SPC AC 200552

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   1252 AM CDT Thu Jun 20 2024

   Valid 201200Z - 211200Z

   ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS ACROSS THE
   NORTHEAST AND OVER THE CENTRAL HIGH PLAINS...

   ...SUMMARY...
   Storms may produce scattered damaging gusts over parts of the
   Northeast on Thursday. Scattered severe storms are also expected
   over parts of the central High Plains.

   ...Northeast...

   Upper anticyclone currently centered over the middle Atlantic will
   gradually build west across the CONUS during the day1 period. As
   this occurs, some flattening of the height field is expected over
   southern QC into New England. Modest 500mb flow will extend along
   the international border, but much weaker mid-level winds will be
   noted over the lower Great Lakes into southern New England. This
   stronger flow over Canada will encourage a surface cold front to
   advance south into northern ME, arcing west along the international
   border into southern lower MI by 18z, with subsequent southeast
   movement expected across the Northeast into the early evening. This
   boundary will serve as the focus for convection later today as
   surface temperatures quickly soar through the 80s into the lower
   90s. Additionally, while deep-layer shear will not be particularly
   strong along the northern fringe of the upper High, PW values are
   seasonally moist with 1.5-2 inches expected south of the frontal
   zone. Damaging winds are the primary risk with convection today.

   ...Central High Plains...

   Weak 12hr mid-level height rises are forecast across much of the
   interior western US through early evening as an upper trough
   repositions itself from WA/OR into CA. This will ensure a corridor
   of modest southwesterly flow at mid-levels from the Four Corners
   region into the central High Plains. Within this flow a few weak
   disturbances may track across the central Rockies into the Plains.
   Each of these features could encourage robust convection. Latest
   guidance suggests a weak lee cyclone will evolve over northeast CO
   by late afternoon in response to one of these features. LLJ is also
   forecast to respond across western KS into SD. As a result, an
   easterly boundary-layer component will be noted across the NE
   Panhandle into eastern WY. NAM forecast sounding for CYS at 22z
   exhibits negligible inhibition but MLCAPE on the order of 2000 J/kg
   with 35kt surface-6km bulk shear. HREF guidance strongly suggests
   convection will evolve over the higher terrain of south-central
   WY/CO fairly early, then propagate downstream, growing upscale into
   a possible MCS within the warm-advection zone. Convection should
   advance into the central Plains after sunset aided by strengthening
   LLJ. Initial storm mode should be supercellular and large hail is
   expected. Severe wind gusts could materialize along the leading edge
   of the MCS if it matures as expected.

   ..Darrow/Thornton.. 06/20/2024

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