760 
FXUS62 KCHS 221127
AFDCHS

Area Forecast Discussion
National Weather Service Charleston SC
627 AM EST Fri Feb 22 2019

.SYNOPSIS...
A backdoor cold front will slide south across the area through
this evening, then shifting back north as a warm front late
Saturday and and Saturday night. A cold front will then move
through Sunday followed by high pressure into mid week. Low
pressure will then likely affect the area late next week. 

&&

.NEAR TERM /THROUGH TONIGHT/...
As of 620 AM: IR satellite and ongoing upper air observation
indicated a large patch of mid clouds, bases between 8.5-9.0
kft, across the CHS Tri-county area. The passage of the mid
clouds has improve sfc visibilities, resulting in patchy dense
fog. Elsewhere, light south winds and patchy fog remained inland
with widespread dense fog along the coast. I will keep the
current placement and timing of the Dense Fog Advisory.

As of 355 AM: Latest sfc analysis indicated a wavy stationary 
front across the Deep South, curving from near the Fall Line of 
SC and east across the Grand Strand. This front is expected to 
slide south over the SC Lowcountry this morning as a backdoor 
cold front. The front should accelerate southward through the 
daylight hours as cold air damming strengthens across the 
central and western Carolinas. The front is timed to push south 
of the forecast area by early this evening. In the wake of the 
front, sfc pressures should gradually rise and winds will remain
steady from the northeast.

Fog: Sfc observations, Web cams, and IR satellite indicated 
widespread sea fog across the shelf waters and over the coastal 
counties. Inland, IR satellite and observations indicated a large 
ragged patch of mid clouds. These clouds and a few weak showers have 
kept fog limited to a half mile visibility or greater. Near term 
guidance has indicated over the past several runs that dense fog 
will develop inland. However, the persistent mid clouds has not 
shown signs of dissipating or moving significantly. I will keep the 
Dense Fog Advisory for the coastal zones until 10 AM. The case for 
fog tonight is low given NE flow and dewpoints falling into the 50s. 

Temps and PoPs: Temperatures will remain challenging giving the 
dense fog along the coast and the approach and passage of the 
backdoor cold front. As a result, the high temperatures forecast 
this afternoon will feature mid 60s across Berkeley County to low 
80s along the Altamaha River. Using a blend of MOS, low temperatures 
will range in the low to mid 50s, with a corridor of around 60 along 
the Altamaha River.

&&

.SHORT TERM /SATURDAY THROUGH MONDAY/...
Moderate to high confidence this period. Weak high pressure wedging 
is likely to give way to a warm front later Saturday and Saturday 
night before a cold front moves through Sunday. Cooler high pressure 
will then move in for Sunday night and Monday. 

Some showers are expected Saturday and Saturday night with the warm 
frontal passage but not much rain is likely. A better chance of rain 
is then expected Sunday with some higher amounts, although mainly 
less than a quarter of an inch. Can't completely rule out a few 
storms but chances appear too low to mention at this time given the 
generally weak instability. Lastly, fog could develop Saturday night 
and/or push inland from the Atlantic into Sunday.

Temperatures will likely stay above normal through the period, with 
the warmest temperatures on Sunday when record highs/high mins are 
possible depending on the timing of the cold front.

&&

.LONG TERM /MONDAY NIGHT THROUGH THURSDAY/...
Moderate to high confidence through Tuesday with low confidence 
thereafter. Dry high pressure will likely persist through at least 
Tuesday before a coastal trough and/or upper disturbance possibly 
bring a chance of rain back to the forecast through mid week and 
eventually a low pressure system later in the week. So chances are 
pretty good that at least a portion of the mid-late week period will 
be unsettled but timing and rain amounts are quite uncertain. 
Temperatures should be near to above normal through at least 
Wednesday before possibly dropping below normal.

&&

.AVIATION /11Z FRIDAY THROUGH TUESDAY/...
A marine layer will likely persist between the sfc and 1 kft.
The 12Z CHS sounding indicated a weak inversion from the sfc to
around 1000 ft agl. The capped marine layer should support fog 
and low clouds at KCHS into the early daylight hours. I will 
initialize the 12Z KCHS TAF with LIFR conditions, then 
transitioning to IFR conditions through 14Z with daytime mixing.
KSAV will begin the 12Z TAF period with IFR conditions. 
LIFR vis and ceilings mat develop over KSAV between 12-14Z. 
Based on latest MOS and forecast soundings, conditions at KSAV 
should return to VFR by 14Z. A backdoor cold front is timed to 
reach KCHS by 19Z and KSAV around sunset. The approach and 
passage of the cold front should support at least MVFR ceilings,
ENE winds, and scattered showers.

Extended Aviation Outlook: Restrictions likely Saturday into Sunday 
afternoon at KCHS/KSAV. Strong winds also possible Sunday. More 
restrictions possible toward the middle of next week.

&&

.MARINE...
Today: Sfc observations, Web cams, and IR satellite indicated 
widespread sea fog across the shelf waters and over the coastal 
counties. The sea fog will continue across the marine zones until a 
back door slides south across the waters today. Based on the timing 
of the front, a Dense Fog Advisory is in effect for the Charleston 
Harbor and Charleston County near shore waters until 10 AM, AMZ352 
until 1 PM, and AMZ354 until 4 PM. In the wake of the front, winds 
will shift and strengthen from the NE. Wave heights are forecast to 
range from 2-3 ft within 20 NM to 3-4 ft beyond 20 NM.

Tonight: High pressure will strengthen following the passage of the 
cold front, resulting in a 2 mb pressure gradient. NE winds are 
forecast to strengthen to 15 to 20 kt across AMZ350 with seas 
building to 4 to 5 ft this evening. At this time, conditions are 
expected to remain below Small Craft Advisory levels. The case for 
fog tonight is low given NE flow and dewpoints falling into the 50s.

Saturday through Wednesday: A warm front will move north through the 
area later Saturday and Saturday night with a cold front to move 
through later Sunday. Conditions could reach marginal Advisory 
levels near the Gulf Stream Sunday and then again late in the 
period. Otherwise, some dense sea fog could develop Saturday in the 
GA waters and then across the SC waters Saturday night into Sunday 
until the cold front passes.

&&

.TIDES/COASTAL FLOODING...
It's not out of the question that we could reach Coastal Flood 
Advisory levels at Charleston Harbor with the high tide this 
evening. The 5.9 ft MLLW astronomical tide would only require a 
1.1 ft anomaly which is quite possible based on the latest 
forecast for 15-20 kt NE winds along the Charleston County 
coast.

&&

.CLIMATE...
Record Highs for February 24...
KCHS: 81/2017 and previous
KCXM: 81/1930
KSAV: 86/2012

Record High Mins for February 24...
KCHS: 60/1992 and previous
KCXM: 63/2018
KSAV: 64/1980

&&

.CHS WATCHES/WARNINGS/ADVISORIES...
GA...Dense Fog Advisory until 10 AM EST this morning for GAZ117-119-
     139-141.
SC...Dense Fog Advisory until 10 AM EST this morning for SCZ048>052.
MARINE...Dense Fog Advisory until 1 PM EST this afternoon for AMZ352.
     Dense Fog Advisory until 10 AM EST this morning for AMZ330-350.
     Dense Fog Advisory until 4 PM EST this afternoon for AMZ354.

&&

$$
NEAR TERM...NED
SHORT TERM...RJB
LONG TERM...RJB
AVIATION...NED/RJB
MARINE...NED/RJB
TIDES/COASTAL FLOODING...
CLIMATE...RJB