SPC AC 081729

   Day 2 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   1229 PM CDT Wed Jul 08 2020

   Valid 091200Z - 101200Z

   ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS THURSDAY INTO
   THURSDAY NIGHT ACROSS CENTRAL PORTIONS OF THE GREAT PLAINS AND
   ACROSS PARTS OF THE UPPER MIDWEST...

   ...SUMMARY...
   Clusters of strong to severe thunderstorms may develop across
   portions of the central Great Plains and Upper Midwest Thursday into
   Thursday night, posing a risk for strong wind gusts and large hail.

   ...Synopsis...
   Models indicate that a significant mid/upper-level trough will dig
   south/southeast of the Gulf of Alaska through the northeastern
   Pacific during this period, with a deepening embedded mid-level low.
    Downstream, mid-level heights are forecast to tend to rise across
   much of the western and central U.S., with mid-level subtropical
   ridging becoming increasingly prominent, centered along an axis from
   the lower Colorado Valley into the southern plains Red River Valley.
    A belt of westerly flow to the north may take on more of a
   southwesterly component across the Pacific Northwest through the
   southern Hudson/James Bay vicinity.  It appears that one significant
   embedded short wave trough will tend to migrate from near the
   central Canadian/U.S. border area into northwestern Ontario, while
   another progresses inland of the Pacific Northwest through the
   northern intermountain region by late Thursday night.

   Downstream of the mid-level ridging, weak, broadly cyclonic
   mid/upper flow likely will persist across and east of the
   Mississippi Valley into the western Atlantic.  It appears that this
   regime will include one or two convectively generated or enhanced
   perturbations migrating into/across the Mississippi Valley, and one
   notable perturbation migrating northward along the Mid Atlantic
   coast.

   In addition to the mid-level perturbations, models are strongly
   suggestive that the lingering remnants of overnight convection
   across parts of the central plains and mid Missouri Valley will be
   accompanied by substantial convective outflow.  It appears that the
   leading edge of this air mass may be steadily advancing
   southeastward and southward into/through a large portion of the
   central plains and lower Missouri Valley at the outset of the 
   period.  This could substantively impact the potential for
   convective redevelopment across a broad area from the central plains
   into the Upper Midwest, where the development of moderate to strong
   boundary-layer instability would otherwise be possible by Thursday
   afternoon.

   ...Parts of central Great Plains into Ozark Plateau...
   Predictability of severe weather potential for this period appears
   generally low due to a combination of weak/uncertain mid/upper
   support beneath building mid-level ridging, potential for sizable
   inhibition beneath a plume of very warm elevated mixed-layer air,
   and the uncertain extent and southward advancement of the
   conglomerate convective outflow boundary.

   Currently, model output appears to suggest that the corridor of
   stronger differential surface heating, ahead of the southward
   advancing convective outflow, is most likely to become focused
   across parts of southern Kansas into northern Oklahoma and adjacent
   southwestern Missouri/northwestern Arkansas by Thursday afternoon. 
   Perhaps aided by favorable shear, near or just south of a belt of
   modest northwesterly mid-level flow associated with the convective
   perturbation, this may be accompanied by either the intensification
   of ongoing convective development, or new intense convective
   development, in the presence of large CAPE.  

   Isolated supercells are possible initially, before convection likely
   rapidly grows upscale, perhaps into another organizing cluster of
   storms which could pose a risk for severe wind and hail,

   Meanwhile, sizable spread is evident among the various models
   concerning potential for late afternoon and evening storm
   development across the higher plains.  However, particularly near
   the Front Range, there appears potential for storms to propagate off
   the higher terrain into moistening and destabilizing low-level
   upslope flow, accompanied by at least some risk for severe hail and
   wind.

   ...Upper Midwest...
   Moderate boundary-layer destabilization (including CAPE in excess of
   2000 J/kg) probably will become supportive of considerable
   thunderstorm development ahead of the approaching convectively
   generated mid-level perturbation.  Deep-layer shear may remain weak,
   but there may be potential for storms consolidate into an  upscale
   growing cluster which could pose increasing risk for potentially
   damaging wind gusts late Thursday afternoon and early evening.

   ..Kerr.. 07/08/2020

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