U.S. Severe Weather Forecast (Convective Outlook)

Day Three

acus01 kwns 231606 
Storm Prediction Center ac 231604 

Day 1 convective outlook 
National Weather Service Storm Prediction Center Norman OK 
1104 am CDT Mon Oct 23 2017 

Valid 231630z - 241200z 

..there is a slight risk of severe thunderstorms this afternoon 
into tonight for the Carolinas and southern Virginia... 

..there is a marginal risk of severe thunderstorms from North Florida to 
the middle Atlantic region... 

Severe storms are possible from northern Florida to the mid-Atlantic 
this afternoon and evening. The primary threat will be damaging 
wind gusts, along with potential for a few tornadoes. 

..Carolinas and vicinity through tonight... 
A midlevel shortwave trough over the Tennessee Valley this morning will 
eject northeastward to the southern Appalachians this 
afternoon/evening, and continue north-northeastward to the central 
Appalachians and upper Ohio Valley tonight, in response to an 
amplifying upstream trough over the MS valley. An associated 
surface cyclone in central in will move north-northeastward and 
deepen across lower Michigan and the upper Great Lakes by tonight, as a 
trailing cold front progresses eastward to the southeast Atlantic 
coast and mid-Atlantic region. Ahead of the cold front, the moist 
warm sector will spread northward across NC today and Virginia by tonight, 
with the more unstable warm sector limited to areas east-southeast 
of a secondary low across western NC this afternoon/evening. 

Regional 12z soundings and radar/satellite imagery show a widespread 
band of clouds/rainfall preceding the cold front, where surface 
heating will be limited and lapse rates will remain rather poor. 
Still, the rich low-level moisture (boundary-layer dewpoints of 
68-72 f) will contribute to weak surface-based buoyancy into the 
western Carolinas (sbcape 250-500 j/kg), and somewhat greater 
buoyancy into SC (1000-1500 j/kg). At the same time, low-level 
shear will become quite strong (0-1 km srh of 250-500 m2/s2), which 
will support strengthening of convection in a band with some breaks 
along the front this afternoon from eastern Georgia into the western and 
central Carolinas. Embedded supercells and lewp structures will be 
capable of producing a few tornadoes and damaging gusts, likely 
beginning by 18-20z in the vicinity of the Savannah River, and then 
spreading eastward-northeastward through this evening. 

Weaker low-level lapse rates and near-surface-based buoyancy will 
tend to limit the severe-storm threat with northward extent tonight 
into Virginia. Still, a line of storms in the presence of very strong 
low-midlevel flow/shear and moist profiles will pose some threat for 
isolated damaging winds and a tornado or two. 

.Thompson/leitman.. 10/23/2017 


Mesoscale Discussion

acus11 kwns 231708 
sels mesoscale discussion 
Storm Prediction Center mesoscale discussion 231707 

Mesoscale discussion 1737 
National Weather Service Storm Prediction Center Norman OK 
1207 PM CDT Mon Oct 23 2017 

Areas affected...extreme eastern Georgia...SC...western NC 

Concerning...severe potential...Tornado Watch likely 

Valid 231707z - 231900z 

Probability of watch issuance...80 percent 

Summary...severe threat will increase across SC and western NC into 
the mid-afternoon hours. Tornado Watch will likely be issued by 
18-19z to account for this threat. 

Discussion...water-vapor imagery suggests a well-defined short-wave 
trough is ejecting across the Tennessee Valley/northern Gulf states into 
the southern Appalachians region. Ahead of this feature, an 
elongated corridor of convection, roughly 100 mi wide, extends ahead 
of the front from southern GA, north into eastern Kentucky. While 
instability should remain weak across the warm sector, partial 
partial breaks in cloud cover have contributed to somewhat steeper 
low-level lapse rates along an axis from extreme southeast Georgia into 
central SC where values are on the order of 6 c/km. While buoyancy 
across this region is only subtly greater than areas near the front, 
shallow discrete convection has recently developed across coastal SC 
and radar data suggests stronger updrafts are showing signs of 
sustained rotation. With time frontal convection should intensify 
and embedded circulations are certainly possible given the observed 
shear, in addition to the discrete activity evolving ahead of the 
frontal band. Tornado threat is expected to increase over the next 
few hours and a watch will likely be issued by 18-19z to account for 
this developing threat. 

.Darrow/Thompson.. 10/23/2017 

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


Latitude...Lon 32908234 35038237 36478135 36047980 33018044 32908234