SPC AC 311630

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   1130 AM CDT Fri Mar 31 2023

   Valid 311630Z - 011200Z

   ...THERE IS A HIGH RISK OF SEVERE THUNDERSTORMS FOR FAR SOUTHEAST
   IOWA...WEST-CENTRAL ILLINOIS...FAR NORTHEAST MISSOURI AND IN PARTS
   OF EASTERN ARKANSAS...NORTHERN MISSISSIPPI...AND SOUTHWESTERN
   TENNESSEE...

   ...SUMMARY...
   A severe weather outbreak appears increasingly likely, centered on
   this afternoon and evening, across a large portion of the
   Mississippi Valley. At least a few long-track, strong to potentially
   violent tornadoes are probable, particularly over portions of the
   Mid-Mississippi Valley to the Mid-South. Swaths of intense damaging
   wind gusts along with very large hail are expected as well.

   ...Synopsis...
   A dangerous severe weather outbreak is likely across a large
   portion of the central states this afternoon into tonight. 
   Long-track, strong to potentially violent tornadoes are forecast
   across a broad region of the MS Valley.  The propensity of
   model-supporting evidence and observational trends lends confidence
   in the upgrade to bimodal High Risks for the mid MS Valley and lower
   MS Valleys.  

   ...Midwest and the Mid-MS to OH Valleys...
   Morning water-vapor imagery shows a potent 500-mb speed max ejecting
   east-northeast across the TX Panhandle/western OK to the south of a
   mid-level low over northern NE.  A 992-mb cyclone over western IA
   will deepen through this afternoon as it matures into the Upper MS
   Valley before occluding this evening.  Visible-satellite
   imagery/surface observations show a plume of increasingly rich
   low-level moisture rapidly advecting northward into the lower MO
   Valley with surface dewpoints rising into the lower 60s F.  Strong
   heating in combination with low-level moistening and cooling
   mid-level temperatures, are expected to result in a relatively large
   corridor featuring 1000-3000 J/kg MLCAPE across a moderate to
   strongly unstable warm sector.  

   In agreement with prior forecast thinking, it seems the primary
   severe evolution remains likely to evolve beginning around 19Z to
   the southeast of the deep cyclone across central IA and broadening
   in coverage through the late afternoon along the
   north/south-oriented cold front arcing southward into eastern MO. 
   Very steep mid-level lapse rates and an elongated mid to upper-level
   hodograph will initially support a broken band of supercells capable
   of producing very large hail.  The risk for tornadoes will likely
   focus initially with supercells near and southeast of the surface
   low.  As this activity rapidly spreads into an increasingly
   favorable low-level SRH environment in the eastern half of IA and
   the MS Valley, several to numerous tornadoes are expected to form. 
   Forecast soundings within the High Risk show large 0-3-km MLCAPE
   co-located with intense speed shear and large low-level and
   elongated hodographs.  Within this arcing band of quasi-discrete
   supercells, several long-track strong to potentially violent
   tornadoes are forecast for this afternoon into the evening.

   Eventual upscale growth into an intense squall line with embedded
   cellular elements is expected to evolve during the evening into
   tonight across IL and rapidly sweep eastward across the OH
   Valley/southern Great Lakes.  Some risk for tornadoes will probably
   spread east with the squall line coincident with a 70-kt 700-mb
   speed max overspreading the region.  Severe gusts (60-90 mph) are
   possible with surges/bowing structures and embedded cells within the
   band of storms.  Have expanded the Moderate Risk due to significant
   wind gusts across eastern IL into IN.  By late tonight, the evolving
   QLCS will outpace weakening surface-based instability around the
   central OH Valley.  Extreme low-level wind fields could support a
   waning wind/brief tornado threat approaching the central
   Appalachians. 

   ...Ark-La-Tex to the Lower MS/TN Valleys...
   Multiple rounds of severe convection are expected to unfold,
   intensifying this afternoon in AR and Ark-La-Tex, before continuing
   east across the Lower MS into the TN Valleys through tonight. 

   Rapid airmass modification is currently underway across the lower MS
   Valley with mid 60s deg F dewpoints surging north-northeast across
   southern AR and towards the AR/TN/MS region.  Initially, a capped
   warm sector via an EML will act to limit storm development (isolated
   early afternoon storms with mainly a hail risk).  However, as
   buoyancy increases (1500-2500 J/kg MLCAPE) and shear profiles
   intensify, very favorable hodographs are forecast to develop by
   mid-late afternoon  across southern/eastern AR into adjacent
   portions of MS/TN (effective SRH 300-600 m2/s2).  Scattered
   thunderstorms will likely develop by late afternoon with these
   rapidly evolving into supercells.  Strong to potentially violent
   tornadoes are forecast with a few long-tracked supercells. 

   Upscale growth into a QLCS with embedded supercells appears probable
   during the late evening and overnight as frontal convergence
   strengthens. Tornadoes and significant damaging wind swaths will
   remain possible well into the night across at least into the TN
   Valley, until warm-sector low-level flow becomes more veered towards
   the end of the period.

   ..Smith/Lyons.. 03/31/2023

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