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Area Forecast Discussion (AFD)

Issued by Charleston, SC (CHS)

FXUS62 KCHS 172320

Area Forecast Discussion
National Weather Service Charleston SC
620 PM EST Thu Jan 17 2019

High pressure will prevail this evening with a weak upper
disturbance moving through tonight. A storm system will impact
the area Saturday night into Sunday, followed by cold and dry
high pressure early next week. A strong cold front should then
sweep through the area during the middle of next week. 


A band of showers, generally widely scattered or scattered in
nature will push through the area this evening. This is in sync
with a short wave moving across along with our lowest
condensation pressure deficits and some difluence from the upper
jet. Not expecting much QPF, but about a 2-3 hour period of
showers, mainly north and west sections through 06Z. There is
quite a bit of thick cloud cover that also arrives, leading to
some bouncing around of hourly temps. But overall since it will
be mostly cloudy or become mostly cloudy, and there is a SW
synoptic flow, minimum temps tonight will be quite a bit warmer
than the past couple of nights. 

Previous discussion...
The mid-levels will consist of a disturbance passing over the 
region this evening, then moving offshore overnight. At the
surface, broad high pressure will be centered over FL while a
weak cold front approaches from the northwest. The front is not
expected to reach our area overnight. However, a band of
pre-frontal precipitation should. The highest POPs are across 
the Charleston Tri-County area this evening, topping out in the 
chance category. QPF here should be minimal, a few hundredths at
most. This band will weaken/dissipate as it moves across our 
area this evening, allowing for drier conditions after midnight.
Mostly cloudy skies should keep lows in the 40s.


Friday: Dry high pressure will be in place under a mainly zonal flow 
aloft. Latest guidance indicates little moisture advection to the 
region, but soundings hint at some low-lvl moisture producing some 
clouds during the day. Temps will gradually modify, peaking in the 
mid 60s for most areas away from the immediate coast. Overnight, 
cloud cover is expected to increase as moisture gradually deepens 
under a h25 jet aloft. Low temps will be noticeably warmer than the 
precious night, only dipping into the upper 40s well inland to lower 
50s for most locations.

Saturday and Sunday: Conditions will quickly change over the weekend 
as a low pressure system deepens over the Central United States and 
tracks over the Ohio River Valley. A cold front extending south of 
this low will be the main contributer to rainfall late into the 
start of next week. Sfc temps will warm substantially ahead of the 
front on Saturday, approaching the lower 70s over most locations 
during the afternoon. These temps should support some mixing into a 
40-50 kt low-lvl jet ahead of the approaching cold front, suggesting 
gusty southerly winds up to 20-30 mph at times late Saturday into 
early Saturday night, especially for locations along and east of the 
I-95 corridor. The bulk of precip will arrive late Saturday, 
beginning in western most zones late Saturday evening into early 
Saturday night along the approaching cold front. Expect precip to 
quickly spread from west to east across the region Saturday night as 
the cold front sweeps through the area and eventually shifts 
offshore around daybreak Sunday. Instability will be weak given the 
timing of fropa, suggesting little to no threat of thunderstorms, 
except well offshore near the Gulf Stream. Rainfall amounts will 
also be limited due to the quickly moving front, but most areas 
should experience around 1/2 inch rainfall accumulation totals by 
late Sunday morning. Much drier and cooler air will follow the 
front, putting an end to all precip early Sunday afternoon. High 
temps should peak early Sunday, in the mid 50s inland to around 60 
near the coast, before cooling during the afternoon as strong cold 
air advection occurs behind the departing front. Given the setup, 
another day of gusty winds are possible with west winds peaking 
around 20 mph until enhanced low-lvl wind fields shift further 
offshore. Winds could also approach marginal Lake Wind Advisory 
levels on Lake Moultrie Sunday morning before conditions improve 
during Sunday afternoon.


High forecast confidence through Monday night before confidence 
lowers into mid week. High pressure will be building from the 
northwest behind an upper trough pushing offshore to start the 
period. The high should be centered near NC/VA by Tuesday with a 
coastal trough developing off the Southeast U.S. coast. Thereafter 
the forecast becomes more uncertain as some model guidance is more 
progressive with the high shifting offshore than others, but for now 
we trended toward the slower ECMWF solution which matches better 
with the GFS ensemble mean. So we are indicating small rain chances 
returning Tuesday night and increasing Wednesday before diminishing 
behind a cold front into Thursday. 

Most, if not all, locales should be below freezing Monday morning 
with inland areas in the mid to upper 20s and wind chills near 20 
degrees. It's not out of the question that coastal areas reach Wind 
Chill Advisory levels of 15 degrees, especially near Charleston 
where the colder temperatures are expected. Highs Monday should only 
range from around 40 near the Santee River to 50 near the Altamaha 
River. Temperatures Monday night look a bit milder especially near 
the coast due to more onshore winds but interior areas could reach 
similar temperatures as Monday morning. Thereafter temperatures will 
moderate into mid week to near or above normal levels before 
possibly returning back below normal again Thursday behind the cold 


Isolated to scattered -SHRA will be over or near the terminals
prior to 06Z in association with energy aloft, but no
significant impacts will occur. Thus we have VFR ceilings at
both KCHS and KSAv through 00Z Saturday. 

Extended Aviation Outlook: VFR conditions should prevail at both CHS 
and SAV terminals on Friday. Flight restrictions are likely at both 
terminals Saturday evening into early Sunday as numerous to 
widespread showers shift over the region with a cold front. IFR 
conditions are possible at both terminals. Also, breezy/gusty 
conditions are expected later Saturday into early Monday.


Tonight: Broad high pressure will remain centered over Florida,
as a weak cold front drifts into the Southeast late. Initially7
winds from the S or SW will c lock around to the SW and W during
the late evening and overnight, mainly due to land breeze
influences. Speeds will average only 5-10 kt, with seas no more
than 1 or 2 ft. 

Friday through Tuesday: Winds/seas will remain well below Small 
Craft Advisory levels on Friday with high pressure over coastal 
waters. However, conditions will change over the weekend with the 
arrival a cold front from the west Saturday night into early Sunday. 
Ahead of the front, there is a risk of sea fog developing over 
nearshore coastal waters as a southerly wind brings higher levels of 
moisture over cooler waters. However, any sea fog should be short-
lived as low-lvl winds increase ahead of the cold front advancing 
towards the waters Saturday afternoon. Winds/seas will increase/ 
build later Saturday, with Small Craft Advisory conditions beginning 
in offshore Georgia waters Saturday evening, then over most coastal 
waters Saturday night through Sunday. Winds could even gust near 
gale force across the offshore Georgia waters Saturday night into 
early Sunday night while 50 kt low-lvl wind fields pass over the 
coastal waters, but the timing of greatest low-lvl mixing is 
somewhat offset with the low-lvl wind maxima during the period. 
Conditions will then improve Monday as winds turn offshore and 
diminish with weakening cold air advection.


Tides/Coastal Flooding...Astronomical tide levels will be high early 
next week and strong north/northeast winds will help contribute to 
even higher levels which could lead to minor saltwater flooding 
around high tide, especially starting Monday. Thus, Coastal Flood 
Advisories will be possible.