SPC AC 061957

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0257 PM CDT Sat Jun 06 2020

   Valid 062000Z - 071200Z

   ...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS LATE THIS
   AFTERNOON AND EVENING ACROSS PARTS OF NORTHWEST NEBRASKA...WESTERN
   AND CENTRAL SOUTH DAKOTA AND ADJACENT PORTIONS OF SOUTHEASTERN
   MONTANA AND SOUTHWESTERN NORTH DAKOTA...

   ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS LATE THIS
   AFTERNOON ACROSS PARTS OF THE COLORADO ROCKIES...COASTAL NEW ENGLAND
   AND PARTS OF THE FLORIDA GULF COAST...

   ...SUMMARY...
   Damaging winds, large hail, and a couple of tornadoes are expected
   this late afternoon into this evening from the central Rockies to
   the northern Plains.  Damaging gusts remain possible this afternoon
   in parts of New England, and a couple of tornadoes may occur along
   the west coast of Florida.

   ...20Z Outlook Update...
   Changes to categorical and probabilistic lines have been made to
   account for the progression of synoptic and sub-synoptic features,
   and associated impacts concerning destabilization and associated
   convective development.

   There remains considerable signal within model output that a
   significant surface pressure perturbation (supported by the remnant
   impulse from a mid-level closed low emerging from the subtropical
   eastern Pacific and associated vigorous convective development) may
   remain the focus for the primary convective development through the
   remainder of this period.  As it crosses the Colorado/Wyoming
   Rockies into a more strongly heated and potentially unstable
   environment within deepening lee surface troughing across the north
   central high plains, considerable renewed thunderstorm
   intensification is expected late this afternoon.  As activity
   ingests boundary-layer air characterized by higher moisture content
   and larger CAPE across parts of northwestern Nebraska and western
   South Dakota, further intensification and organization is possible
   into early evening, in the presence of a strengthening low-level
   jet.  This probably will be accompanied by increasing risk for
   strong surface gusts until it encounters drier/more stable air which
   remains entrenched northeast of the middle Missouri Valley.

   ..Kerr.. 06/06/2020

   .PREV DISCUSSION... /ISSUED 1133 AM CDT Sat Jun 06 2020/

   ...Portions of central Rockies today...
   An pronounced midlevel shortwave trough will eject
   north-northeastward from AZ to eastern UT/western CO from late this
   morning through this afternoon.  Unusually moist profiles, per the
   1.07" PW observed at GJT in the 12z sounding, and forcing for ascent
   in advance of the ejecting trough will support early thunderstorm
   development near the Four Corners, and then storms will spread
   quickly north-northeastward through the afternoon across eastern
   UT/western CO.  A narrow zone of cloud breaks immediately ahead of
   the ejecting trough will contribute to destabilization, in an
   environment with strong deep-layer vertical shear that will support
   a mix of supercells and bowing line segments.  Damaging winds will
   be the main concern this afternoon, though isolated large hail and
   an isolated tornado will also be possible.

   ...Central/northern High Plains late this afternoon into tonight...
   Bands of thunderstorms will likely form over the mountains today and
   then spread northeastward over the central High Plains in
   conjunction with the ejecting midlevel trough.  Surface
   heating/mixing in cloud breaks will contribute to inverted-v
   profiles in the lee of the mountains from eastern CO northward into
   eastern WY, with some surface-based buoyancy.  Midlevel flow will
   increase to 60-70 kt this afternoon, and combined with the
   inverted-v profiles, will favor efficient production of severe
   outflow winds.

   Richer low-level moisture is present farther east into central NE,
   where boundary-layer dewpoints are in the upper 60s.  Some elevated
   convection is ongoing in a zone of warm advection from central NE
   northward across the central Dakotas, and these storms could pose a
   threat for isolated large hail.  The prospects for surface-based
   storms this afternoon/evening are a little less certain.  A narrow
   corridor of richer moisture will be maintained along the warm front
   from central into western SD this afternoon.  Storm development in
   this corridor is less certain given potential capping concerns prior
   to the arrival of the more linear/outflow-driven convection from the
   High Plains later this afternoon evening.  Assuming storms can form,
   forecast profiles will favor supercells capable of isolated very
   large hail and a couple of tornadoes.

   ...New England this afternoon...
   Surface heating/destabilization is occurring in advance of a cold
   front and surface trough, as a midlevel trough approaches from the
   west-northwest.  An increase in storm coverage and strengthening
   midlevel flow will favor a threat for damaging winds with broken
   bands of storms (cells and line segments).  Please see MD 850 for
   additional information.

   ...FL today and northern Gulf coast end of period...
   Tropical storm Cristobal is moving northward across the central Gulf
   of Mexico, and is forecast to make landfall in LA tomorrow (please
   refer to the latest NHC advisories regarding storm specifics).  The
   eastern convective bands and 35-45 kt low-midlevel flow are
   affecting the west coast of FL now, where a few embedded supercells
   and a couple of tornadoes will be possible today.  The convection
   will gradually expand inland with muted surface heating this
   afternoon, as well as gradually northward along the FL west coast. 
   Please see MD 851 for additional information.  Toward the end of the
   period, there could be enough strengthening of low-level flow/shear
   for a low-end supercell/tornado threat approaching the north central
   Gulf coast.

   CLICK TO GET WUUS01 PTSDY1 PRODUCT

   NOTE: THE NEXT DAY 1 OUTLOOK IS SCHEDULED BY 0100Z