SPC AC 011257

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0657 AM CST Sat Dec 01 2018

   Valid 011300Z - 021200Z

   ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS ACROSS PARTS OF
   THE MIDDLE MISSISSIPPI VALLEY AND CENTRAL GULF COAST STATES...

   ...SUMMARY...
   Severe storms are possible today across parts of the middle
   Mississippi Valley and central Gulf Coast States.

   ...Synopsis...
   In mid/upper levels, the synoptic- to large-scale pattern for the
   CONUS is transitioning toward a blocky phase with strong ridging
   over the northeastern Pacific, and broadly cyclonic flow from West
   to East Coast by the end of the period.  This will occur as a
   pronounced 500-mb cyclone -- initially centered over KS -- deepens
   somewhat and moves northeastward to southeastern IA.  Meanwhile, an
   upstream shortwave trough -- now apparent in moisture-channel
   imagery offshore OR and northwestern CA -- will move southeastward
   across the Sierra to the southern NV/northwestern AZ region by 12Z. 
   A subtle, slow-moving southern-stream perturbation over the
   north-central/northeastern Gulf will move northeastward across the
   central/northeastern Gulf Coast region today.

   At the surface, the 11Z analysis showed a deep/988-mb surface low
   over east-central KS, with Pacific cold front arching diffusely
   southeastward/southward over AR and northwestern LA behind earlier
   convection, and then across southeast and deep south TX.  Two warm
   fronts were drawn:
   1.  Synoptic warm front from the low generally eastward over central
   MO, southern IL, and eastern KY, and
   2.  Gulf/marine boundary from near MEM southeastward over eastern MS
   to near MOB then southeastward offshore from the west-central FL
   Panhandle.

   The surface low is forecast to shift northeastward through the
   period largely in step with the mid/upper-level cyclone, with which
   it nearly is vertically stacked already.  The cold front should move
   eastward to northern GA, southeastern AL, the western FL Panhandle,
   and the west-central/southwestern Gulf by 12Z, decelerating as it
   becomes largely parallel to winds aloft.  The poleward/synoptic warm
   front will proceed northeastward to the southern upper Great Lakes
   and lower Great Lakes.  The southern/marine warm front is expected
   to move slowly northeastward/inland today, reaching south-central AL
   and southwestern GA this afternoon into early evening.

   ...Middle Mississippi Valley...
   A curving, northwest/southeast-aligned arc of strong-severe
   thunderstorms (including a few supercells) is expected to form
   midday through afternoon and move northward to northeastward across
   the outlook area, offering a sporadic, full-spectrum severe threat. 
   Activity may affect portions of extreme southeastern IA,
   northeastern MO, and western/central IL before weakening in the
   evening with loss of boundary-layer instability.

   East of the cold front, behind the morning clouds/precip, and south
   of the northern/synoptic warm front, a relatively narrow corridor of
   favorable destabilization is expected.  Low/middle-level lapse rates
   will steepen considerably through the day from a combination of
   cooling/DCVA aloft related to the mid/upper cyclone, low-level
   warm/moist advection and areas of direct insolation/diabatic
   heating.  The result should be a corridor of 500-1500 J/kg MLCAPE,
   with forecast soundings showing surface-based effective-inflow
   parcels even for temperatures in the low 60s F.  Strong deep shear
   also is expected across the outlook area beneath 60-90-kt 500-mb
   winds, while backed flow near the warm front yields favorably
   enlarged low-level hodographs.

   ...Gulf Coast States...
   Thunderstorms will pose a risk of damaging gusts, hail and a few
   tornadoes in two main regimes that may have some spatial overlap
   over the course of the entire period, even as they are somewhat
   separated at any given time.  The aforementioned southern-stream
   perturbation has been associated with an area of convection now
   south of the AL and western FL Panhandle coasts, that has persisted
   for about 29 hours since originally forming over the west-central
   Gulf.  This activity has included occasional supercells, based on
   longer-range radar characteristics, in turn indicating a favorable
   kinematic and thermodynamic environment south of the warm-frontal
   zone as well.  Backed flow near the marine front, as well as
   favorably large boundary-layer theta-e , will support a buoyancy/
   shear parameter space suitable for supercells with any of that
   convection that can move ashore along or south of the warm front.

   Farther west, isolated to widely scattered severe storms may develop
   along/ahead of the surface cold front, with damaging gusts being the
   main concern and a tornado or two also possible.  Buoyancy and shear
   conditions in the foregoing warm sector will be favorable for
   supercells, with surface dew points mid 60s to low 70s F and diurnal
   heating supporting MLCAPE 1500-2500 J/kg, amidst 50-60-kt effective-
   shear magnitudes.  One concern will be some veering of prefrontal
   surface winds which will act to reduce both low-level shear/SRH and
   low-level convergence, potentially mitigating convective coverage/
   longevity.  Amount and persistence of low-level forcing for ascent
   is uncertain, but may best support sustained updrafts in prefrontal
   confluence/convergence zones across southern MS/AL into central AL. 
   As such, relatively maximized tornado potential is maintained across
   that area and extended to parts of the Gulf Coast discussed above.

   ..Edwards/Broyles.. 12/01/2018

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