SPC AC 281237

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0637 AM CST Tue Feb 28 2017

   Valid 281300Z - 011200Z

   ...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS LATE THIS
   AFTERNOON INTO TONIGHT FROM NORTHERN ARKANSAS NORTHEASTWARD ACROSS
   PARTS OF MISSOURI...ILLINOIS...AND INDIANA...

   ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS SURROUNDING THE
   ENHANCED RISK AREA...FROM THE ARK-LA-TEX TO THE OHIO VALLEY...

   ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS IMMEDIATELY
   SURROUNDING THE SLIGHT RISK AREA...

   ...SUMMARY...
   Severe thunderstorms are expected to develop from portions of the
   lower Mississippi Valley to the Ohio Valley through tonight.  Large
   hail, damaging winds, and a few strong tornadoes are possible.

   ...Synopsis...
   A large-scale mid-upper trough over the Great Basin and lower CO
   River valley will progress eastward to the MS Valley by the end of
   the period.  An associated surface cyclone will develop
   northeastward from the MO/IA border to Lower MI overnight, as a cold
   front moves eastward from the Plains to the MS Valley.  Ahead of the
   front, a relatively broad/unstable warm sector is expected from
   parts of the middle MS and lower OH Valleys southward, where
   vertical shear will increase with time as the mid-upper trough
   approaches from the west.  Several rounds of strong/severe storms
   may occur across a broad area, with all severe storm hazards
   possible this afternoon through tonight, though uncertainty persists
   with warm sector development through this evening.

   ...Northern AR/southern MO to the lower OH Valley this evening...
   To the southwest of ongoing elevated storms over northern MS and
   western TN, rich low-level moisture (boundary layer dewpoints of
   65-70 F) is spreading northward from east TX/LA toward eastern
   OK/AR.  Though a subtropical cirrus plume may tend to limit surface
   heating in some areas, afternoon temperatures well into the 70s
   along the moist axis will drive MLCAPE values upward to 1500-2500
   J/kg.  Simultaneously, vertical shear will become increasingly
   favorable for supercells in the warm sector, with effective bulk
   shear at or above 70 kt, and effective SRH of 300-400 m2/s2 expected
   by this evening.

   The primary uncertainty for diurnal convection in the open warm
   sector will be a focus for initiation.  Only a weak cap is expected
   this afternoon over the warm sector given soundings modified for
   upper 70s and upper 60s dewpoints, and the primary source of weak
   ascent during the afternoon will be low-level warm advection. 
   However, uncertainty remains substantial, as represented by
   inconsistency among multiple convection-allowing model forecasts. 
   If storms can form in this regime across central/northern AR and
   southern MO during the afternoon/evening, they will likely become
   primarily discrete supercells and move rapidly northeastward at
   50-60 kt across southeast MO toward southern IL.  The combination of
   vertical shear, buoyancy, and low-level moisture will conditionally
   support a risk for strong tornadoes, while isolated very large hail
   will also be possible.  

   ...East of the surface cyclone into IL this afternoon/evening...
   A few near-surface-based storms could form this afternoon and
   persist into this evening immediately east of the surface cyclone,
   along the warm front into central/northern IL as boundary layer
   dewpoints increase to near 60 F beneath steep midlevel lapse rates. 
   Cloud breaks will dictate the degree of surface heating into
   northeast MO, with potentially moderate buoyancy (MLCAPE 1000-1500
   J/kg) along the north edge of the warm sector into central/northern
   IL.  If storms form within the north edge of the warm sector, strong
   deep-layer and low-level shear will favor supercells with all severe
   hazards possible.

   ...MO/AR to the OH Valley overnight...
   As the surface cyclone develops northeastward to the Great Lakes,
   the surface cold front will progress eastward to the Ark-La-Miss and
   lower OH Valley by Thursday morning.  One or more bands of
   convection are likely along or just ahead of the front tonight into
   Wednesday morning, with an accompanying risk for damaging winds and
   hail.  The wind and tornado risk will be maximized with embedded
   bowing segments and/or supercell structures within or ahead of the
   convective band/front, in an environment with strong vertical shear
   and sufficient low-level moisture/buoyancy.  There will also be some
   risk for more discrete storm development farther south into western
   and central AR late tonight.

   ..Thompson/Kerr.. 02/28/2017

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