National Weather Service Raw Text Product

598 
FXUS62 KCHS 182018
AFDCHS

Area Forecast Discussion
National Weather Service Charleston SC
418 PM EDT Sat Apr 18 2020

.SYNOPSIS...
High pressure will prevail tonight. A warm front will north 
through the area Sunday and Sunday Night ahead of a strong storm
system. A cold front will push offshore Monday morning with 
high pressure prevailing through mid-week before another storm 
system impacts the area late week. 

&&

.NEAR TERM /UNTIL 6 AM SUNDAY MORNING/...
Tonight: The cold front is situated just upstream of the 
forecast area, and is expected to move to the coast this 
evening. Isolated showers could develop along the front this 
evening down along the Altamaha, so we carry a slight chance of 
showers for a few hours. Otherwise, the overnight should be 
quiet and dry. Skies will be mostly clear for most of the night,
especially the further north you go. Clouds will start to 
increase as sunrise approaches, but shouldn't have much impact 
on temperatures. Lows are forecast to reach the low 50s well 
inland, ranging to the low/mid 60s along the coast. 

&&

.SHORT TERM /6 AM SUNDAY MORNING THROUGH TUESDAY/...
Two rounds of severe thunderstorms are possible through Monday 
morning with damaging winds and tornadoes being the primary 
hazards.

Sunday: A stationary front near the Georgia/Florida border will
begin to propagate slowly north through the day in response to 
a strengthening low-level jet ahead of low pressure moving near 
the Red River Valley. A mid-level shortwave embedded in the 
subtropical jet is forecast to cross the area during the 
afternoon as the warm front crosses the Altamaha River. The 
shortwave is expected to aid in the development of numerous to 
widespread showers/tstms as it begins to interact with a plume 
of PWATs in excess of 1.50 inches. It is unclear exactly how far
north the warm front will progress with diabatic processes 
associated with rain falling north of the strongest thermal 
gradient acting to slow its northward progression. This will 
have an impact on the severe weather potential as the bulk of 
the strongest instability should remain along and south of the 
sharpening warm front. At this time, the slower GFS solution is 
favored which matches the latest HREF trends. Although surface 
winds look to turn southerly across the entire area by 
afternoon, the strongest low-level baroclinicity looks to remain
south the I-16 corridor suggesting that is where the main warm 
front will reside. This is where the best severe risk should be 
centered Sunday afternoon, highest near the Altamaha River. If 
the warm front ends up being farther north, then the risk could 
spread into southern South Carolina. Damaging winds and possibly
an isolated tornado (0- 1km helicity increasing to 200-300 
m2/s2) will be possible during this time with the target window 
looking to be in the 1PM-6PM timeframe. Pops will be raised to 
70-90% for the area, highest over Southeast Georgia, with highs 
warming into the mid-upper 70s. 

Sunday Night into Monday: The initial round of severe 
weather/rain should move off the coast by early evening with 
quiet conditions prevailing through at least midnight. Once this
activity ends, the warm front will quickly jump to the north as
low-level jetting increases even more. There could be some 
isolated to perhaps scattered showers and possibly a few tstms 
that develop within the warm sector. The main concern will 
center on the early Monday time period as a mature QLCS/squall 
line with embedded supercells marches across southern parts of 
the Deep South. The line is currently timed to enter the far 
western zones one to two hours prior daybreak and offshore by 
mid-morning Monday. The atmosphere should steadily destabilize 
through the night as both temperatures and dewpoints rise as 
mixing intensifies within the warm sector. By early Monday, 
MLCAPE will range from 500-1000 J/kg juxtaposed with 0-6km bulk 
shear of 50-70 kt. This should help in QLCS maintenance as the 
line approaches. Similar to Monday's event, embedded supercells 
within the QLCS will pose en enhanced risk for damaging winds 
and tornadoes as 0-1km helicity maxes out between 300-500 m2/s2.
There could be a few strong tornadoes (EF-2 or greater) with 
model SigTor values increasing to 7-11 units. 

