SPC AC 140520

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   1120 PM CST Tue Dec 13 2022

   Valid 141200Z - 151200Z

   ...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS TODAY INTO
   TONIGHT ACROSS PARTS OF SOUTHERN AND EASTERN LOUISIANA AND
   MISSISSIPPI...SOUTHWESTERN ALABAMA AND MUCH OF THE FLORIDA
   PANHANDLE...

   ...SUMMARY...
   Severe thunderstorms are expected to develop across parts of the
   central and eastern Gulf Coast states today through tonight.  Some
   will become capable of producing locally damaging wind gusts and
   tornadoes, a few of which could be strong.

   ...Synopsis...
   It appears that blocking will become a bit more prominent within the
   westerlies across the mid-latitude eastern Pacific through the
   western Atlantic during this period, with mid-level highs forecast
   to continue to build offshore of the British Columbia coast and
   across the northern Quebec vicinity.  Several mid-level lows will
   linger at slightly lower latitudes, including one which may continue
   to deepen across the middle Missouri Valley vicinity, within larger
   scale cyclonic flow encompassing much of the interior U.S.

   An occluding surface cyclone may slowly weaken beneath the mid-level
   low, which may only very slowly shift eastward today through
   tonight.  It appears that this will mostly be in response to a
   vigorous short wave perturbation rapidly pivoting around its
   southern and eastern periphery, from west Texas through the Upper
   Midwest/Great Lakes vicinity.  Models suggest that this will be
   accompanied by secondary cyclogenesis along a lead surface front
   advancing east of the lower Mississippi Valley by midday.  The
   surface low is generally forecast to track from central Mississippi
   northward through the Ohio Valley, with a more significant cold
   front overtaking the weaker lead front in its wake.

   Additional cyclogenesis may initiate across the Piedmont of Alabama
   into Georgia late this evening into the overnight hours.  It appears
   that this will be focused near the intersection of the eastward
   advancing cold front and a developing warm front, as a wedge of
   potentially cold air boundary-layer air over the Southeast gradually
   erodes from the southwest.  The cool, dry air is still expected to
   be entrenched across much of Georgia, northern Florida and Alabama
   into eastern Mississippi at 12Z this morning.  However, beneath a
   strengthening southerly low-level jet (including 40-50+ kt around
   850 mb) off the northern Gulf of Mexico, substantive northeastward
   boundary-layer moistening appears possible in a narrow pre-frontal
   plume across southeastern Mississippi, toward west central Alabama,
   by this afternoon, and perhaps across the Florida Panhandle toward
   the Georgia Piedmont tonight.

   ...Lower Mississippi Valley/Eastern Gulf Coast states...
   Aided by at least the glancing influence of the short wave impulse
   emerging from west Texas, thunderstorm development, which probably
   will be ongoing at 12Z this morning across the northwest Gulf Coast
   and lower Mississippi Valley vicinity, is expected to persist and
   perhaps increase through midday.  This likely will include both
   along the initially slow moving lead cold front, from northern
   Mississippi into southwestern Louisiana coastal areas, and ahead of
   the front near the intensifying low-level jet axis.  

   Clockwise-curved low-level hodographs conducive to tornadic
   development probably will be maximized within the unstable
   boundary-layer air across parts of southeastern Louisiana through
   the Hattiesburg and Meridian vicinities of southeastern Mississippi
   by mid to late afternoon.  This is where potential for a couple of
   longer-lived/longer track supercells currently appears greatest,
   accompanied by the potential to produce a strong tornado or two,
   before gradually spreading eastward into portions of southern
   Alabama.

   Upstream, along the cold front, which may begin to surge east of the
   lower Mississippi Valley by this afternoon, a more widespread
   line/cluster of thunderstorms may gradually evolve.  However,
   tornadic and damaging wind potential with this activity is more
   uncertain, as forecast soundings suggest that this convection may
   tend to be increasingly undercut by cold surface-based air.

   Farther east, across the Florida Panhandle, northward into the
   Georgia Piedmont, strengthening wind fields/shear and large-scale
   forcing for ascent may maintain convection with potential to produce
   locally damaging wind gusts and tornadoes late this evening into the
   overnight hours.  The extent of this potential will hinge on the
   degree of northeastward boundary-layer destabilization, which
   remains unclear at this time.

   ..Kerr/Lyons.. 12/14/2022

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