SPC AC 021252

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0752 AM CDT Thu Jul 02 2020

   Valid 021300Z - 031200Z

   ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS ACROSS PARTS OF
   THE GREAT PLAINS TO CENTRAL GULF COAST...AND IN NORTHERN NEW
   YORK/NEW ENGLAND...

   ...SUMMARY...
   Clusters of strong to locally severe thunderstorms may develop today
   across parts of the Great Plains to central Gulf Coast, and in
   northern New York/New England.  Severe gusts and hail are the
   hazards.

   ...Synopsis...
   In mid/upper levels, the pattern will be dominated by a
   well-developed longwave ridge that should remain quasistationary,
   along an axis from northern MX across west TX to the upper
   Mississippi Valley and northwestern ON.  A complex synoptic cyclone,
   with several embedded vorticity maxima, will continue to cover much
   of western Canada from the Prairie Provinces to the Pacific Coast.
   The southern sector of the associated cyclonic flow will persist
   from northern CA across the northern Rockies to MT.  An embedded
   shortwave trough -- apparent in moisture-channel imagery over
   western MT -- will eject northeastward to SK by 00Z, with a trailing
   500-mb vorticity lobe embedded in confluent flow over
   north-central/northeastern MT.

   Meanwhile, a smaller/weaker mid/upper cyclone is centered near Cape
   Cod, with trough southwestward across the Carolinas and GA to
   coastal AL/MS.  The cyclone is losing closure and will be a
   still-weaker, open-wave trough by 00Z, offshore from New England and
   the Mid-Atlantic.  Behind it, shortwave ridging will weaken while
   crossing New England/NY.

   At the surface, 11Z analysis showed a low between AMA-LBL, with a
   diffuse warm-frontal zone arching across central KS, then blending
   with aggregated convective outflows southeastward across AR and
   southern MS.  The central Plains part of this boundary should shift
   northward across KS and perhaps into southwestern NE today, while
   strong diurnal heating helps to maintain low pressure to its south
   over western KS.  That portion of this boundary from the Ozarks to
   Gulf Coast should move slowly eastward as well, serving as a
   convergence max and relative focus for convection in that corridor.

   ...Great Plains to lower Mississippi Valley...
   Isolated to widely scattered thunderstorms are expected to develop
   today within a lengthy corridor from the Northern Plains' border
   with Canada to the Mississippi Coast and vicinity.  Damaging gusts
   and severe hail are possible from the most intense cells among any
   sustained convection in this corridor, even as environments vary
   considerably between the two ends.  Any concentrations of strong-
   severe convection should be widely spaced within this corridor, and
   strongly tied to diurnal heating and assorted boundary-localized
   lift processes.  As such, too much uncertainty remains in potential
   gaps in the threat to carve the swath into smaller segments at this
   time, or to focus an area of 15% hail/wind probabilities.  Some of
   this area may need an upgrade as mesoscale threats become better-
   defined today.  For now, two candidates, as relative concentrations
   of afternoon/evening potential are apparent:

   1.  Parts of the central High Plains (eastern CO toward western KS
   and/or southern NE).  A moist axis already was drawn at 11Z through
   the warm-frontal zone and into northeastern CO, with seasonally high
   surface dew points in the mid 60s F extending into CO and 70s over
   portions of western KS.  In concert with strong afternoon heating
   and steep midlevel lapse rates, an axis of 3000-4500 J/kg MLCAPE may
   develop from southeast KS to northeastern CO.  Though deep/speed
   shear will be weak, strong directional shear may support transient
   supercell structures, while weak inhibition may encourage upscale
   clustering into early evening.  More of northern KS and southern NE
   has been included in this update for that possibility.

   2. Western Dakotas, where weaker (but still adequate) low-level
   moisture will combine with steep low/middle-level lapse rates to
   support an axis of relatively uninhibited, 2000-3000 J/kg
   preconvective MLCAPE.  Somewhat stronger deep shear is possible near
   the eastern rim of enhanced midlevel flow related to the
   Northwestern cyclone and aforementioned shortwave trough/vorticity
   lobe to the west.  Well-mixed subcloud layers will encourage
   maintenance of any severe hail/gusts.  Convective coverage is
   uncertain along a supportive surface trough/confluence axis, but
   some guidance indicates enough may develop for more than marginal
   probabilities.

   ...New England and northern NY...
   Widely scattered thunderstorms should develop today and move
   southeastward across the outlook area, offering locally gusty winds
   and hail near severe levels.  As aforementioned shortwave-ridge
   weakening occurs, a belt of difluent northwest flow aloft will
   overspread the outlook area.  Steep low-level lapse rates are
   expected to result from diurnal heating, combining with residual
   moisture to contribute to peak MLCAPE in the 1000-1500 J/kg range. 
   Though low/middle-level flow and shear will be modest, stronger
   upper winds (e.g., 50-80 kt flow near anvil level) will aid in
   multicellular organization.  The already marginal severe potential
   should diminish considerably after dark, with loss of supportive
   boundary-layer lapse rates.

   ..Edwards/Gleason.. 07/02/2020

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