SPC AC 200558

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   1158 PM CST Thu Jan 19 2017

   Valid 201200Z - 211200Z

   ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS ACROSS PARTS OF
   LA/MS/AL AND THE WESTERN FL PANHANDLE...

   ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS ACROSS PARTS OF
   THE LOWER SABINE/MS RIVER VALLEYS INTO THE GULF COAST STATES...

   ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS ACROSS PARTS OF
   COASTAL SOUTHERN CA...

   ...SUMMARY...
   Severe storms may develop Friday night into early Saturday across
   the lower Mississippi Valley and adjacent Gulf coastal areas, with a
   few tornadoes, large hail, and damaging winds all possible. Strong
   thunderstorms with at least some severe-weather potential are also
   possible near the southern California coast Friday afternoon.

   The potential for higher severe hail/tornado probabilities (and a
   corresponding Enhanced Risk) exists for late in the Day 1 period
   across parts of LA/MS/AL, but a possible upgrade has been deferred
   to a later outlook due to:
   1. Considerable uncertainty in convective coverage/mode across the
   warm sector late Friday into early Saturday per latest guidance,
   2. Relatively high potential for spatial error in the placement of
   higher probabilities, and
   3. The opportunity for later near-term guidance and observations to
   provide additional details on the mesoscale evolution of this event.

   A detailed discussion follows below.

   ...Synopsis...
   A large-scale upper trough over the western CONUS will acquire an
   increasingly negative tilt as it shifts northeastward across the
   Plains Friday. One upper jet initially across the southern Plains
   will develop eastward across the lower to mid MS Valley, TN/OH
   Valleys, and adjacent Gulf Coast through early Saturday. A separate
   upper jet is forecast to move from the eastern Pacific across CA and
   parts of the Southwest through the period.

   A shortwave trough embedded within the synoptic-scale
   mid/upper-level trough will likely move from northern Mexico across
   TX and into the lower MS Valley from Friday evening through 12Z
   Saturday (end of Day 1 period). An elevated mixed layer with steep
   mid-level lapse rates originating from the higher terrain of
   northern Mexico appears likely to shift northeastward into much of
   the southern Plains and parts of the lower MS Valley in tandem with
   the embedded shortwave trough.

   At low levels, Gulf moisture will stream northward across the lower
   MS Valley and adjacent areas ahead of the approaching shortwave
   trough Friday night into early Saturday. A southerly/southwesterly
   low-level jet is forecast to strengthen to 40-50 kt across parts of
   LA/MS/AL, mainly after 21/03Z. Higher surface dewpoints/moisture
   will likely be slower to advance northward across the same area due
   to a convectively reinforced boundary currently located over the
   northern Gulf of Mexico from prior thunderstorms. The primary
   surface cyclone is expected to remain well displaced to the
   northwest (across the central/northern Plains) from the warm sector
   across the lower MS Valley and Gulf Coast states.

   ...East TX/Lower MS Valley and Adjacent Gulf Coast States...
   While isolated thunderstorms may occur near to just offshore parts
   of coastal TX/LA/MS during the day Friday along a remnant boundary,
   the better opportunity for scattered to numerous thunderstorms
   appears to be Friday evening after 21/00Z. This later development
   would be tied to increasing large-scale ascent associated with an
   embedded shortwave trough over the southern Plains approaching the
   region, as well as low-level warm air advection/ascent with a
   strengthening low-level jet.

   Considerable uncertainty remains regarding convective coverage and
   mode late in the period. Regardless, forecast soundings across
   LA/MS/AL show slowly steepening mid-level lapse rates, strengthening
   low- to mid-level winds, and increasing low-level moisture Friday
   night into early Saturday morning that will support supercell
   structures. The threat for large hail, some potentially greater than
   2 inches in diameter, will exist with any initially discrete
   supercell storms that can form this environment, although the better
   mid-level lapse rates will be arriving toward 06-12Z Saturday. In
   addition, damaging winds and a few tornadoes will also be possible
   across this region, particularly southern LA/MS/AL where higher
   surface dewpoints may support surface-based convection.

   ...Coastal Southern CA...
   Numerous showers and isolated thunderstorms may develop Friday
   afternoon across coastal southern CA and adjacent eastern Pacific
   tied to ascent with the nose of an upper-level jet moving over this
   region. Forecast soundings indicate weak instability may combine
   with marginal bulk shear values to support some organization with
   this convective activity. Locally strong to damaging winds appear to
   be the primary threat.

   ..Gleason/Picca.. 01/20/2017

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