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Area Forecast Discussion (AFD)

Issued by Charleston, SC (CHS)

                            
000
FXUS62 KCHS 182332
AFDCHS

Area Forecast Discussion
National Weather Service Charleston SC
632 PM EST Fri Jan 18 2019

.SYNOPSIS...
High pressure will move offshore through tonight. A powerful
storm system will impact the area Saturday night into Sunday,
followed by cold, dry high pressure early next week. A strong
cold front will affect the area Wednesday night into Thursday. 

&&

.NEAR TERM /UNTIL 6 AM SATURDAY MORNING/...
Weak isentropic lift on the 290K surfaces, plus modest low level
moisture transport and some forcing for ascent due to a 120-130
kt upper jet across northern Georgia and South Carolina, will
continue to produce isolated to scattered showers. Based on
recent and anticipated trends we have expanded the risk for
showers further northeast and east, and also bumped PoP to as
high as 30%. Still not expecting too much QPF, but most
communities tonight will get a little rain on the order of a
trace to maybe 0.02 inches. Cloud cover has been a little more
extensive than previously forecast, so we have increased the
coverage to 80-100%, and this in turn will delay the fall of
temps early tonight. The build down of stratus could be enough
to result in the formation of fog after midnight, especially
near and west of US-301 in Georgia and far inland South
Carolina. Not yet confident enough to add mention to the
forecast though.

Previous discussion...
This Evening and Tonight: High pressure under a zonal flow aloft 
will gradually slide offshore as a subtle warm front develops to the 
west/southwest. Most guidance suggests a few showers developing over 
inland areas of Southeast Georgia this evening, the spreading north 
into Southeast South Carolina later tonight as the warm front lifts 
north. The front should then continue to shift north of the area by 
daybreak Saturday. Low temps are expected to remain more mild 
than the previous night as a light southerly wind prevails under
abundant clouds near and to the south of the warm front. In 
general, temps should only dip to around 50 degrees inland to 
lower 50s near the coast. Despite mild overnight temps, sfc 
dewpts will also be around 50 overnight, which suggests at least
some potential for patchy fog to develop late tonight should 
breaks in cloud cover occur. At this time, abundant cloud cover 
should limit fog potential and will remain out of the forecast.

&&

.SHORT TERM /6 AM SATURDAY MORNING THROUGH MONDAY/...
Saturday: A warm front will be located across northern South 
Carolina by daybreak with Southeast South Carolina and Southeast 
Georgia poised to remain solidly within the warm sector of a storm 
system organizing over the Mid-Mississippi Valley. While a stray, 
isolated shower could brush the far western and northern portions of 
the forecast area Saturday morning within a region of weak 
isentropic ascent, most of the day will remain rain-free and 
quite warm with highs warming into the lower 70s away from the 
beaches. Rain chances will increase rapidly from west-east 
during the evening hours as a broad area of rainfall ahead of an
approaching cold front moves across the region. Strong moisture
transport of 250-280 g/kg out of the Gulf of Mexico coupled 
with region of strong, deep- layered DPVA/forcing for assent 
ahead of a powerful upper trough digging across the eastern 
United States will support a band of moderate to locally heavy 
rainfall along/ahead of the cold front with rainfall amounts 
averaging around 0.5 inch. There looks to be enough surface and 
elevated instability to maintain a mention of a slight chance of
tstms, but severe weather is not expected this far to the east.
Rain chances will quickly diminish from west-east as daybreak 
Sunday approaches and the cold front shifts offshore. Pops will 
be raised to 100% with overnight lows ranging from the upper 40s
well inland to the mid 50s along the lower South Carolina 
coast. 

Sunday and Monday: The strong cold front will clear the coast by 
sunrise Sunday with intense cold air advection quickly spreading in 
from the west. High temperatures Sunday will recover little from the 
morning lows, only warming 3-4 degrees as H8 temperatures fall 
during the day. Skies will be slow to clear as extensive cold air 
cumulus is likely to develop as the H5 cold pocket (-21C) associated 
powerful upper trough swings through. This should keep skies rather 
cloudy for much of the day with rapidly clearing occurring Sunday 
evening after insolation ceases with sunset. Lows Monday morning 
will bottom out in the upper 20s to lower 30s for most locations 
with a few mid 20s possible across some sheltered areas. Although 
winds will be diminishing, north winds of 5-10 mph will yield 
minimum wind chills from the upper teens to mid 20s around sunrise 
Monday. This is above both the inland and coastal Wind Chill 
Advisory thresholds of 10 and 15 degrees respectively. Monday looks 
to remain quite chilly as the cold H8 temperatures occur during this 
time. Despite sunny skies, highs will only warm into the lower 40s 
across the Charleston Tri-County area to the upper 40s/near 50 
across Southeast Georgia.

