SPC AC 211259

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0759 AM CDT Thu May 21 2020

   Valid 211300Z - 221200Z

   ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS OVER PORTIONS OF
   THE CENTRAL/SOUTHERN HIGH PLAINS...

   ...SUMMARY...
   A few severe thunderstorms are expected this afternoon and evening
   in the southern and central High Plains. Other marginally severe
   thunderstorms may occur in the Carolinas and Florida.

   ...Synopsis...
   A persistently blocky, high-amplitude upper-air pattern over the
   CONUS will begin to break down this period, especially the Rex
   pattern in the East.  The initially cut-off cyclone -- its core
   region currently over middle TN and central KY -- has been filling
   since yesterday, based on 500-mb height rises analyzed since 21/00Z.
    This will continue as the mid/upper low drifts toward the CVG area
   through the rest of the period.  An associated shortwave trough --
   evident in moisture-channel imagery from central NC southward across
   central/eastern SC and adjoining Atlantic waters -- will pivot
   mostly northward today, while making little longitudinal progress.

   Large-scale troughing will continue over the west, but with internal
   reconfiguration on the synoptic and subsynoptic scales.  An embedded
   cyclonic circulation center now near the MT/SK/AB border confluence
   will move northward to central AB and deepen, while another cyclone
   now over coastal BC digs southeastward across the Pacific Northwest
   as an intense shortwave trough.  Though heights will fall over the
   northern High Plains with the departure of the border cyclone, a
   southern perturbation -- initially located over AZ -- will keep
   cyclonic flow in place across the central/southern High Plains as it
   pivots eastward to portions of CO/NM by 00Z.  this trough then
   should reach western KS and the TX Panhandle by the end of the
   period.  Meanwhile, a weaker/southern-stream shortwave trough -- now
   over west-central/northwest TX -- will move east-northeastward to
   eastern OK and east TX by 00Z, then into the Mid-South region by
   12Z.

   The surface analysis showed an occluded/cold front from southern SK
   across western SD to northwestern KS, to a low over southeastern CO.
    The front extended southwestward from there across eastern NM.  A
   warm front was drawn from the low southeastward across southwestern
   OK, north-central TX and east TX, becoming quasistationary across
   the coastal FL Panhandle region, then a weak cold front over
   southeastern GA to a low near CHS.  A warm front was drawn from
   there across coastal southern NC.  The southern Plains warm front
   will move slowly northward into southern OK and the Arklatex region.
    The SC low should shift northward across central NC through the
   period, while the warm front moves inland over central/eastern NC. 
   A dryline will mix eastward to northwest TX and the Permian Basin
   from an initial position near the TX/NM border and lower Pecos
   Valley.

   ...South-central High Plains...
   Scattered thunderstorms should develop across portions of
   southwestern and extreme western KS this afternoon in a zone of
   maximized low-level convergence near the frontal triple point, which
   also will be located near the northern extension of the dryline. 
   Early-stage supercells will be capable of all severe hazards,
   including large/damaging hail and a couple tornadoes possible.  This
   activity should evolve upscale into the evening, yielding a more
   wind-dominant severe hazard with time.  However, any discrete to
   semi-discrete supercell remaining still will pose a tornado threat
   as low-level hodographs enlarge, before nocturnal/diabatic cooling
   increases MLCINH too strongly.  

   Moist advection and diurnal heating, beneath steep midlevel lapse
   rates, will contribute to MLCAPE rising into the 2000-3000 J/kg
   range in a narrow, southeast/northwest-oriented corridor close to
   the warm front.  Though midlevel winds will not be particularly
   intense, limiting effective-shear magnitudes to under 40 kt,
   low-level shear will be favorable, especially in the warm-frontal
   zone.  A well-mixed subcloud layer will support maintenance of
   severe gusts/hail to the surface.  A small MCS resulting from the
   early activity may progress as far as south-central KS, the eastern
   OK Panhandle, northeast TX Panhandle, and northwestern OK with its
   wind threat, before weakening overnight.

   ...Southern Plains/dryline area...
   Isolated to scattered thunderstorms are expected to form near the
   dryline this afternoon, offering severe gusts and hail.  

