SPC AC 211631

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   1131 AM CDT Thu Jun 21 2018

   Valid 211630Z - 221200Z

   ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS OVER A PORTION OF
   THE LOWER MISSISSIPPI VALLEY AND WESTERN TENNESSEE VALLEY REGIONS...

   ...SUMMARY...
   Severe thunderstorms with isolated damaging wind will be possible
   today over a part of the lower Mississippi Valley and western
   Tennessee Valley. Storms capable of isolated severe gusts and hail
   are possible across parts of the High Plains and north-central
   Rockies later this afternoon into the early evening.  Storms
   producing a few locally strong to damaging gusts are also possible
   over the lower Ohio Valley as well as the North Carolina/Virginia
   vicinity.

   ...MO/AR into KY/TN/MS...

   Late morning surface analysis indicates an occluded front extending
   east from a surface low over southwest IA. This boundary then
   transitions to a cold front that stretches across western IL into
   south central MO and northwest AR. Partial clearing is occurring
   from AR into western TN and southeast MO in wake of morning showers.
   Mid-level lapse rates are weak, but diabatic warming of the very
   moist boundary layer should boost MLCAPE to 1500-2000 J/kg. Storms
   should initiate along the cold front from northeast AR into
   southeast MO this afternoon and gradually increase in intensity as
   they move through the destabilizing warm sector. A 50 kt mid-level
   jet max on the southern periphery of the upper low circulation is
   moving southeast and will overtake the frontal zone this afternoon,
   resulting in up to 45 kt effective bulk shear supportive of a few
   rotating updrafts. Some storms may also evolve into broken line
   segments with damaging wind expected to be the main threat. Despite
   weak mid-level lapse rates, a few instances of hail are also
   possible, especially given possibility of left splits and potential
   contribution of pressure perturbations to updraft strength. Though a
   brief tornado cannot be ruled out, low-level hodographs are
   relatively small which should temper overall tornado threat.  

   ...Northern Rockies and High Plains...

   A pronounced shortwave trough is moving across ID late this morning,
   and will continue through the northern Rockies this afternoon.
   Scattered thunderstorms are expected to form over much of this
   region, with development also across eastern WY and southward into
   northeast CO this evening.  Forecast soundings suggest modest CAPE
   in most areas, but sufficient vertical shear to promote the risk of
   a few rotating storms. Isolated large hail and locally damaging wind
   gusts will be possible in the strongest cells. It remains uncertain
   where coverage of severe events will be sufficient for a categorical
   upgrade, so will maintain marginal risk this update.

   ...IA/IL...

   An upper low centered over southern IA will track southeastward into
   eastern MO today.  Strong convergence is occurring along an occluded
   boundary wrapping westward into IA.  Given the ambient vorticity
   under the upper low, and proximity to the surface low/boundaries, an
   isolated non-supercell tornado or two is not out of the question in
   this zone. However, overall setup does not appear as favorable as
   yesterday.

   ...VA/NC...

   Scattered thunderstorms are expected to form over the higher terrain
   of western VA/NC as well as in warm sector this afternoon and track
   eastward across the MRGL risk area.  Ample low level moisture is
   present, along with the potential for strong heating and MLCAPE
   values of 1000-1500 J/kg.  Locally damaging wind gusts will be
   possible, although weak mid-level lapse rates may temper the overall
   severe risk.

   ...NM to TX Panhandle...

   A few strong thunderstorms are expected to form this afternoon off
   the higher terrain of the Raton Mesa, spreading southeastward toward
   the TX Panhandle this evening.  A few instances of large hail and
   damaging winds will be possible with these storms. It still appears
   the coverage of storms will remain very isolated, so will maintain
   the MRGL risk category this update.

   ..Dial/Bentley.. 06/21/2018

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