SPC AC 201729

   Day 2 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   1229 PM CDT Thu Apr 20 2017

   Valid 211200Z - 221200Z

   ...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS FRIDAY INTO
   FRIDAY NIGHT ACROSS PARTS OF CENTRAL AND EASTERN OKLAHOMA...AND
   ADJACENT NORTHEASTERN TEXAS...INTO WESTERN ARKANSAS...

   ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS ACROSS SURROUNDING
   AREAS OF OKLAHOMA...NORTHEAST TEXAS...NORTHWESTERN
   LOUISIANA...ARKANSAS AND SOUTHWESTERN MISSOURI...

   ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS ACROSS
   SURROUNDING AREAS OF THE SOUTHERN PLAINS...MISSISSIPPI AND TENNESSEE
   VALLEYS INTO THE MID ATLANTIC COAST REGION...

   ...SUMMARY...
   One or two clusters of severe storms may evolve Friday through
   Friday night across parts of the south central Plains into the Ozark
   Plateau.  Strong thunderstorms may also impact portions of the
   Tennessee Valley into the Mid Atlantic Coast region, with at least
   some risk for severe weather.

   ...Synopsis...
   Mid/upper flow appears likely to remain progressive through this
   period, with much of the U.S. under the influence of a generally
   zonal belt of westerlies emanating from the mid-latitude Pacific. 
   Within this regime, one significant wave and embedded modest cyclone
   are expected to shift east of the lower Great Lakes region through
   the St. Lawrence Valley.  Upstream troughing, comprised of at least
   a couple of significant smaller scale perturbations, appears likely
   to redevelop east southeast of the U.S. intermountain region. 
   Models suggest that the lead impulse within this latter regime will
   turn east of the Colorado Rockies early Friday, before continuing
   eastward through portions of the middle Mississippi Valley and Ozark
   Plateau by 12Z Saturday, as another impulse digs from the northern
   intermountain region through Wyoming/Colorado Rockies.

   In lower levels, weak secondary surface cyclogenesis may take place
   along a southeastward advancing frontal zone, across or east of the
   northern Mid Atlantic Coast.  Somewhat stronger surface wave
   development is possible along the front across the south central
   Plains into the Ozark Plateau.  Substantive further inland
   moistening off the western Gulf of Mexico appears unlikely, but
   moisture already present probably will contribute to sufficient
   instability to support considerable thunderstorm development along
   and ahead of the cold front, from parts of the southern Plains into
   the Mid Atlantic Coast region.

   ...South central Plains into Ozark Plateau...
   Forcing associated with lower/mid tropospheric warm advection may be
   supporting considerable thunderstorm activity across the region at
   the outset of the period.  This may be sustained through much of the
   day, with strongest activity expected to become focused across parts
   of eastern Oklahoma into Arkansas, on the nose of a plume of warmer
   and more strongly capping elevated mixed layer air emanating from
   the Mexican Plateau region.  This regime probably will be conducive
   to the risk for severe hail, particularly as a 50-70+ kt cyclonic
   500 mb jet noses into/through northern Oklahoma by Friday afternoon.

   Exit region forcing associated with this latter feature may remain
   mostly displaced to the north of the destabilizing pre-frontal
   boundary layer, but in the wake of possible upscale growing early
   day convective development, low/mid-level warm advection may support
   renewed vigorous convective development across parts of central into
   southeastern Oklahoma by late Friday afternoon.  This seems most
   likely near the southwestern flank of early day convectively
   generated outflow, and the cold front/dryline.

   Inflow of a moist unstable boundary layer characterized by CAPE at
   least on the order of 1000+ J/kg, and perhaps up to 2000 J/kg, may
   support at least isolated supercell development initially.  This may
   be accompanied by a window of opportunity for a tornado or two, in
   the presence of sizable clockwise curved low-level hodographs
   supported by 30-40 kt southerly 850 mb flow, before forcing
   associated with warm advection aids upscale growth into an
   organizing convective system.  As this occurs, severe weather
   potential should transition to a risk for primarily potentially
   damaging wind gusts as activity tends to develop southeastward
   toward the Red River before weakening Friday night.

   ...Tennessee Valley into Mid Atlantic Coast region...
   Severe weather potential with possible thunderstorm activity
   along/ahead of the southward advancing surface front remains
   somewhat unclear for Friday/Friday evening.  This corridor may
   remain to the south of the stronger flow fields associated with the
   trough crossing the Great Lakes region into the St. Lawrence Valley.
   And lower/mid tropospheric lapse rates are not expected to be
   particularly steep.  Still, though, models suggest at least pockets
   of weak to moderate boundary layer destabilization are possible
   across the Tennessee Valley into the lee of the central
   Appalachians.  It may not be out of the question that the
   environment will become conducive to at least widely scattered
   thunderstorm development capable of producing marginally severe hail
   and/or potentially damaging surface gusts.

   ..Kerr.. 04/20/2017

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