SPC AC 280544

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   1144 PM CST Sat Feb 27 2021

   Valid 281200Z - 011200Z

   ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS FROM THE ARKLATEX
   TO MIDDLE TENNESSEE...

   ...SUMMARY...
   Strong to severe thunderstorms are likely from parts of Texas into
   Middle Tennessee on Sunday. Damaging wind gusts and a few tornadoes
   should be the main hazards, but some large hail may also occur.

   ...Synopsis...
   Several mid-level shortwave troughs are embedded within a larger
   trough across the central CONUS. A stronger shortwave will move
   through the Upper Midwest/Great Lakes through the day. A compact
   shortwave will develop into a closed low across the Southwest
   throughout the day while several weaker perturbations will move
   through moderate southwesterly flow extending from the Southern
   Plains to the Ohio Valley. At the surface, an area of low pressure
   will begin the day in Iowa before deepening as it moves
   northeastward into the Great Lakes. As this surface low strengthens,
   a cold front will sharpen and advance southeastward from the Midwest
   to the southern Plains. Numerous thunderstorms, some severe, are
   expected along and ahead of this cold front.

   ...Southern Plains to the Ohio Valley...
   A shortwave trough seen on Water Vapor across northern Mexico early
   this morning will move quickly northeastward through the morning.
   Weak ascent associated with this mid-level shortwave is expected to
   overspread a moist and unstable warm sector across
   north-central/northeast Texas and southeast Oklahoma this morning
   which will likely initiate scattered convection. Supercells are
   possible given moderate instability (1000-1500 J/kg), steep
   mid-level lapse rates (~7.5 C/km) and effective shear around 65 kts.
   All severe weather hazards will be possible with these early storms.
   This includes the potential for a tornado or two given the moderate
   low-level shear.

   Temperatures ahead of the cold front are expected to increase into
   the low 70s with dewpoints in the low to mid 60s from the southern
   Plains as far northeast as southwest Kentucky by mid afternoon. This
   should yield MLCAPE around 1000 to 1500 J/kg from the southern
   Plains northeastward to far southern Kentucky. This destabilization
   of the air mass, combined with the tightening low-level frontal
   circulation should support widespread storm development along the
   front by early to mid afternoon. Storms may initially struggle to
   become severe due to some warmer temperatures near 700mb and the
   propensity for storms to initially move to the cool side of the
   boundary and become elevated. However, through time, storms are
   expected to grow upscale into a forward propagating MCS.

   This line of storms will likely pose a threat for damaging winds
   given the strong low-level flow and a broad region of weak to
   moderate instability downstream. 00Z CAM guidance suggests the most
   robust linear segments may occur from northeast Arkansas into
   western Tennessee where height falls and cooling temperatures aloft
   may assist in stronger updraft development. Low level flow is
   expected to increase above 40 knots at 0.5 km in this region which
   will support a greater threat for severe wind and a favorable
   low-level shear profile for potentially a few tornadoes embedded
   within the line. By late evening, instability is expected to weaken
   substantially which will likely support the line weakening by the
   early overnight hours.

   ...South Texas...
   A few storms are expected to form late in the period (after 07Z)
   near the Edwards Plateau and move eastward. Moderate instability
   (1500 to 2000 J/kg) is expected in this region with steep mid-level
   lapse rates around 8 C/km. Supercells will be the likely storm mode
   given effective shear around 60 to 65 knots. Low level flow will
   remain quite weak and thus large hail will be the primary threat
   with this activity.

   ..Bentley/Darrow.. 02/28/2021

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