SPC AC 081254

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0754 AM CDT Wed Jul 08 2020

   Valid 081300Z - 091200Z

   ...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS OVER PORTIONS
   OF CENTRAL NEBRASKA TO NORTHERN KANSAS...

   ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS IN PARTS OF
   NORTHERN NEW ENGLAND...AND SURROUNDING THE ENHANCED AREA
   NORTHEASTWARD TO MINNESOTA...

   ...SUMMARY...
   Severe thunderstorms, with wind as the main hazard, are most
   probable late this afternoon into evening over parts of the central
   Plains.

   ...Synopsis...
   In mid/upper levels, a nearly zonal belt of flow is expected across
   the northern CONUS and southern Canada, perturbed primarily by
   shortwave troughing associated with a cyclone over southern portions
   of SK/AB.  The cyclone should move east-northeastward across
   southern/central SK and adjoining west-central/southwestern MB
   through the period, while the shortwave trough shifts eastward to
   ND.  Farther east, a shortwave trough -- initially apparent in
   moisture-channel imagery over southwestern QC and southeastern ON --
   should shift eastward across northern NY and New England through
   this evening.

   Meanwhile, a broad area of height weakness aloft will persist over
   the Southeast.  A slow-moving, embedded shortwave trough over the
   Carolinas, and adjacent Atlantic waters, sometimes may exhibit
   circulation closure in mid/upper levels, but will be more pronounced
   in the lower troposphere.  An associated/weak surface low was
   analyzed at 11z over coastal SC.  Through the period, this low is
   forecast to move/redevelop northeastward along or just offshore from
   the southern NC coastline to near the southern Outer Banks.  Low-
   level and deep shear are forecast to remain too weak inland for a
   severe threat, given the current forecast track of the low and
   character of the accompanying low-level flow.  Otherwise, see NHC
   tropical-weather outlooks and WPC heavy-rainfall products for more
   on this system.

   Elsewhere at the surface, a low was drawn near PIR, on the south end
   of a frontal segment diffused by convective outflow.  The cold front
   continued southwestward from there across southeastern WY.  By 00Z
   the front should move to north-central/west-central MN, east-
   central/southeastern SD, and southwestern NE, with a trough/dryline
   from there southward to a heat-enhanced low over southeastern CO. 
   By 12Z, the front should reach that low over southwestern KS, and
   extend from there across eastern NE, northwestern IA, and eastern
   MN.

   ...Central Plains...
   Widely scattered to scattered thunderstorms are expected to develop
   this afternoon along and ahead of the cold front, predominantly
   across NE but with some contribution from the upslope-flow regime
   across the NE Panhandle/southeastern WY region as well.  Large hail
   and damaging gusts are possible relatively early in the convective
   cycle, while low/middle-level lapse rates and SBCAPE are maximized
   and storm modes are still relatively discrete.  Weaknesses in
   midlevel flow are likely to lead to clustering together of
   convection rather quickly, though a few short-lived supercells are
   possible beneath initially strong anvil-level winds.  With midlevel
   lapse rates forecast near 8 deg C/km, steepening low-level lapse
   rates throughout the day amidst diurnal heating, and a favorably
   moist warm-sector boundary layer, peak MLCAPE values of 2500-3500
   J/kg are possible, decreasing gradually with southward extent and
   time, in proximity to stronger ridging and warmer temperatures
   aloft.

   With time, a quasi-linear convective system (QLCS) is expected to
   aggregate and organize a cold pool, propelling it southeastward
   across remaining parts of NE and into KS with severe wind the main
   threat.  A preponderance of synoptic and convection-allowing
   guidance reasonably concentrates the convective upscale growth in
   what now is the 30% wind area, though this likely will need
   additional refinement as MCS size and track become even less
   uncertain.  By that stage, forward-propagational, forced ascent,
   perhaps fronting an organized rear-inflow jet, should be the main
   factors permitting the complex to stay well-organized deep into the
   night, as the foregoing moist boundary layer only slowly stabilizes.
   Considerable uncertainty lingers regarding how far southeast the
   complex will be able to maintain organized severe potential --
   driven largely by internal dynamic processes.  However, the
   probability lines were expanded slightly southeastward to allow more
   room for MCS motion toward the eastern KS/western MO area before the
   end of the period.

   ...Upper Mississippi Valley and vicinity....
   A small MCS is observed moving east-southeast across portions of
   central MN, along but mainly on the cool side of the warm front. 
   This activity poses a threat for sporadic hail/gusts near severe
   limits the next few hours as it gradually shifts away from the axis
   of a modest, slightly weakening/veering LLJ.

   Widely scattered to scattered thunderstorms should develop again
   thus afternoon along/ahead of the cold front, with a threat mainly
   for severe gusts and hail.  Low-level warm advection (in mass
   response to the progressive Canadian cyclone) and diurnal heating
   should aid in airmass recovery, behind the morning complex and ahead
   of the cold front.  Where the boundary layer is initially relatively
   undisturbed or can recover, steep midlevel lapse rates and rich
   low-level moisture will combine with diabatic heating to support
   MLCAPE reaching the 3000-4500 J/kg range, though a lack of stronger
   midlevel flow will limit effective-shear magnitudes to under 40 kt
   in areas most likely to experience sustained heating.  The northern
   extent of favorable surface-based destabilization is uncertain, and
   may not extend entirely to the Canadian border; however, mesoscale
   uncertainties on timing/location are too great to alter the outlook
   are substantially at this time.

   Isolated to briefly scattered thunderstorms may develop this
   afternoon into early evening on the trailing portion of the surface
   cold front, from southern MN to northeastern NE, offering a threat
   for locally severe gusts/hail.  However, this regime may represent a
   relative minimum in convective organization/duration compared to
   areas farther northeast and southwest.

   ...Northeast...
   Isolated to scattered thunderstorms are expected to develop from
   later this morning through afternoon, offering the threat for 
   damaging to severe gusts.  In northern areas, isolated severe hail
   is possible as well, with some supercell potential.

   Coverage likely will be more dense across northern New England in
   and near the 15% wind-probability area, near both a weak surface
   trough and confluence line related to the approaching mid/upper
   perturbation and an outflow/differential-heating boundary left
   behind by ongoing/morning convection and precip.  The latter
   boundary may augment low-level shear/lift and enlarge hodographs
   locally, as flow and subtle yet supportive large-scale lift/
   destabilization increase with the approach of the mid/upper
   perturbation.  Otherwise, vertical shear by most measures should
   weaken with southward extent, as more pulse/multicellular convective
   character predominates.  Steepest midlevel lapse rates should be
   over central/northern ME and perhaps northern VT/NH, while low-level
   moisture should remain greater farther south where lift is weaker. 
   Still, diurnal heating and available moisture may support 1500-2000
   J/kg MLCAPE across northern New England, away from convective
   outflow, trending to the 1000-1500 J/kg range in the interior
   northern Mid-Atlantic region.

   ..Edwards/Broyles.. 07/08/2020

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