SPC AC 090609

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0109 AM CDT Fri Apr 09 2021

   Valid 091200Z - 101200Z

   ...THERE IS A MODERATE RISK OF SEVERE THUNDERSTORMS FOR PARTS OF
   EXTREME SOUTHEAST ARKANSAS INTO FAR NORTHEAST LOUISIANA INTO CENTRAL
   MISSISSIPPI...

   ...SUMMARY...
   A complex severe-weather scenario is expected to unfold across the
   Mid-South into the Southeast late this morning through the afternoon
   and overnight hours. Significant severe storms, including widespread
   damaging winds, and at least a few tornadoes are expected across
   portions of northeast Louisiana into Mississippi, especially
   overnight. Large hail is also expected to be a concern, mainly
   during the day across parts of eastern Oklahoma into the Arklatex
   region.

   ...Synopsis...
   Broad cyclonic mid-level flow will prevail across the eastern half
   of the CONUS through the period, with multiple embedded impulses
   supporting deep moist convection across several areas spanning from
   the Southeast towards the Mid Atlantic and extending into portions
   of the Ohio Valley. Scattered strong storms (a few of which may be
   severe) are expected to first initiate by mid morning into early
   afternoon within a warm-air advection regime across portions of AL
   into GA and the FL Panhandle as a weak mid-level impulse exits the
   area. Later in the day, this same mid-level impulse is expected to
   support an organized cluster of storms across VA into NC. However,
   the main severe threat will begin during the afternoon hours and
   continue into the overnight period as multiple, potentially highly
   organized rounds of storms are expected to traverse portions of the
   Southeast, with all facets of severe possible. More isolated severe
   is possible along portions of the cold front, mainly during the day,
   across portions of the Ohio Valley.

   ...Portions of the Southeast - Midday...
   Storms are expected to gradually expand in coverage and intensity
   across portions of eastern AL into western GA with the onset of
   diurnal heating. Deep-layer wind fields are not expected to be
   overly strong, with small looping hodographs supporting multicells
   and occasional transient supercell structures. Modestly steep (7
   C/km) low and mid-level lapse rates will support damaging gusts and
   large hail with the more organized updrafts.

   ...Portions of the Southern Plains into the Mid-South...
   As the first in a series of mid-level impulses traverses portions of
   the Mid-South area by early afternoon, the first rounds of
   convective initiation are expected across the warm sector, from
   southeast KS, eastern OK into southern MO and western AR. 7-8.5 C/km
   low and mid-level lapse rates atop mid to upper 60s surface
   dewpoints are expected to foster MLCAPE over 2000 J/kg in advance of
   a dryline, though convective inhibition is expected to limit the
   coverage of convection southwestward to a degree across portions of
   east-central TX into southwest LA. Relatively weak wind fields,
   especially below 3 km, suggest that multicellular clusters with an
   occasional transient supercell will be the predominant storm mode
   initially, with a few clusters potentially growing upscale into MCSs
   via cold pool mergers. The steep low-level lapse rates will promote
   an organized damaging-wind threat with any of the more intense
   linear segments that form, while large hail (including a few stones
   exceeding 2 inches in diameter) and perhaps a tornado may accompany
   the longer-lived, more discrete updrafts. Currently, confidence is
   lowest in the coverage of severe hail (especially 2+ inch stones)
   given the possibility for linear modes, and downgrades to hail
   probabilities may be needed in future outlooks.

   ...Parts of the Southeast - late afternoon and overnight...
   By late afternoon into early evening, the main mid-level impulse and
   associated forcing for ascent is expected to eject into the
   Southeast, with a potentially more organized MCS expected to develop
   somewhere near the Arklatex area and progress across Mississippi
   into Alabama overnight. Despite the potential for multiple rounds of
   multicellular clusters across the Southeast (a few of which may
   support damaging winds, large hail, or a brief tornado), a healthy
   fetch of steep low to mid-level lapse rates is expected to accompany
   the nocturnal MCS, promoting a widespread damaging-wind threat. The
   approach of the main upper support will also result in a
   considerable low-level mass response and subsequent strengthening of
   the low-level flow. Over 200 m2/s2 0-1km SRH will support organized
   QLCS circulations embedded within the MCS and associated tornado
   threat. The MCS will continue into central AL through 12Z Saturday,
   with a damaging-wind threat persisting.

   ...Virginia into North Carolina...
   Modest ascent associated with a passing mid-level impulse is
   expected to be timed with the diurnal heating cycle across portions
   of VA into NC, with organized thunderstorm development during the
   afternoon. 7+ C/km low and mid-level lapse rates are expected to
   emanate east of the Appalachians supporting vigorous updraft
   development and sustenance. Relatively weak tropospheric wind
   profiles suggest that multicellular convection will be the dominant
   mode, with damaging gusts and large hail possible. However, a warm
   front is expected to be draped across southern VA during the day.
   Any storms that can anchor to the warm front may acquire more
   transient supercell structure with a locally heightened threat for
   wind and hail, and a brief tornado cannot be completely ruled out.
   The warm front will continued to be monitored for a possible Slight
   risk upgrade in later outlooks.

   ...Portions of the Ohio Valley...
   A cold front is expected to persist across the Ohio Valley during
   the day, with modest upper support overspreading the region. While
   convective coverage will be relatively sparse in nature, any storms
   that develop will be atop a dry boundary layer and associated steep
   lapse rates. With up to 60 kt 700 mb flow expected atop this dry
   boundary layer, efficient downward momentum transport may support a
   couple damaging gusts with the more robust convection, mainly during
   the mid to late afternoon hours.

   ..Squitieri/Jewell.. 04/09/2021

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