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Area Forecast Discussion (AFD)

Issued by Charleston, SC (CHS)

FXUS62 KCHS 091831

Area Forecast Discussion
National Weather Service Charleston SC
131 PM EST Sun Dec 9 2018

A strong low pressure system will move through the area today.
High pressure will then gradually build from the west and
persist through Thursday. A storm system will affect the area
Friday and Friday night, followed by high pressure. 


The back edge of the steady moderate to locally heavy rains will
clear Charleston county area by 2 pm as low pressure just off 
the coast lifts to the northeast. Behind the low there will be a
continue influx of colder air on northwest and north winds, and
temps will fall into the lower and middle 40s (if they already 
haven't done so). Given the saturated low levels of the 
atmosphere we have added mention of patchy drizzle to the 
forecast in wake of the ending rains. 

We extended the Lake Wind Advisory for Lake Moultrie until 7 pm
to account for good mixing of the colder air across the lake.

Tonight: Wrap around moisture will persist in the wake of the 
departing surface low. Model consensus is strong that isolated 
to perhaps scattered light rain showers can be expected. The 
forecast features rain chances in the 20-30 percent chance 
range. Lows are expected to fall into the upper 30s for most 
areas with some mid 30s possible well inland.


PVA associated with the broad upper trough will linger into 
Monday evening before the trough axis shifts overhead late 
Monday night. Moist isentropic ascent associated with the 
secondary surface low expected to develop off the NC coast will 
maintain a chance for light showers mainly across southern SC 
through Monday evening. NVA should cause any residual 
precipitation to end by late Monday night. Extensive cloud cover
and low-level cold air advection will keep temperatures in the 
low to mid 40s on Monday, with Monday night lows in the mid 30s.

Dry high pressure will move east through the area Tuesday and 
Wednesday. Dry weather and warming temperatures expected.


A powerful closed upper low is expected to approach the area 
Friday, pushing a cold front through the area along with 
numerous showers and perhaps some tstorms over the waters. 
Increasing southerly flow ahead of the front will yield above 
normal temperatures Thursday through Friday. There are model 
differences regarding the speed of the storm system and whether 
precipitation might linger into Saturday before drier high 
pressure builds in.


Abundant moisture will remain place under a strong inversion 
behind low pressure that will lift to the northeast off the 
North Carolina coast tonight and Monday. This will result in IFR
ceilings through the 18Z TAF cycle. Although not explicitly 
shown, a little drizzle is possible at times at both KSAV and
KCHS. This might reduce visibilities once in awhile if it does 

Extended Aviation Outlook: MVFR or lower ceilings likely to 
persist through Monday evening as an upper trough lingers. VFR 
conditions thereafter.


The surface low has passed north of the Charleston County 
waters, and as a result winds will shift to the NW at 20-25 kt 
and gusty in its wake. Small Craft Advisory is in effect for all
waters, except for a Gale Warning on the Charleston County
Atlantic waters. We could briefly see winds near gale force on 
the Georgia waters, but not enough duration or coverage for 
raising a Gale Warning.

Tonight: Winds will gradually come down tonight, but will still
be a solid 15-20 knots by sunrise Monday. Seas will also 
gradually diminish, becoming 3-5 feet in the nearshore waters 
and 4-6 feet in the outer waters by late tonight. Small Craft 
Advisories will be needed across all waters. 

A fairly strong NNW gradient will persist Monday through 
Tuesday as a secondary coastal low develops off the NC/SC coast 
before moving east. Small Craft Advisory conditions are expected
across the Charleston County nearshore waters and Georgia 
offshore waters during this period due to a combination of 25 kt
wind gusts and seas 6 ft or greater.

Relatively quiet marine conditions anticipated Wednesday 
through Thursday as high pressure dominates. Southerly flow will
strengthen Thursday night through Friday ahead of a strong cold
front. Seas will rapidly build due to the long fetch, with SCA 
conditions likely over all waters outside Charleston Harbor by 
Friday afternoon.


Rainfall records for 9 December:

KCHS: 0.68" so far today (old record 0.64"/1948) 
KCXM: 0.99"/1948 
KSAV: 2.44"/1885


SC...Lake Wind Advisory until 7 PM EST this evening for SCZ045.
MARINE...Small Craft Advisory until 7 PM EST this evening for AMZ330-
     Gale Warning until 7 PM EST this evening for AMZ350.
     Small Craft Advisory until 10 AM EST Wednesday for AMZ374.