U.S. Severe Weather Forecast (Convective Outlook)

Day Three

acus01 kwns 241249 
Storm Prediction Center ac 241247 

Day 1 convective outlook 
National Weather Service 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.... 

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

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 Alabama and Georgia. 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 Florida and the 
eastern Gulf, will shift eastward across SC south of the low, and 
eastward over the Florida 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. 

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 radiosonde observation 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 


Mesoscale Discussion

acus11 kwns 240232 
sels mesoscale discussion 
Storm Prediction Center mesoscale discussion 240232 

Mesoscale discussion 0552 
National Weather Service Storm Prediction Center Norman OK 
0932 PM CDT sun Apr 23 2017 

Areas affected...southeast Georgia...central/eastern SC 

Concerning...severe potential...watch unlikely 

Valid 240232z - 240500z 

Probability of watch issuance...20 percent 

Summary...thunderstorms are expected to persist into late tonight, 
especially across portions of South Carolina. The strongest cells 
will be capable of locally damaging wind and perhaps a tornado or 
two. Watch issuance is considered unlikely at this time. 

Discussion...thunderstorms have recently increased in coverage and 
intensity across portions of eastern Georgia into west-central SC. This 
increase occurred in conjunction with a collision of the 
westward-moving sea breeze and an eastward-moving outflow boundary 
emanating from earlier convection over Georgia. The resulting composite 
boundary extends southward from a weak surface low over eastern GA, 
with another nearly-stationary boundary extending eastward from the 
low across central SC. 

A few cells have exhibited weak rotation across portions of 
west-central SC in Aiken and Barnwell counties, though these are 
likely slightly elevated and up to now have not appeared to pose an 
appreciable severe risk. Cells further south have shown less 
organization, but as they advance eastward into a somewhat more 
unstable airmass across the coastal plain of SC, some increase in 
intensity is possible with a risk of locally damaging wind in the 
short term. 

Later this evening, a modest increase in the low-level jet is 
anticipated in response to a well-defined upper low that will move 
slowly east-southeastward into Georgia. As this occurs, cells that are 
able to interact with the frontal boundary (which may move slowly 
northward as a warm front with time) may pose a threat of a brief 
tornado or two, though widespread convection and relatively weak 
deep-layer shear will likely keep the severe threat relatively 
limited. While a low-end severe threat will exist for much of the 
night, the magnitude of the threat at any particular location and 
time is currently expected to remain too limited for watch issuance. 

.Dean/guyer.. 04/24/2017 

..please see www.Spc.NOAA.Gov for graphic product... 


Latitude...Lon 32718163 33238157 33728142 34058052 33997956 33827892 
33227913 32787973 32368044 31878113 31948167 32278171