Area Forecast Discussion
Issued by NWS Charleston, SC

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FXUS62 KCHS 190301
AFDCHS

Area Forecast Discussion
National Weather Service Charleston SC
1001 PM EST Tue Feb 18 2020

.SYNOPSIS...
A warm front will lift north this evening before a cold front
stalls across the area Wednesday. Low pressure will move up the
coast Thursday, then cold high pressure will build in Friday
through the weekend. Another storm system will impact the area
early next week.

&&

.NEAR TERM /UNTIL 6 AM WEDNESDAY MORNING/...
19/02z surface analysis showed the warm front moving slowly
north across the South Carolina Lowcountry and should clear the
Santee River over the next few hours. The initial surge of
nocturnal rain activity has pushed north with only isolated to
perhaps low-end scattered showers poised to push from southwest
to northeast across the region overnight. Rain chances will
increase again from the west late as activity associated with
the cold front draws closer. Pops at daybreak will run from 80%
well inland to 20% along the coast.

The overnight fog forecast is very complicated. Webcams and
surface observations show sea fog lurking along the upper
Georgia and far southern South Carolina coast while farther
inland stratus build-down in the wake of the warm front has
resulted in dense fog in the Sylvania-Statesboro area and
extending southeast into portions of the Savannah Metro Area.
Guidance is similar in expanding the sea fog north into coastal
South Carolina as winds turn more southerly with time and will
slowly merge with fog resulting from stratus build-down across
interior Southeast South Carolina over the next few hours.
Although not all areas are currently seeing dense fog,
consistent short-term model trends with support from NARRE-TL
output have pushed confidence high enough to hoist a Dense Fog
Advisory for all of southern South Carolina and the Georgia
counties bordering the Savannah River. Staggered advisory ending
times were utilized as the southern end of the fog bank in
Georgia is expected to erode from the south after 1 or 2 AM.

Adjusted overnight lows up a degree or two for many areas.

&&

.SHORT TERM /6 AM WEDNESDAY MORNING THROUGH FRIDAY/...
A backdoor cold front will slowly drop through the area on
Wednesday. Fairly strong isentropic ascent will exist during the
morning along with substantial moisture. As the front drops
farther south, a veering profile will decrease the available
moisture and also weaken the isentropic ascent. Fairly
widespread rain during the morning should decrease in coverage
during the day, though we still anticipate scattered rain
through afternoon. A large temperature gradient will exist as
cooler air moves into the north while areas closer to the
Altamaha River remain in the warm sector. Northern areas will
struggle to reach the lower 60s while farther to the south we
could see some mid 70s.

Wednesday night will begin with relatively little rain coverage
but this will quickly change through the overnight hours. A
plethora of upper shortwaves will move in from the west
overnight into Thursday while surface low pressure develops
along the GA/SC coast. Extensive isentropic ascent will
overspread the area after midnight Wednesday night, continuing
through Thursday night. Pockets of heavier rain will be possible
Thursday afternoon and evening as some right entrance region jet
divergence exists. Highs on Thursday will only be in the upper
40s across southern SC and lower 50s in southeast GA due to a
combination of cold air advection and evaporational cooling.

A potent upper trough will drop in Thursday night, pushing the
stalled front along with the surface low farther to the east.
Cold, dry high pressure will build in at the surface. Models
continue to show a possible window over the northern Tri-County
area late Thu night during which a sub-freezing airmass may
coincide with enough lingering moisture to produce a little
snow. The moisture should get stripped out pretty quickly once
the cold air moves in so we do not anticipate heavy enough
precipitation to produce any accumulations. However, a few
flurries or light snow are not out of the question. We therefore
added slight chance for light snow across Dorchester, Berkeley,
and far northern Charleston Counties after midnight Thu night.

Friday will be cold and dry with highs in the mid to upper 40s.
Breezy north winds of 10-20 mph expected.

&&

.LONG TERM /FRIDAY NIGHT THROUGH TUESDAY/...
Models are in fairly good agreement in the general pattern through
early next week. High pressure will drift over the Carolinas
Saturday and eventually into the Atlantic on Sunday. Low pressure
will then lift out of the Southern Plains and into the eastern U.S.
early next week, bringing the next chance of rain to the area.
Temperatures will gradually moderate through the period.

&&

.AVIATION /03Z WEDNESDAY THROUGH SUNDAY/...
KCHS: Low stratus and dense sea fog are expected to overspread
the terminal later this evening. Guidance remains mixed on
exactly how low vsbys will get, but the 00z TAFs will show a
trend of cigs/vsbys dropping below alternate minimums by 03z
with TEMPO conditions right, but not below, airfield minimums
04-08Z. After 08-09z, conditions may begin to improve as low-
level wind fields increase ahead of a cold front. Rain looks to
move into the terminal by 13z, but should end by 17z. Low cigs
will persist. Cold frontal passage will occur around 17z with
northerly winds prevailing thereafter.

KSAV: Low stratus will fill back in later this evening as a warm
front lifts north. Dense sea fog working up the Savannah River
on an easterly wind could briefly impact operations through
about 03z, but it remains unclear how low vsbys will get.
Limited vsby to 4SM for now, but vsbys as low as 1/2SM can not
be completely ruled out. Winds will turn more southerly after
03z behind a warm front which should end any dense fog concerns.
Low stratus and light fog will linger through the night. Rain
chances will increase 13-17z as a cold front approaches.
Guidance is similar in keeping core of the heaviest rains north
of the terminal for now. Cold frontal passage will occur around
19-20z with northerly winds prevailing thereafter.

Extended Aviation Outlook: Flight restrictions likely through
Thursday night due to a combination of low ceilings, occasional
fog, and rain. Gusty northeast winds are expected Thursday
afternoon through Friday.

&&

.MARINE...
Tonight: Expanded the Marine Dense Fog Advisory to include all
nearshore legs and Charleston Harbor through 9 AM.

Wednesday through Sunday: Northeast winds will develop on
Wednesday and increase through the day as high pressure builds
from the north and low pressure takes shape along the coast.
Small Craft Advisory conditions are likely across all waters by
late Wednesday or Wednesday night. Gales are then possible
Thursday afternoon into Friday as a very tight gradient develops
with the low pressure system moving northeast while cold air
builds from the north.

Marine conditions will improve on Saturday, and Advisories
should be able to come down for all waters by Sunday morning. No
additional concerns are expected thereafter.

Sea Fog could develop and/or linger into early Wednesday
afternoon due to surface dewpoints in the lower 60s.

&&

.TIDES/COASTAL FLOODING...
Increasing northeast winds late this week could result in minor
coastal flooding with the morning high tides beginning Friday
morning, especially along the SC coast.

&&

.CHS WATCHES/WARNINGS/ADVISORIES...
GA...Dense Fog Advisory until 5 AM EST Wednesday for GAZ100-101-
     116>118.
     Dense Fog Advisory until 9 AM EST Wednesday for GAZ088-119.
SC...Dense Fog Advisory until 9 AM EST Wednesday for SCZ040-042>045-
     047>052.
MARINE...Dense Fog Advisory until 9 AM EST Wednesday for AMZ330-350-352-
     354.

&&

$$


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