SPC AC 211827

   Day 3 Convective Outlook AMEND 1
   NWS Storm Prediction Center Norman OK
   0127 PM CDT Thu Sep 21 2023

   Valid 231200Z - 241200Z

   ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS ACROSS PARTS OF
   THE CENTRAL/SOUTHERN PLAINS INTO PARTS OF MO/IA/SOUTHERN MN...

   AMENDED TO ADD SEVERE PROBABILITIES OVER THE COASTAL MID-ATLANTIC

   ...SUMMARY...
   Severe thunderstorms are possible on Saturday across parts of the
   central and southern Plains into Iowa and Missouri.

   ...Coastal Mid-Atlantic...
   Recently issued track of Potential Tropical Cyclone 16 brings it
   onshore over coastal NC early Saturday morning, and then northward
   into the Chesapeake Bay by Saturday evening. Recent guidance
   suggests the warm sector to the north and east of the system's
   center should penetrate far enough inland to support modest buoyancy
   and deeper convective cores. Given the strong low to mid-level wind
   fields, any updrafts that are able to deepen and mature could
   produce tornadoes. Mesoscale details, in particular how much
   buoyancy develops inland, remain uncertain enough to introducing
   only a Marginal/Level 2 risk with this amendment. 

   -- PREVIOUS DISCUSSION --

   ...Great Plains into parts of MN/IA/MO...
   A seasonably strong mid/upper-level cyclone is forecast to move
   eastward across SD on Saturday. The primary surface low attendant to
   this mid/upper cyclone is forecast to move from the central Plains
   into the Dakotas as the system begins to occlude, while secondary
   surface low development is possible along the trailing front across
   the southern Plains. 

   Moderate to locally strong buoyancy is expected to develop across a
   broad area of the Great Plains on Saturday, east of the cold front
   and outside of any areas where outflow persists from morning
   convection. One notable midlevel jet maximum is forecast to move
   from the central Plains into the Dakotas, while a southern-stream
   jet maximum will begin to impinge upon parts of the southern Plains.

   Organized convection will be possible Saturday into Saturday night
   across a broad region from the Great Plains into parts of MN/IA/MO,
   with a mixture of supercells and organized clusters possible
   along/east of a cold front. Initial development is most likely
   closer to the mid/upper-level low across the central/northern
   Plains, with storm development into parts of the southern Plains
   expected by early Saturday evening. All severe hazards will be
   possible, though primary storm mode and evolution of mesoscale
   details remain uncertain at this forecast range. 

   ...Eastern NC into the Mid Atlantic...
   A surface low is forecast to move from near or just offshore of
   eastern NC into parts of the Mid Atlantic on Saturday. Depending on
   the westward extent of the track, richer low-level moisture may
   spread into coastal areas of the Mid Atlantic region, potentially
   accompanied by increasing low-level flow/shear and some threat for a
   brief tornado or two. Depending on the ultimate track and intensity
   of this system, severe probabilities may eventually be needed for
   some part of this region.

   ..Mosier/Dean.. 09/21/2023

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   NOTE: THE NEXT DAY 3 OUTLOOK IS SCHEDULED BY 0730Z