SPC AC 041251

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0751 AM CDT Sat Jul 04 2020

   Valid 041300Z - 051200Z

   ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS FROM PARTS OF
   SOUTHWESTERN/CENTRAL MONTANA TO NORTHWESTERN MINNESOTA...AND OVER
   PARTS OF THE SOUTH-CENTRAL HIGH PLAINS...

   ...SUMMARY...
   Severe thunderstorms are most probable this afternoon and evening
   from parts of southwestern/central Montana to northwestern
   Minnesota, and over parts of the south-central High Plains.

   ...Synopsis...
   In mid/upper levels, the amplitude of ridging over the north-central
   CONUS will diminish through the period.  This will occur as a series
   of variably amplified shortwave perturbations cross the northern
   Rockies, and international-border region between MT and Lake
   Superior.  A small, initially closed cyclone was evident in
   moisture-channel imagery over southern BC, with shortwave trough
   southwestward across coastal WA and offshore OR.  This will become a
   fully open-wave trough today, reaching central MB, southeastern BC,
   and northern/southwestern OR by 00Z.  By 12Z, the trough should
   deamplify somewhat, assume a less-positive tilt, and be aligned
   along the SK/MB border then southwestward over western MT.  In
   downstream difluent flow aloft, a perturbation now over ND will
   eject across northern MN and adjoining ON by 00Z, with its southern
   lobe crossing the Boundary Waters region overnight.

   Farther south, a broad/elongated perturbation -- with several
   embedded vorticity maxima -- was apparent over southeastern UT,
   eastern AZ, eastern UT, and western/central CO.  This feature should
   move slowly eastward across the remainder of CO and much of northern
   NM through the period.  To the east and southeast, a very broad area
   of modest cyclonic flow, with several embedded (and mainly
   convectively originated) vorticity maxima, will persist from the
   Ozarks to the north-central Gulf and vicinity of the FL/GA line.

   At the surface, 11Z analysis showed a quasistationary frontal zone
   oriented northeast/southwest across southern MB, to north-central ND
   and into an extensive area of convective outflow covering much of
   the western Dakotas.  A low was drawn over the Black Hills with a
   trough southwestward across eastern WY.  These boundaries should
   remain near their present locations through most of today, except
   where modulated by convective processes, given the lack of robust
   upper-level influences.  A weak cold front should develop over
   central/southwestern MT today and move eastward, reaching
   northeastern to south-central MT by 00Z.

   ...Northern Plains...
   Multiple episodes or clusters of thunderstorms are expected to
   affect the outlook area through the period, offering a threat for
   severe gusts and hail.  The most probable among the scenarios
   involves convection forming this afternoon ahead of the Northwest
   shortwave trough, on the higher terrain of south-central/
   southwestern MT and vicinity, then moving northeastward across MT
   and organizing to severe levels near and behind the trailing surface
   frontal zone.  Activity may overtake that front and encounter richer
   theta-e this evening into tonight.

   Surface dew points off the mountains should range from the 40s over
   west-central MT to low/mid 60s in eastern MT, with upper 60s and 70s
   possible along and ahead of the outflow boundary over the Dakotas. 
   Activity moving off the higher terrain over the western/central
   parts of the outlook area will encounter generally increasing
   low-level moisture with eastward extent, as well as a diabatically
   destabilized and well-mixed boundary layer over central MT. 
   Forecast soundings suggest MLCAPE should range from around 500-1000
   J/kg in the west to over 2000 J/kg near the eastern MT border. 
   Vertical shear should strengthen somewhat ahead of the shortwave
   trough, with time series of forecast soundings and planar progs
   showing a rapid lowering of the level of the 50-kt isotach through
   the upper/middle troposphere with time, over western/central MT. 
   Despite weak near-surface winds in most areas, 40-50 kt effective-
   shear magnitudes are possible, suggesting supercell potential. 
   Outflows may aggregate with the formation of a forward-propagating
   band or arc of convection this evening, converting the predominant
   threat to wind with time.

   Strong-severe thunderstorms also may develop near the outflow
   boundary and related surface convergence/differential-heating zone
   over eastern ND late this afternoon, with convection potentially 
   persisting into early evening.  Weaker deep shear will exist than
   over the northern High Plains; however, low-level shear/vorticity
   may be maximized near the boundary to support at least transient/
   heavy-precip supercell character within a more-dominant
   multicellular regime.  Steep low/middle-level lapse rates and rich
   moisture will contribute to peak MLCAPE in the 3000-4500 J/kg range.
   Mesoscale uncertainties preclude a more-focused unconditional
   outlook at this time.  However, given the potential CAPE/shear
   parameter space available to any storm(s) that form, tornado
   production and/or localized very large hail cannot be ruled out.

   ...Central/southern High Plains...
   Isolated to scattered thunderstorms are expected to develop this
   afternoon over the eastern ranges and foothills of the Rockies, from
   the Laramie Mountains southward into the Sangre de Cristos,
   including the Raton Mesa and Palmer Divide.  Some of this activity
   should persist eastward across the adjoining High Plains with a
   threat for severe gusts and large hail.  The greatest concentration
   of convection and associated severe potential may occur for a few
   hours late this afternoon into early evening ahead of the shortwave
   trough and associated vorticity maxima, over portions of
   southeastern CO, and northeastern NM, shifting into parts of the TX
   Panhandle.  After an initial hail/wind blend of threats, some of
   this activity may aggregate outflows to produce a cold pool that
   would forward-propagate eastward for a few hours before weakening. 
   To reflect this relative concentration, 15% wind/hail areas have
   been introduced.

   Although deep-layer flow will be modest, strong directional shear
   and favorable storm-relative boundary-layer winds are evident in
   progs across the corridor from southeastern CO to the northern TX/NM
   border area.  Strong/sustained diabatic heating is most probable
   north of an area of clouds/precip now located over east-central NM,
   northward to southeastern CO as well.  Forecast soundings suggest
   steep low-level lapse rates, well-mixed subcloud layers, and
   sufficient moisture to support 1000-1500 J/kg MLCAPE, with large
   DCAPE.  Evening cooling/stabilization of the foregoing boundary
   layer will reduce both convective coverage and severe potential,
   with time and eastward extent tonight.

   ..Edwards.. 07/04/2020

   NOTE: THE NEXT DAY 1 OUTLOOK IS SCHEDULED BY 1630Z