western TN, with lingering uncertainties
   regarding 1) their specific position, 2) airmass recovery to their
   north, and 3) strength of lift along them during most-favorable
   convection-initiation period of ambient peak diabatic warming this
   afternoon.  Where outflow is not too intense and diabatic heating
   can be sustained under cloud breaks, peak preconvective MLCAPE may
   range from 2500-3500 J/kg over western parts of the outlook area,
   and 1000-2000 J/kg in the east, all amidst modest deep shear
   (effective-shear magnitudes generally around 30 kt or less).

   Another round of convection may develop this afternoon over the High
   Plains of northeastern NM and the adjoining TX Panhandle/South
   Plains region, and move eastward toward northwest TX, western OK and
   the Red River region with at least isolated strong-severe gusts and
   hail possible.  The inflow layer will consist of diurnally modified,
   upslope-aimed trajectories from the outflow of the western of the
   two prior MCSs.  As such, uncertainties exist regarding
   thermodynamic contribution to longevity and organization, despite
   favorable veering of winds with height and potentially 40-50-kt
   effective-shear magnitudes where surface flow remains backed into an
   easterly component.  A broad area of 25-35-kt 850-mb flow, and
   related theta-e advection and moisture transport, may support enough
   recover overnight to sustain strong convection across OK and/or
   north TX.  As such, a spatial continuum remains in the outlook,
   despite the major difference in timing of ongoing/eastern and
   late/western-origin convective regimes.

   ..Edwards/Kerr.. 08/30/2020

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