One major difference noted between this event and last Monday's
event are wind fields are not as strong and veered. In fact, 
model hodographs do not depict a sharp right kinking 
configuration like they did a week ago. This suggests a somewhat
lower risk for long- track tornadoes, but conditions still look
favorable for a few strong, shorter track tornadoes, especially
south of the Savannah River, including the Savannah Metro Area,
and arching up along the lower South Carolina coast to south of
the Charleston Metro Area. The latest SPC Day 2 WFO guidance 
places the entire region in the Enhanced Risk (3/5) category 
with a hatched area for strong tornadoes, which looks reasonable
at this time.

Pops will be increased to 80-90% all areas with rain chances 
diminishing Monday morning as the cold front pushes offshore. 
Lows in the mid-upper 60s will occur Sunday evening with 
temperatures slowly rising into the 70-73 range overnight. Highs
Monday will warm into the mid-upper 70s with a few lower 80s 
possible in the Savannah- Darien-Reidsville corridor.

&&

.LONG TERM /TUESDAY NIGHT THROUGH SATURDAY/...
Another storm system is likely to move through the region with 
another risk for some severe thunderstorms, mainly Thursday 
afternoon/evening. High pressure should then briefly return 
before another storm system possibly impacts the area late in 
the week. After a brief return back below normal Wednesday 
temperatures should be near to above normal.

&&

.AVIATION /20Z SATURDAY THROUGH THURSDAY/...
VFR conditions are expected to prevail at KCHS and KSAV through
tonight. Gusty winds will continue for a few more hours this 
afternoon. The cold front will push through this evening, 
turning winds north and then northeasterly with time. Sunday 
morning, the front will lift back north as a warm front. Showers
are expected along the warm front beginning first across 
southeast Georgia. Can't rule out thunder as well, but 
confidence isn't high enough to include in the TAF at this time.
Best chance for showers at KCHS will be beyond 18z. MVFR 
ceilings will likely accompany the showers, but again more into 
the afternoon hours on Sunday. 

Extended Aviation Outlook: Shower/tstm impacts are likely 
Monday morning with low vsbys, gusty winds and IFR cigs 
possible. Vsbys/cig issues possible again Thursday as another 
storm system impacts the area.

&&

.MARINE...
Tonight: The Small Craft Advisory for the Charleston Harbor has
been extended through 6 pm to account for continued wind gusts 
into the 25-30 knot range as reported by the Weatherflow sensors
at Shutes Folly and Fort Sumter. Otherwise, winds will steadily
veer overnight behind the southward sinking cold front. By 
sunrise northeast winds will pinch a bit across the South 
Carolina waters with 10-15 knots expected. Seas will drop off, 
but should still average 2-4 feet, and up to 5 feet in the outer
waters early this evening. 

Sunday and Monday: Southeast winds will turn more southerly 
through the day as a warm front sharpens to the south. Winds 
will increase to 20-25 kt by late evening as the warm front 
pushes north of the Santee River and increase to 25-30 kt with 
gusts to 35 kt over the Georgia offshore and Charleston County 
early Monday morning. Small Craft Advisories are likely all legs
and if confidence increases for frequent gusts to gale Gale 
Watches could be needed. Convective wind gusts in excess of 50 
kt and waterspouts could accompany a squall line has it emerges 
off the coast mid-late Monday morning.

Tuesday through Thursday: Small Craft Advisory condition are 
possible Thursday and Thursday Night as another storm system 
impacts the area.

&&

.CHS WATCHES/WARNINGS/ADVISORIES...
GA...None.
SC...None.
MARINE...Small Craft Advisory until 6 PM EDT this evening for AMZ330.

&&

$$

NEAR TERM...BSH
SHORT TERM...ST
LONG TERM...RJB
AVIATION...BSH/ST
MARINE...BSH/ST