SPC AC 161255

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0655 AM CST Wed Dec 16 2020

   Valid 161300Z - 171200Z

   ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS ACROSS PARTS OF
   COASTAL NORTH CAROLINA...

   ...SUMMARY...
   Severe thunderstorms are possible across parts of coastal North
   Carolina, and the Gulf coastal areas of west-central/northwestern
   Florida.

   ...Synopsis...
   In mid/upper levels, the split-flow pattern -- now regulated by the
   strong trough extending from eastern IA to south-central TX -- will
   become more unified as the trough moves east and deamplifies
   slightly.  Still, a potent shortwave trough should extend by 00Z
   across portions of OH eastern KY/TN, to near CSG. The trough should
   become even more compact and move over the NJ/NYC area between
   09-12Z.

   At 11Z, surface lows were drawn just south of the AL Coast, and over
   Atlantic shelf waters southeast of CHS.  A warm front extended from
   each low, southeastward to east-southwestward over waters of the
   northeastern Gulf and Atlantic, respectively.  A secondary warm
   front/marine-modification boundary, north of the synoptic front and
   near the west edge of the Gulf Stream, extended from the Atlantic
   low northeastward, parallel to the southeastern NC coast then east
   of HSE near buoy 41001.  The latter boundary will move more slowly
   until the main warm front overtakes it from the deepening low
   eastward.

   The Gulf low should move inland obliquely across the central/
   eastern FL Panhandle today and weaken, while the Gulf warm-frontal
   segment moves eastward and only gradually diffuses. Meanwhile the
   Atlantic low will move north-northeastward over eastern NC and
   evolve into the dominant, deepening cyclone center over the southern
   Delmarva region by 00Z.  The main cold front will extend from there
   across the FL coastal bend region and over the eastern Gulf, with a
   combined warm front over Atlantic waters east of the Hampton Roads
   region.  By 12Z, the primary low should be deeper and occluded
   offshore from the Mid-Atlantic, with the cold front moving
   southeastward over central FL.

   ...Eastern NC...
   Scattered thunderstorms should move northeastward over the outlook
   area this afternoon, offering threats for damaging gusts and a few
   tornadoes.  The outlook follows the expected path of the warm
   sector, east through south of the surface low across easternmost
   portions of NC.  Boundary-layer theta-e in relatively convectively
   uncontaminated parts of the warm sector should contribute to the
   development of surface-based effective-inflow parcels with 500-800
   J/kg MLCAPE.  This is most probable over the Outer Banks and sounds.
   Favorable destabilization is more uncertain/marginal with westward/
   inland extent, given more prior precipitation in the low-level
   trajectories.

   Deep-tropospheric wind fields should increase throughout the day as
   the mid/upper trough and associated strong height gradients
   approach.  This, along with backed low-level winds along and south
   of the warm front, will yield favorable shear for supercells (e.g.,
   effective-shear magnitudes 45-55 kt and effective SRH 150-300 J/kg,
   in forecast soundings with surface-based parcels).  The threat
   should ramp up around midday/18Z and diminish markedly after about
   00Z.

   ...Coastal FL Panhandle to Tampa Bay region...
   An ongoing area of precip -- with isolated, embedded, nonsevere
   thunderstorms north of the surface warm front -- will cross the
   eastern Panhandle and northern FL this morning through midday. 
   Behind that, the northern and eastern fringes of a narrow,
   marginally destabilized slice of warm-sector air will brush along
   coastal areas of the central/eastern FL Panhandle, coastal bend, and
   northwestern FL.  Greater inland penetration of favorably buoyant
   air is possible this afternoon over parts of central FL.  In all
   cases, weak deep-layer lapse rates near the decaying warm-frontal
   zone should keep MLCAPE generally below 1000 J/kg.  Though this is
   not optimal for FL severe, some onshore progression of supercells
   and/or bowing segments from the Gulf may occur while shear still is
   favorable, with a conditional threat for a damaging to severe gust
   or tornado before activity weakens inland.

   Prefrontal surface flow will veer through the afternoon, amidst
   larger-scale mass response to the NC coastal cyclogenesis.  With
   time, this gradually will reduce hodograph size and low-level vector
   shear, and with that, supercell potential.  Destabilization should
   be more marginal with eastward/inland extent across northern FL,
   especially before the surface veering occurs to advect higher-
   theta-e Gulf air farther inland.

   ..Edwards/Smith.. 12/16/2020

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