Lake Winds: Strong post-frontal cold air advection will support at 
least gusts to 25 kt on Lake Moultrie Sunday in the wake of a strong 
cold front. A Lake Wind Advisory will likely be needed. Unsure how 
much southerly mixing will occur over the cold waters Saturday Night 
ahead of the front. Depending on how strong the pre-frontal wind 
fields get, the advisory could be needed as Saturday night. Waves 
will peak 1-2 ft, mainly central and eastern portions of the lake.

&&

.LONG TERM /MONDAY NIGHT THROUGH FRIDAY/...
High pressure will prevail Monday night through Tuesday night,
then another system will likely bring another round of
precipitation to the area Wednesday night into Thursday.

&&

.AVIATION /00Z SATURDAY THROUGH WEDNESDAY/...
KCHS: A little light rain or showers will occur over or near the
terminal at times tonight as a developing warm front lifts north
through the area tonight. While no significant impacts will
occurfrom any rains, there will be the development of low 
stratus/stratocumulus overnight and Saturday morning, enough to 
result in at least MVFR ceilings. VFR will prevail in the "warm 
sector" behind the warm front for late Saturday morning into the
early evening, and deep mixing will tap into a strengthening 
low level jet in advance of a powerful storm system in the 
Tennessee Valley. This allows for southerly winds to reach as 
high as 20 or 25 kt beginning around 18Z.

KSAV: An area of light rains will impact the terminal this
evening, mainly resulting in MVFR ceilings, but also perhaps a
period of MVFR visibilities. This will occur in association with
a developing warm front that lifts north of the area overnight
and Saturday morning. During this transition there will be the
lowering of stratus/stratocumulus down into the low-end MVFR
range, possibly even IFR at times. VFR conditions will then
return during late Saturday morning through the end of the valid
00Z TAF cycle, and similarly to what occurs at KCHS, gusty
southerly winds will climb near 20 or 25 kt beginning around
18Z.

Extended Aviation Outlook: Extended Aviation Outlook: There is high
confidence in IFR and MVFR cigs/vsbys impacting the the terminals
Saturday night as a large area of rain associated with a cold 
front pushes through. Expect rapidly improving conditions Sunday
morning as the rain pushes offshore, although clouds will linger
through the day. Gusty winds near 20 kt will occur Sunday within
a strong, post-frontal cold air advection regime.

&&

.MARINE...
This Evening and Tonight: High pressure will prevail over the 
coastal waters into the evening and early tonight, before a warm 
front approaches the coast, then slowly lifts north across the area 
later tonight. Conditions will remain well below small craft 
advisory levels through daybreak Saturday. In general, SE and S
winds will remain at or below 5-10 kt. Seas will range between 
1-2 ft. 

Saturday and Sunday: Hazardous marine conditions will dominate for 
much of this period. Pre-frontal low-level jetting ahead of an 
approaching cold front will support south winds increasing to 20-25 
kt with gusts 30 kt nearshore waters and 25-30 kt with gusts to 35 
kt Georgia offshore waters beginning Saturday evening then 
persisting into Sunday in a west strong, post-frontal cold air 
advection regime in the wake of a cold front. There is some 
uncertainty on the gust potential Saturday evening across the 
nearshore waters within the pre-frontal warm air advection regime, 
but suspect there will be enough suppression across the cold shelf 
waters within 20 NM to keep frequent gusts below gale force. Warmer 
waters over the Georgia offshore waters will more easily support 
gusts 35 kt to perhaps 40 kt, especially beyond 40 NM. Small Craft 
Advisories will be posted for all nearshore zones with a Gale Watch 
for the Georgia offshore leg beginning Saturday evening then ending 
Sunday evening as conditions improve. Opted to not issue a 3rd 
period Small Craft Advisory for the Charleston Harbor as the mixing 
profile seems a bit marginal. Seas will peak 4-8 ft nearshore waters 
and as high as 10 ft over the Georgia offshore waters.

Monday through Wednesday: High pressure will build over the waters 
later Monday into mid/late week, resulting in quiet conditions.

&&

.TIDES/COASTAL FLOODING...
Tide levels will be increasing this weekend through the middle
of next week due to astronomical factors. A positive tidal
anomaly is also possible due to wind forcing, potentially 
leading to minor coastal flooding with the morning high tides. 
The greatest chance for this to occur will be Saturday morning and 
Tuesday morning. Blow out tides are also possible with low tide 
Sunday and Sunday night/Monday morning with levels dropping to -1 to 
-2 ft MLLW.

&&

.CHS WATCHES/WARNINGS/ADVISORIES...
GA...None.
SC...None.
MARINE...Small Craft Advisory from 10 PM Saturday to 1 AM EST Monday 
     for AMZ350-352-354.
     Gale Watch from Saturday evening through Sunday evening for 
     AMZ374.

&&

$$
NEAR TERM...
SHORT TERM...ST
LONG TERM...JRL
AVIATION...
MARINE...
TIDES/COASTAL FLOODING...