SPC AC 211640

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   1140 AM CDT Fri Apr 21 2017

   Valid 211630Z - 221200Z

   ...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS ACROSS PARTS OF
   SOUTHEAST OKLAHOMA AND THE ARKLATEX...AND ADJACENT NORTH TEXAS...

   ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS FROM THE
   MID-MISSISSIPPI VALLEY AREA WEST-SOUTHWEST INTO PARTS OF OKLAHOMA
   AND NORTH TEXAS...

   ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS EXTENDING FROM
   THE MID-ATLANTIC REGION WEST-SOUTHWEST INTO THE SOUTHERN PLAINS...

   ...SUMMARY...
   Severe thunderstorms capable of producing damaging gusts, large hail
   and a few tornadoes are expected through tonight from parts of the
   southern Plains into the Ozark Plateau.  Isolated severe
   thunderstorms may also impact portions of the Mid-South into the
   Mid-Atlantic with wind and hail.

   ...Synopsis...
   While an upper-level trough moves across Ontario/Quebec and the
   adjacent northeast U.S. and a second over the northeast Pacific
   reaches the West Coast late, the main trough associated with the
   primary convective/severe risk today is currently shifting into the
   central U.S.  An initial perturbation within this trough will shift
   across Kansas/Oklahoma with time, reaching the Ozarks overnight. 
   Meanwhile, a strong northwesterly mid- and upper-level jet on the
   back side of the trough will maintain broader cyclonic flow over the
   central U.S. through the period, in the wake of the passage of the
   initial/aforementioned smaller-scale perturbation.

   At the surface, a cold front currently extends from NY/PA
   southwestward across the mid-South to Arkansas, and then westward
   roughly along the Red River Valley.  Here, convectively-induced low
   moving along the front will allow some brief retreat of the boundary
   northward across the southeast Oklahoma/southern Arkansas vicinity
   as the low moves quickly eastward through the afternoon.  In the
   wake of the low, a southward surge of the convectively reinforced
   front is expected across the southern Plains through the end of the
   period.

   ...Southern Plains eastward across the Ozarks/mid MS Valley...
   A fast-moving/slightly elevated MCS continues moving across Oklahoma
   at this time, just north of the surface front now draped across
   southern Oklahoma.  This early/well-organized cluster of storms
   complicates the forecast substantially, as current model guidance --
   notably CAM runs and in particular the latest HRR -- have failed to
   depict this MCS almost entirely.  Convective outflow on the southern
   fringe of this line of storms is shifting quickly southeast across
   southwest and south central Oklahoma, where it has overtaken the
   synoptic front and resulted in a convectively reinforced frontal
   surge which should continue into north Texas with time.

   Ahead of this fast-moving system -- extrapolated to reach western
   Arkansas and southeast Oklahoma by 18Z, a moist/slowly destabilizing
   airmass is in place near and south of the synoptic front.  Slight
   northward advance of the front will occur ahead of the MCS, but
   eventual southward suppression is expected as the MCS passes and
   convective outflow interacts with the front.

   The eventual location of the outflow/front conglomerate will be
   significant, as it will serve as a northern fringe of
   later-to-be-realized surface-based severe threat expected to evolve
   across the Arklatex into north Texas.  Redevelopment of convection
   south of the ongoing MCS -- near and ahead of the advancing
   front/outflow is expected, and with moderate instability in
   conjunction with a favorably sheared environment, supercells are
   anticipated along with attendant risks for very large hail, locally
   damaging winds, and a few tornadoes.  Risk should spread east across
   the Arklatex and southeast across north Texas this evening, before
   diminishing in conjunction with diurnal cooling/stabilization. 
   Given potential for greatest severe risk to extend farther south and
   east than prior outlooks indicated due to ongoing convective
   evolution, the ENH risk area is being adjusted southward a bit
   toward the DFW metroplex, and eastward into the Arklatex.

   Farther north, the ongoing MCS should continue across Arkansas
   today, possibly continuing across the mid-Mississippi Valley later
   this afternoon and evening.  With some severe risk -- including
   potential for locally damaging winds and hail -- extending east of
   the river, either with this MCS or else with later/convective
   redevelopment, will extend SLGT risk east into parts of KY and TN. 

   ...Mid-Atlantic region westward into the TN Valley...
   Scattered showers and a few thunderstorms are ongoing at this time
   along the cold front, which currently extends from central
   NY/western PA southwestward across eastern Kentucky/middle
   Tennessee.  As this front advances very slowly southeastward today,
   heating through scattered to broken cloud cover will support
   continued/modest destabilization, with mixed-layer CAPE 500 to 1000
   J/kg expected in the pre-frontal warm sector this afternoon.  Flat
   to weakly anticyclonic flow aloft from roughly the mountains east
   through evening suggests isolated coverage of stronger storms,
   though 30 to 40 kt mid-level west-southwest flow will contribute to
   shear sufficient for organized -- possibly weakly rotating --
   updrafts.  As such, risk for hail and/or locally damaging winds will
   be possible with a few sustained storms within the MRGL risk area.

   ..Goss.. 04/21/2017

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