   By mid/late afternoon, the area may be back under neutral mid/upper
   forcing, as low-level thermodynamic processes near the dryline
   dominate the contribution to convective potential.  
    Strong surface heating will occur in the zone of prolonged diurnal
   clearing behind the initial shortwave trough.  Steep
   low/middle-level lapse rates and surface dew points commonly in the
   60s F will contribute to MLCAPE reaching the 3000-4000 J/kg range,
   above a well-mixed boundary layer.  These conditions will support
   intense updraft and downdraft development, with locally severe gusts
   probable in the most intense cells.  Though convective coverage is
   uncertain, and organization will be limited by a lack of deep shear,
   some upscale clustering/aggregation may occur that would extend a
   severe-wind threat eastward toward parts of central TX.  At this
   time, that potential still appears too uncertain to apply to the
   15%/unconditional wind area.

   ...FL...
   Scattered thunderstorms should form during the afternoon, from the
   Everglades northward across areas around Lake Okeechobee toward the
   MCO area.  The main concern will be damaging gusts, with isolated
   severe downdrafts possible.  Initiation foci will be sea breezes,
   then increasingly with time, outflows from nearby convection, and
   especially collisions of these various boundaries.  Strong diurnal
   heating will occur inland to strengthen the sea-breeze circulations.
    

   Favorable lapse rates, with 500-mb temperatures around -11 to -12 C,
   will overlie a regime of strong boundary-layer heating, with surface
   dew points upper 60s to lower 70s F in southern FL, trending to mid
   60s in central areas of the peninsula.  This should yield peak
   preconvective MLCAPE 1500-2500 J/kg.  A deep convective column and
   well-mixed subcloud layer will support the potential for strong,
   potentially/locally severe downdrafts in the most intense cells. 
   Weak low/middle-level flow and lack of substantial shear will foster
   dominant multicellular mode.  Activity should diminish during the
   early evening as the air mass stabilizes, due to a combination of
   diabatic surface cooling and relatively extensive coverage of
   outflow air. 

   ...Carolinas...
   Multiple rounds of thunderstorms are expected across the region
   through this evening, a few of which may produce damaging to briefly
   severe gusts.  Though height rises are expected with time as the
   inland cyclone fills and retreats slowly away, the area will remain
   under peripheral cyclnic flow aloft, with subtle large-scale ascent
   from the shortwave trough.  Environmental low-level shear generally
   is expected to be modest, localized storm-scale and boundary
   interactions may support brief/isolated potential for a tornado as
   well.  

   Activity will be focused along a combination of low-level
   confluence/convergence zones and outflow/differential-heating
   boundaries, with only subtle lift required to initiate convection
   given the weak diurnal MLCINH expected.  Rich low-level moisture
   will continue in this airmass, with PW in the 1.5-2-inch range and
   surface dew points upper 60s to low 70s F, supporting peak MLCAPE in
   the 1500-2000 J/kg range.  Mid/upper winds will be modest, limiting
   deep shear and contributing to mainly multicell modes.  Still,
   veering of flow with height in low levels will impart considerable
   curvature to mostly small hodographs that, with localized boundary
   interactions, may yield brief/small mesocyclones. 

   ...Arklatex region...
   Widely scattered to scattered thunderstorms may develop this
   afternoon over parts of northeast TX and southeast OK, near and
   south of the front.  The main concern will be strong/isolated severe
   downdrafts, though short-lived, messy supercell(s) may be encouraged
   by backed flow and relatively maximized vorticity in low levels in
   the frontal zone.  This activity will be encouraged by a combination
   of frontal lift with:
   1.  Large-scale DCVA/ascent ahead of the southern shortwave trough,
   with some localized enhancement ahead of an embedded MCV now evident
   just southwest of the Metroplex;
   2.  Weakening CINH related to diurnal heating, amidst a richly moist
   warm sector characterized by upper 60s to low 70s F surface dew
   points.

   ...Lower Rio Grande Valley...
   A small cluster of thunderstorms south of Laredo may produce
   isolated severe hail/gusts for another couple hours as it moves into
   a very moist air mass with favorable buoyancy (2500-3500 J/kg
   MLCAPE), but also some MLCINH as sampled by the 12Z BRO RAOB.

   ..Edwards.. 05/21/2020

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