U.S. Severe Weather Forecast (Convective Outlook)

Day Three

acus01 kwns 200559 
Storm Prediction Center ac 200558 

Day 1 convective outlook 
National Weather Service Storm Prediction Center Norman OK 
1258 am CDT Tue Mar 20 2018 

Valid 201200z - 211200z 

..there is an enhanced risk of severe thunderstorms from the 
northern Florida Peninsula to parts of the coastal Carolinas... 

..there is a slight risk of severe thunderstorms from the central 
Florida Peninsula to the Outer Banks... 

..there is a marginal risk of severe thunderstorms from Florida to 
parts of eastern North Carolina... 

Strong to severe thunderstorms, capable of damaging winds, large 
hail, and a few tornadoes, will be possible from the Florida 
Peninsula northward to portions of the coastal Carolinas today. 

In concert with considerable 850-700mb cold-air advection over the 
western/central Gulf Coast tonight, a mid-level trough currently 
over the Southern Plains will deepen as it advances towards the 
southeast later this morning. Accordingly, 500mb 
west/southwesterlies will strengthen up to 80-90 kt at the base of 
the trough, in tandem with this jet Max reaching the eastern Gulf 
Coast by mid-day. 

The surface response will feature a couple zones of cyclogenesis. An 
initial cyclone should continue lifting northeast across the 
Carolinas during the morning, generally following a 
northeastward-advancing baroclinic zone. Meanwhile, in association 
with increasing background ascent from the aforementioned shortwave 
trough, a secondary cyclone will likely organize over South Carolina 
during the afternoon. Trailing to the south/southwest of this low, a 
trough will advance towards the southeast US coast through the 
afternoon, likely pushing offshore by early evening. 

..Florida Peninsula to parts of the coastal Carolinas... 
Ahead of the surface trough, a plume of mid/upper 60s dew points 
will filter north, behind prior convection from the night before. 
Atop this wedge of moistening air, mid-level lapse rates are 
forecast to steepen considerably, owing to eastward advection of a 
relatively pristine eml. 00z lch and lix soundings sampled this eml, 
and in the absence of significant Gulf convection overnight, these 
steep lapse rates should be maintained as this plume reaches the 
southeast later tonight. The resultant thermodynamic profile is 
expected to Foster a corridor of MLCAPE values around 1000-2000 j/kg 
from the northern half of the Florida Peninsula to far 
southern/coastal North Carolina by mid-day. 

Supported by increasing background ascent and low-level confluence 
near/ahead of the surface trough, thunderstorms are forecast to 
initially organize from northeast Florida to the Georgia/South 
Carolina coasts by early afternoon. Despite relatively veered 
low-level flow (resulting in straight hodographs), a robust 850mb 
jet (characterized by 50-60kt southwesterlies) will favor ample 
low-level shear. In turn, a mixture of bowing segments and embedded 
supercell structures is possible. Although the lack of more discrete 
modes casts some uncertainty on higher-end severe potential, the 
strength of low-level shear/flow should be favorable for a few 
tornadoes and damaging gusts, while steep lapse rates aloft will 
enable a large-hail threat with more organized updrafts. These cells 
will likely move off the southeast coast by early evening, ending 
the severe threat here. 

Farther south, a secondary confluence band may yield a broken line 
of bowing segments and supercell structures, and these cells would 
approach the western peninsula by late afternoon/early evening. 
Despite weaker background ascent here, mid-level cooling/moistening 
associated with the deepening trough will likely encourage a few 
stronger cells. Any persistent/organized updrafts here will also be 
capable of large hail, damaging winds, and a couple tornadoes 
through the evening hours. 

.Picca/Gleason.. 03/20/2018 


Mesoscale Discussion

acus11 kwns 200855 
sels mesoscale discussion 
Storm Prediction Center mesoscale discussion 200854 

Mesoscale discussion 0155 
National Weather Service Storm Prediction Center Norman OK 
0354 am CDT Tue Mar 20 2018 

Areas affected...eastern Georgia and southern SC 

Concerning...Severe Thunderstorm Watch 16... 

Valid 200854z - 201030z 

The severe weather threat for Severe Thunderstorm Watch 16 

Summary...damaging wind threat persists across eastern Georgia and 
southern South Carolina through early this morning (until at least 

Discussion...mosaic radar imagery and individual radars indicated a 
leading band of storms moving through southern SC at 40 kt, which 
should move off the SC coast by 10z. A second, faster-moving line 
of storms located in east-central Georgia (in Emanuel County at 0830z) 
will advance to the east at 40-45 knots. Surface and objective 
analyses indicated continued moisture advection in advance of these 
storms across eastern Georgia and southern SC, sustaining sufficient 
destabilization for these storms to remain surface based. Clx radar 
showed an outflow boundary associated with the lead storms in SC 
trailing westward through southern Screven County Georgia to the storms 
in Emanuel County. Given strong westerly winds (50 kt) across 
central Georgia per area wsr-88ds and an unstable downstream environment, 
bands of storms will continue to develop and pose a threat for 
damaging winds into the early morning. 

.Peters.. 03/20/2018 

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


Latitude...Lon 32558275 32718223 32848200 32828153 32758101 32908064 
33358031 33498002 32937892 31218095 31308139 31888216 
31948272 32118295 32488308 32558275