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Area Forecast Discussion (AFD)

Issued by Charleston, SC (CHS)

FXUS62 KCHS 190558

Area Forecast Discussion
National Weather Service Charleston SC
1258 AM EST Sat Jan 19 2019

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. 


As of 1255 AM: KCLX detected a few patches of small showers
north of the Savannah River. I will update the forecast to
increase PoPs over the next 2-3 hours.

Previous Discussion:
A warm front will continue to develop across the area overnight,
gradually lifting north and reaches through all counties except
the Charleston quad-county district by daybreak. There might be
enough isentropic ascent on the 290K and 295K surfaces, along
with some forcing from the upper jet that moves through the SE
states to produce a few light showers in spots. But nothing more
than 10 or 20 PoPs are shown in the forecast, with little to no

A bigger concern will be the build down of stratus that will
lead to at least patchy fog, mainly across locations near and 
inland of I-95. This is where the lowest condensation pressure 
deficits will occur. Since geostrophic winds are around 15 kt 
the potential for dense fog is low. 

Temps tonight will be at their warmest in around 2 weeks, given
considerable cloudiness, dew points as high as 50-55F, and 
little to no dew point depressions.


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 

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.


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.


Prior to the 6Z TAFs, LIFR ceilings and a small shower was
observed over KCHS. Based on observation trends and model
guidance. I will indicate LIFR ceilings through the late night
hours, ceilings lifting to MVFR by 9Z. As a warm front
approaches, KSAV should see at least periods of MVFR ceilings 
by 9Z. The boundary layer should steadily deepen after sunrise,
resulting in VFR conditions by 15Z. Greater momentum transfer
and strengthening llvl winds should result in gusty south winds
by 18Z. A steepening pressure gradient should maintain gusty
winds through the rest of the TAF period. A few pre-frontal
showers should spread across the region this evening. 

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.


Tonight: High pressure will prevail over the coastal waters 
before a warm front developing lifts slowly north across the 
area later tonight. Marine conditions will be good overall,
with SE and S winds to remain at or below 5-10 kt, while seas 
will range between 1 and 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.


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 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.


MARINE...Small Craft Advisory from 10 PM this evening to 1 AM EST 
     Monday for AMZ350-352-354.
     Gale Watch from this evening through Sunday evening for AMZ374.