SPC AC 280539

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   1139 PM CST Mon Feb 27 2017

   Valid 281200Z - 011200Z

   ...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS FROM AR TO
   IL/IN...

   ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS FROM EASTERN OK TO
   OH...

   ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS FROM NORTHEAST
   TX TO WESTERN PA...

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

   ...Discussion...

   Ridging appears to be responsible for delaying meaningful convection
   across the MS Valley late this evening.  However, LLJ is beginning
   to increase across MO/IL and isolated thunderstorms are evolving
   within a corridor from the Bootheel of MO into central IL.  This
   activity should gradually expand in areal coverage as warm advection
   lifts north toward the Great Lakes region.  Given these trends,
   there is increasing confidence that much of the southern
   Plains/lower MS valley will remain convection-free into the
   afternoon hours.  As a result, significant boundary-layer recovery
   is expected into the OH Valley prior to frontal passage.

   Late evening model guidance continues to suggest a corridor of
   strong southwesterly flow aloft will extend from the southern
   Rockies, into the Great Lakes with 500mb flow expected to increase
   in excess of 80kt by early afternoon across western portions of the
   severe risk area.  Large-scale height falls will overspread the
   Mississippi Valley and several convective scenarios may ultimately
   evolve within a broad moistening warm sector.

   1.  Warm-advection corridor (northern IL):

   Strengthening LLJ over IL will aid northward advance of higher
   quality air mass as surface dew points rise through the 40s into the
   50s as far north as northern IL by late morning.  Warm advection is
   expected to induce scattered strong/severe thunderstorms early in
   the period as warm front lifts northward in advance of the primary
   surface wave.  Convection may be aided by a weak mid-level
   disturbance embedded within the stronger southwesterly flow. 
   Initial activity will be elevated in nature and hail is the primary
   risk with these storms.

   2.  Warm sector (AR/MO/IL):

   28/00z sounding from OUN exhibited a steep-lapse-rate environment
   with substantial low-level moisture.  Strong capping and
   neutral-weak subsidence will allow this air mass to advect northeast
   such that surface dew points should rise into the lower 60s across
   much of the warm sector south of I-70.  Breaks in cloud cover during
   the day should allow surface parcels to approach their convective
   temperatures from eastern OK into southwestern MO by mid-late
   afternoon.  Latest thinking is isolated supercells may evolve well
   ahead of the cold front along nose of a secondary branch of LLJ that
   will strengthening across AR during the day.  It's not clear how
   much storm coverage will be noted across this region but
   environmental shear and increasing instability/buoyancy favor robust
   supercells.  Forecast soundings suggest very large hail could
   accompany this activity and a few strong tornadoes are possible,
   especially if discrete structures evolve as it appears they may.

   3.  Cold front:

   Large-scale forcing will contribute to a convectively active cold
   front by early evening as the wind shift surges into an increasingly
   moist/unstable air mass from eastern IA into northern MO. 
   Thunderstorms are expected to mature into a strong squall line that
   will surge east across the mid MS Valley into the OH valley as 500mb
   flow increases to near 100kt by 01/12z.  Damaging winds should be
   noted with this frontal convection and tornadoes may also be
   embedded along the line.  Eastward momentum should easily allow this
   activity to spread across much of OH by sunrise Wednesday morning.

   ..Darrow/Picca.. 02/28/2017

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