SPC AC 241247

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0747 AM CDT Mon Apr 24 2017

   Valid 241300Z - 251200Z

   ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS OVER PARTS OF
   THE EASTERN CAROLINAS....

   ...SUMMARY...
   Thunderstorms will offer marginal severe-weather risk over parts of
   the eastern Carolinas.

   ...Synopsis...
   The most important upper-air feature for severe-storm potential will
   be a mid/upper-level cyclone, now apparent in moisture-channel
   imagery over much of AL and GA.  The associated 500-mb low,
   initially near CSG, should track in a cyclonically curved arc near
   MCN, SAV, CHS and MYR through the period.  As this occurs, the
   related surface low -- analyzed at 11Z between CHS and NBC -- should
   migrate slowly northeastward across the SC coastal plain today. 
   Overnight, this low effectively should merge with an initially
   separate, north-northwestward-moving surface cyclone now over
   Atlantic waters north of the northern Bahamas.  The combined low is
   expected to move inland over eastern NC by 12Z.  A cold front,
   initially extending south-southwestward over northern FL and the
   eastern Gulf, will shift eastward across SC south of the low, and
   eastward over the FL Peninsula, through the period.  A sharply
   defined warm front, initially from the low northeastward over NC
   coastal waters, is expected to move slowly northward/inland through
   the period.  The timing and inland extent of the front's progress
   likely will be impeded more than most model progs indicate, by
   rain/outflow reinforcement of boundary-layer static stability on its
   poleward side. 

   Meanwhile, a series of mostly low-amplitude, mid/upper-level
   shortwave perturbations will traverse and reinforce a large-scale
   trough, and related cyclonic flow covering much of the western and
   central U.S.  Associated cooling aloft, steep lapse rates and
   marginal low-level moisture, as well as low-level warm advection
   tonight in the central Plains, should contribute to a broad swath of
   potential for isolated to widely scattered thunderstorms from the
   northern Great Basin and northern Rockies across the central Plains.

   ...Carolinas...
   As the cold-core region of the mid/upper cyclone approaches, pockets
   of surface heating occur in cloud breaks, and lapse rates steepen
   atop available low-level moisture, a few bands or arcs of
   thunderstorms are expected to develop episodically and move east-
   northeastward across the outlook area.  Activity in the warm sector
   will access pockets of high-theta-e marine air as well as impinge
   upon the warm-frontal zone, while offering isolated hail near severe
   limits, damaging gusts and a conditional/marginal tornado risk.  The
   hail risk may extend somewhat poleward of the warm front with
   elevated convection.  Minor coastward adjustments to the marginal-
   severe probabilities (especially wind) are made over SC to account
   for a slightly less-inland expected penetration of favorable
   surface- and near-surface-based effective-inflow parcels, based on
   the expected track of the surface low.  

   Forecast soundings and the modified 12Z CHS RAOB suggest up to about
   1500 J/kg warm-sector MLCAPE may develop, decreasing quickly along
   and north of the warm front and under persistent convective cloud/
   precip plumes.  Deep shear will remain modest, with a substantial
   component of the mean flow parallel to the convergence zone(s)
   providing convective-scale forcing.  This should contribute to a
   dominant linear mode, with isolated bow/lewp formations and
   ephemeral QLCS mesovortices embedded -- particularly near the warm
   front where low-level vorticity, SRH and storm-relative flow all
   will be relatively maximized.

   ..Edwards/Peters.. 04/24/2017

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