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Area Forecast Discussion (AFD)

Issued by Charleston, SC (CHS)

FXUS62 KCHS 181132

Area Forecast Discussion
National Weather Service Charleston SC
732 AM EDT Wed Sep 18 2019

A cold front will advance through the area early today. High
pressure will then build south into the region later today and
continue into early next week. 


As of 725 AM: Showers and thunderstorm continued across the
lower SC coast and SE GA coast early this morning. Latest runs
of the CAMs indicate that the convection will track to the SW
this morning, pushing across the Altamaha River by 14Z. I will
update the forecast to align PoPs and weather with latest

As of 620 AM: KCLX detected a wind shift, likely associated with
the backdoor cold front, passing across the radar site at 6 AM.
This feature will continue to push southwest through the rest of
this morning. Near and ahead of the front, over the coastal
counties, clusters of showers and thunderstorms were detected by
KCLX. I will update the forecast to adjust PoPs and increase the
mention of thunderstorms.

As of 4 AM: KCLX detected a few showers across the Savannah 
River basin, especially inland of the radar. These showers were 
likely developing ahead of backdoor cold front. Latest runs of 
the HRRR and HREF indicates that convection will significantly 
increase near the mouth of Savannah River around daybreak, 
tracking south across the coastal GA counties through the rest 
of the morning. SPC mesoanalysis indicates that instability will
remain limited to the marine and coastal counties early this 
morning, spreading inland across GA during the daylight hours. 
Based on the timing of the cold front and position of 
instability, I will highlight the GA zones with SCHC to CHC PoPs
for showers and isolated thunderstorms. Drier high pressure 
will build from the north through the day, expected to end 
convection across SE GA by late this afternoon. Using a blend of
MOS, favoring lower values, I will forecast dewpoints to fall 
into the upper 50s across the SC Lowcountry this afternoon. 

Tonight, a wedged ridge of high pressure is forecast to build 
southwest along the east facing slopes of the Appalachians tonight. 
The ridging should support steady NE winds across the CWA tonight. 
The NE winds will likely result in a gradually lowering of low level 
thicknesses and cooling H85 temps across the region. The combination 
of cooling llvl thermal profiles and clearing sky will yield below 
normal low temperatures tonight. Using a blend of cooler MOS, I will 
forecast low temperatures to range from the upper 50s across inland 
SC to the low to mid 60s across SE GA.


Thursday: Surface high pressure will dominate the eastern seaboard. 
Very dry air aloft will reside across the forecast area as Pwats 
will mainly be less than 1 inch. Dry weather is on tap plenty with 
abundant sunshine across most of the area. Atlantic stratocu and 
coastal afternoon cumulus are expected. There could be a potential 
for a few very light showers or sprinkles to brush far south coastal 
GA, but measurable rain potential looks quite low. Cooler temps are 
on tap with highs from the upper 70s north to lower 80s south. The 
coastal corridor will also see breezy conditions, especially closer 
to the beaches and barrier islands.

Friday: Models continue to hint at a weak coastal trough just off 
the GA coast. Latest data also progs a decent amount of isentropic 
ascent into southeast GA. Pwats will increase slightly over 1 inch 
and light patchy showers could certainly develop. Patchy showers 
further inland to the south of I-16 are also possible after mid 
afternoon. We have some slight chance POPs in that area with low-end 
chance POPs along the GA coastal corridor. We should see more clouds 
across the area although northern SC zones may still see a mostly 
sunny day, especially inland. 

Saturday: Deep layered ridging will continue to expand across the 
forecast area. We anticipate a dry day with scattered stratocu 
more prevalent along coastal GA with the onshore flow. Temps
will begin a warming trends with highs in the mid 80s with upper
80s to the west of I-95.


Deep layered ridging will persist into Monday and then tend to break 
down thereafter. Models hint at a weak front and stronger northwest 
flow aloft by mid-week, however the depth of dry-layered air was 
still enough to keep mentionable POPs out of the forecast. Temps are 
expected to run above climo each day with a return to some 90 degree 
high temps early next week inland and southern zones.


A backdoor cold front will slide south across KSAV during the 
early daylight hours. The approach and passage of the cold front
will support a few showers near KSAV this morning, I will 
highlight with VCSH at KSAV. High pressure is expected to build 
across the terminals through the afternoon. The combination of 
mixing and a weak pressure gradient should support gusts near 20
kts at KCHS and KSAV this afternoon. Clearing sky and 
decreasing winds are expected across the terminals this evening.

Extended Aviation Outlook: VFR conditions will prevail.


A backdoor cold front will slide south of the marine zones today. 
Gusty NE winds are forecast across the SC and GA waters today into 
tonight. The combination of wind and sea conditions will be 
highlighted with Small Craft Advisories for all marine zones outside 
of the CHS harbor. The greatest wave heights will occur across the 
outer GA waters, AMZ374, with seas around 10 ft with 11 second 

Northeast winds will dominate throughout the week as high pressure 
anchors to the north. Small Craft Advisory conditions for all 
coastal waters into Friday evening for the nearshore waters and well 
into Saturday night for the offshore Georgia waters. We may need to
issue a SCA for the Charleston Harbor on Thursday as periods of near
25 kt wind gusts are possible. 

The pressure gradient is forecast to weaken during the weekend as 
the surface high bridges closer to the marine waters. Seas will 
gradually subside with time as well. By Monday, winds may become 
light and variable, before becoming south to southwest later in the 

Rip Currents: Moderate Risk today due to lingering elevated long 
period swells and increasing longshore currents due to strengthening 
northeast winds. The rip current risk will remain elevated through 
the end of the week as northeast winds and strong longshore currents 


Low chances for minor coastal flooding around the times of high
tide through late week, especially along the South Carolina 


MARINE...Small Craft Advisory until 8 PM EDT Friday for AMZ350-352.
     Small Craft Advisory until 4 AM EDT Sunday for AMZ374.
     Small Craft Advisory from 11 AM this morning to 8 PM EDT 
     Friday for AMZ354.