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Area Forecast Discussion (AFD)

Issued by Charleston, SC (CHS)

FXUS62 KCHS 181956

Area Forecast Discussion
National Weather Service Charleston SC
356 PM EDT Wed Sep 18 2019

High pressure will provide noticeably cooler and drier
conditions through late week and will prevail through early
next week. A cold front could push into the region around the
middle of next week. 


This Evening and Tonight: High pressure will build across the region 
in wake of a cold front shifting south of the area. However, few to 
scattered showers and perhaps an isolated thunderstorm is possible,
mainly across parts of Southeast Georgia where MLCAPE around
1500 J/kg and PWATs between 1.6-1.8 inches remain. Expect the 
bulk of precip activity to wane this evening, then dissipate 
around sunset due to the loss of daytime heating. Few to scattered
clouds should then clear across the area from north to south 
during the first half of the night. Weak cold air advection 
associated with dry high pressure building across the area should
help maintain a 5-10 mph northeast wind overnight. However, 
cold air advection and drier air will likely support cooler 
temps, with lows eventually dipping into the upper 50s to around
60 well inland (coolest in Southeast South Carolina) and 
mid/upper 60s near the coast. 


Cooler/drier high pressure will prevail through this period. The 
only wrinkle will be for isolated/scattered showers which will cross 
coastal waters and develop onshore within an E/NE low-level flow 
regime. Slight chance PoPs are limited to far southern/coastal 
counties through Thursday, then coverage of slight chance/chance 
PoPs expands somewhat later Thursday night through Friday as a 
coastal trough becomes more pronounced, with highest PoPS across 
southern/coastal counties. Limited instability does not justify any 
mention of thunderstorms through this period. Friday night through 
Saturday, guidance suggests that the surface high will expand and 
will push the chance for showers south of the region. 

Otherwise, high temperatures in the upper 70s north and lower to mid 
80s south will warm into the middle/upper 80s Saturday away from the 
beaches. Thursday night, low temperatures will average 55-60F away 
from t he coast and 65-70F on the beaches. Late Friday night, 
temperatures will fall into the upper 50s to middle 60s inland and 
will bottom out in the upper 60s to middle 70s on the coast. The 
most noticeable change will be the drier air mass featuring daytime 
dewpoints in the 50s/60s through this period. 


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 few showers could potentially bring brief MVFR conditions at the 
SAV terminal late this afternoon into early evening. Otherwise, 
VFR conditions are expected at both CHS and SAV terminals 
through 18Z Thursday. Northeast winds could gust to 15-20 kt 
near 18Z Thursday.

Extended Aviation Outlook: VFR conditions will prevail.


This Evening and Tonight: High pressure will build across coastal 
waters through tonight behind a departing cold front to the south. A 
fair amount of cold air advection associated with a northeast wind 
along with long period swells near 11 seconds will result in ongoing 
Small Craft Advisories for all waters outside the Charleston Harbor 
through tonight. Some guidance suggests winds could increase toward 
daybreak Thursday, necessitating a Small Craft Advisory in the 
Charleston Harbor. In general, northeast winds will gust to
20-25 kts while seas build up to 5-7 ft in nearshore waters and
7-10 ft in offshore Georgia waters.

A tight northeast gradient enhanced by high pressure centered north 
of the waters will maintain hazardous winds/seas through Friday. 
Northeast winds averaging 20-25 knots with gusts to 30 knots will 
contribute to combined seas topping out at 5-8 feet nearshore waters 
and 8-12 feet beyond 20 nm, with worst conditions Thursday through 
Thursday night. Small Craft Advisories are in effect for all waters 
outside Charleston Harbor, and wind gusts could approach SCA levels 
even on the Harbor Thursday.  

Friday and especially Friday night, the pressure gradient/ 
associated winds will relax. While SCA seas could persist into the 
weekend over the outer GA waters, overall we can expect a much more 
tranquil regime this weekend into early next week. Long period swell 
created by offshore TC Jerry could reach coastal waters by early 
next week.  

Rip Currents: High risk of rip currents on all beaches through 
Thursday evening due to strong northeast winds and at least a 
2-3 ft/11 second period swell created by offshore Hurricane 
Humberto. An elevated risk for rip currents will then continue 
through at least the end of this week.


Low chances for minor salt water flooding around the times of high
tide through late week, especially along the South Carolina coast.


GA...High Rip Current Risk through Thursday evening for GAZ117-119-
SC...High Rip Current Risk through Thursday evening for SCZ048>051.
MARINE...Small Craft Advisory until 8 PM EDT Friday for AMZ350-352-354.
     Small Craft Advisory until 4 AM EDT Sunday for AMZ374.