SPC AC 191300

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0700 AM CST Sun Feb 19 2017

   Valid 191300Z - 201200Z

   ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS FROM THE RED RIVER
   REGION OF NORTHWEST TX AND SOUTHWEST OK...ACROSS THE HILL
   COUNTRY...TO THE RIO GRANDE SOUTHWEST OF SAN ANTONIO....

   ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS ELSEWHERE FROM
   CENTRAL OK TO DEEP SOUTH TX...

   ...SUMMARY...
   Severe thunderstorms are possible across parts of the southern
   Plains late today and this evening, developing farther southward
   across the Hill Country and perhaps into much of south Texas
   tonight.  Large hail and sporadic wind damage are possible, and a
   tornado or two cannot be ruled out.

   ...Synopsis...
   In mid/upper levels, a highly amplified yet progressive pattern will
   remain in place through the period.  The well-defined trough now
   extending from the central Appalachians region to the northeastern
   Gulf is forecast to dig east-southeastward across the Atlantic just
   north of the Bahamas.  Meanwhile, in its wake, synoptic ridging will
   move eastward across the Mississippi Valley to the upper Great
   Lakes, Tennessee Valley, Florida Panhandle, and eastern Gulf.

   Upstream, phased troughing is evident in northern and southern
   streams, with the combined trough now apparent in moisture-channel
   imagery from offshore of the Pacific Northwest southeastward across
   NV and AZ, then southward across southern Baja to tropical latitudes
   of the East Pacific.  As additional shortwaves (now well out over
   the North Pacific) reinforce larger-scale troughing off the West
   Coast, the inland trough will elongate meridionally and its
   vorticity pattern will split.  By the end of the period, what now is
   an embedded mid/upper low over northern AZ should eject
   northeastward across the central Rockies as an open wave, reaching
   the central High Plains by 12Z, and still in phase with a
   northern-stream extension across MT to eastern AB.  The remainder of
   the trough over AZ and the Pacific will proceed eastward, reaching
   the southern High Plains, Big Bend region of TX, and west-central MX
   by 12Z.

   At the surface, lee troughing (with several weak/attached lows) is
   already well-established from central MT southeastward to eastern Co
   then south-southeastward across eastern NM.  Cyclogenesis is
   expected this evening over the NE Panhandle or Badlands SD region,
   with the resultant low reaching eastern ND by 12Z.  The surface
   trough to its south will move eastward across the High Plains to a
   position near a JMS-HSI-P28-LRD line by the end of the period.  A
   cold front extending south-southwestward from the Dakotas low will
   overtake the trough from north to south during the late
   overnight/early morning hours.  A dryline, analyzed at 11Z from
   south of MAF into northern Coahuila, will become better-defined from
   south to north today across west TX and the eastern Panhandles as
   moisture return continues in the warm sector to its east.

   ...Southern Plains...
   At least two primary rounds of convection will contribute to severe
   potential across the outlook area this period.  The slight-risk
   probabilities have been expanded in deference to increasing coverage
   and severe potential mainly with the second:

   1.  Isolated to widely scattered thunderstorms may develop gradually
   this afternoon -- as early as about 20Z -- in a low-level
   confluence/convergence zone located east of the dryline/lee trough,
   in central to northwest TX.  This activity should move northward to
   north-northeastward over the rest of northwest and western
   north-central TX and into OK while increasing in coverage.  An
   initially well-defined capping layer at the EML base -- evident in
   12Z soundings -- should erode gradually amidst a combination of
   low-level thetae advection and cooling aloft.  This in tandem with
   surface heating should foster convective development and gradual
   deepening starting in mid/late afternoon, as low-mid 60s F surface
   dew points now over central/south TX, and associated PW near an
   inch, spread northward in the return flow.  MLCAPE around 1000-1500
   J/kg should cover most of the area by late afternoon.

   In forecast soundings, lack of stronger flow around 800-900 mb
   limits hodograph size and SRH over much of north TX and southern OK
   where surface-based effective-inflow parcels are apparent.  Still,
   favorable deep-layer speed/bulk shear will support supercell
   potential, as will stronger low-level shear just along the
   northeastern fringes of the surface-based warm sector.  Any
   sustained cells developing/growing in the warm sector may become
   surface-based and mature atop at least a weakly buoyant boundary
   layer before outpacing that destabilization and crossing into cooler
   air.  As such, marginal tornado probabilities are being introduced,
   subject to additional increase in later outlooks if mesoscale
   developments warrant.  Some models' more-aggressive destabilization
   trends into OK are dependent on lack of convective/cloud cover,
   related stronger heating and faster return-flow moistening than
   currently expected.  Any such mesoscale trends may compel later
   northward extension of probabilities as well.

   2.  Thunderstorms should develop this evening into tonight in a
   south/north-oriented corridor ahead of the dryline, between central
   Texas and the Rio Grande Valley, amidst increasing low-level
   convergence and large-scale forcing for ascent.  Until this activity
   aggregates into a band, a few supercells are possible.  Moist
   advection back into parts of south-central TX during the evening
   should overcome pockets of diurnally mixing-reduced moisture in
   support of nocturnal convective potential. With time overnight,
   amidst deep meridional flow and an increasingly pronounced
   veer-back-veer wind pattern with height, convective modes generally
   should become more messy and quasi-linear in character.  In concert
   with that, hail potential ramps down, and wind-damage and brief/
   conditional tornado potential exist in the form of embedded
   supercells and QLCS-generated vortices.  With eastward extent across
   much of eastern OK and east TX, convection generally will encounter
   a less-favorable vertical thermodynamic profile late tonight, and
   severe potential generally should become more marginal.

   ..Edwards/Peters.. 02/19/2017

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