SPC AC 201243

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0643 AM CST Fri Jan 20 2017

   Valid 201300Z - 211200Z

   ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS THIS EVENING
   ACROSS THE UPPER TEXAS COAST...AND TONIGHT ACROSS THE LOWER
   MISSISSIPPI VALLEY...

   ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS SURROUNDING THE
   GULF COAST SLIGHT RISK AREA...

   ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS THIS AFTERNOON
   ALONG THE SOUTHERN CALIFORNIA COAST...

   ...SUMMARY...
   Severe storms will develop this evening and spread across the lower
   Mississippi Valley tonight through Saturday morning, 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 this afternoon.

   ...Synopsis...
   Broad cyclonic flow is now established across much of the contiguous
   U.S., as a series of embedded shortwave troughs move from the
   Rockies to the MS Valley in advance of a strong mid-upper jet
   approaching CA.  A broad/diffuse cyclone will be maintained across
   the Plains, which will encourage gradual inland progress of the
   warm/moist sector across the Gulf coast.  A lead shortwave trough
   now over northern Mexico will begin to affect the TX coast by this
   evening, with its influence spreading east-northeastward across the
   lower MS Valley by Saturday morning.

   ...TX coast this evening to the lower MS Valley overnight...
   Convection on Thursday shunted the richer low-level moisture
   southward into the northern Gulf, but low-level air mass recovery is
   underway across the northwest Gulf coast.  Today will be quiet in
   terms of convective potential across the lower MS Valley.  However,
   the risk for severe thunderstorms will increase by this evening as
   the lead midlevel trough from northern Mexico interacts with the
   moisture/buoyancy gradient along the TX coast.  The storm
   environment will support supercells capable of producing large hail
   and damaging winds as convection spreads east-northeastward toward
   LA.

   The more pronounced mass response to the midlevel trough will occur
   tonight across the lower MS Valley, where strengthening low-level
   flow/warm advection will contribute to multiple thunderstorm
   clusters near and after midnight.  Boundary layer dewpoints in the
   mid-upper 60s will maintain surface-based buoyancy through the
   overnight hours, while the increasing low-level shear will favor
   supercells and some risk for tornadoes.  Otherwise, steepening
   midlevel lapse rates (emanating from northeast Mexico/south TX) will
   promote large hail formation in the stronger/supercell storms, as
   well as a risk for damaging winds.
     
   ...Southern CA coast this afternoon...
   The latter system will overspread the central/southern CA coast
   today, with focused ascent in the left-exit region of the mid-upper
   jet.  Steepening lapse rates and some increase in low-level moisture
   with onshore flow later today will drive at least weak surface-based
   buoyancy near the southern CA coast.  Deep-layer vertical shear and
   buoyancy will be sufficient for low-topped supercells and organized
   line segments capable of producing isolated damaging gusts, and
   perhaps a brief waterspout/tornado along the coast.

   ..Thompson/Peters.. 01/20/2017

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