018
FXUS62 KCHS 191653
AFDCHS

Area Forecast Discussion
National Weather Service Charleston SC
1153 AM EST Sun Jan 19 2025

.SYNOPSIS...
A cold front will move offshore this morning, followed by
arctic High pressure into Monday night. A storm system
originating in the Gulf of Mexico will move up the Southeast
coast Tuesday through Tuesday night, bringing impactful
weather. Another weaker storm system could move up the coast
Thursday night into Friday.

&&

.NEAR TERM /THROUGH TONIGHT/...
The cold front is beginning to enter far interior areas. A band
of showers just ahead of the front will steadily march east,
clearing the coast by 18-19z. Temperatures are peaking in some
areas now, but will likely peak in the next hour two closer to
the coast. Temperatures will then begin to fall through the
afternoon. Highs were once again nudged down in a few areas per
latest observational and short term model trends.

Today: Baggy low pressure noted over North Carolina this morning
is on track to push offshore of the Outer Banks/Virginia Tidewater
this afternoon with its training cold front propagating rapidly
east and offshore of coastal South Carolina and Georgia. KCLX
shows a lingering band of rain moving east across the Midlands
which will brush portions of southern South Carolina over the
next few hours. Near term pops were adjusted slightly showing
20-40% pops for the next few hours, highest over Berkeley
County. Temperatures will peak by early afternoon then fall
through the afternoon mid-late afternoon with the onset of cold
air advection. Highs were nudged down in a few places based on
the latest frontal timing/near term mode trends. Overall, highs
will range from the lower-mid 60s west of I-95 with mid-upper
60s east of I-95. Enjoy this last bit of warmth today before
bitterly cold weather settles in! A Lake Wind Advisory continues
for Lake Moultrie.

Tonight: Dry and considerably colder high pressure will spread
across the region. The main concern will be how chilly temps become
and whether winds remain up for much of the night. Strong cold air
advection and clear skies support temps dipping into the mid 20s
inland to lower 30s near the beaches. The combination of these temps
and a 10-15 mph wind should support apparent temperatures in the 15-
20 degree range for areas west of I-95 and a Cold Weather Advisory
will likely be needed for later tonight. The scenario becomes more
complex across coastal locations and the Tri-County Area, where the
latest forecast places temps slightly warmer and winds slightly
lighter. Trends will need to be monitored, but if temps are
anticipated to become slightly cooler and/or winds slightly stronger
overnight, a Cold Weather Advisory could be required here as well.

&&

.SHORT TERM /MONDAY THROUGH WEDNESDAY/...
Monday: The mid-levels will consist of a very large trough hovering
over most of the CONUS. NAEFS indicates the 500 mb heights in areas
far to our northwest will be 2-3 standard deviations below normal.
This will yield west southwest flow over our area. At the surface,
strong High pressure will be located over the Middle MS Valley in
the morning. It`ll shift eastward, becoming centered over the Mid-
Atlantic region overnight. The High will bring dry conditions, along
with unseasonably cold temperatures. Despite plenty of sunshine,
highs should only peak in the lower 40s. For the second half of the
night, troughing develops just south of our area, ushering moisture
to the north. Some of this moisture could make it to our coastal
waters late. But land areas should remain dry. Even with increasing
clouds, lows will range from the lower to middle 20s across most
areas, to just below freezing along our GA beaches. Northwest winds
will cause wind chills to drop into the teens across most of our
area, which would prompt Cold Weather Advisories.

Tuesday and Wednesday: The mid-levels will consist of a very large
trough hovering over the eastern half of the CONUS in the morning,
with a shortwave stretching from the Great Lakes Region to near the
4-Corners Region. This shortwave will round the base of the trough
late in the day, passing over the spine of the Appalachians Tuesday
night, which will also be when it`s closest to our area. It`ll then
quickly shift offshore on Wednesday. Additionally, the right
entrance region of a 150+ kt jet streak will be located to our
north. This will provide plenty of lift during the Tuesday evening
and overnight hours. At the surface, troughing will be located just
south of our coast in the morning. Additional, a storm system will
be developing in the Gulf of Mexico. This system will rapidly shift
east northeastward into the afternoon and combine with the troughing
to form a larger system along the Southeast Coast Tuesday evening
and overnight. High pressure will also be located to our northwest,
then north during this same time period. The High will be ushering
unseasonably cold temperatures into our area, while the storm system
will have abundant moisture with it, especially further offshore.
The interaction between these two features will cause winter
precipitation across our area.

The 00Z GFS trended much drier, while the 00Z NAM and ECMWF still
have a decent amount of moisture. WPC also trended their QPF much
lower. We went a little higher than WPC. But the net result was a
lower QPF forecast ranging from ~0.15" far inland to ~0.5" along the
immediate coast, with a sliver of ~0.75" along McIntosh County.

The synoptic models generally have precipitation developing along
the coast late in the morning, then increasing in both coverage and
intensity into the late afternoon as it spreads inland. The highest
POPs will be during the evening through most of the night. The
models still have some differences regarding the amount of warm air
above the surface and the resulting precipitation type that comes
with it. With this in mind, our general forecast is as follows...

During the day, most locations near the coast should be warm enough
for rain. Further inland, pockets of warmer and colder temperatures
just above the surface could bring a rain/snow mix.

During the evening, surface temperatures will drop. Most locations
near the coast could get rain transitioning to sleet, except for
McIntosh County and vicinity, which could have freezing rain.
Locations further inland could have mainly snow.

After midnight, as more colder air comes in at the surface and
aloft, snow is expected everywhere, except for McIntosh County and
vicinity, where there could be a wintry mix.

The winter precipitation quickly comes to an end early Wednesday
morning far inland, and closer to the coast a few hours later.

Total snow and ice will all be highly dependent on everything
mentioned above. At this point, we`re forecasting up to 0.25" of
freezing rain accretion for McIntosh County and vicinity, and 1-2"
of snow everywhere else.

The rest of Wednesday will be dry with clearing skies, as High
pressure builds in from the north.

In addition to winter weather, bitterly cold temperatures will
occur. Tuesday, highs should only peak in the mid to upper 30s for
most areas, to the lower 40s across our southernmost GA counties.
Tuesday night, lows will fall into the 20s late, with wind chills in
the teens. A Cold Weather Advisory will be needed. Wednesday, highs
should again only be in the 30s, aided by an arctic air mass and
made colder by frozen precipitation that`s on the ground. We can`t
rule out some places not rising above freezing during the day.
Tuesday night. There is even a chance for some areas to reach
Extreme Cold Warning criteria (10 degrees F or colder).

A Lake Wind Advisory may also be needed Tuesday night.

&&

.LONG TERM /WEDNESDAY NIGHT THROUGH SATURDAY/...
The long term forecast remains somewhat interesting. The synoptic
models point to another storm system slowly moving along/off the
Southeast Coast Wednesday night into Thursday night. The highest
moisture should be just offshore. However, if any moisture makes it
to land, then things will get interesting. Temperatures during the
day should remain warm enough to support rain. But Wednesday and
Thursday nighttime temperatures will be below freezing, supporting
freezing rain. The NBM forecast has chance POPs across our entire
area during this entire time frame. This seemed way too high, so we
trended them lower, but probably didn`t go far enough. This will
need to get reviewed more with future forecasts. Either way,
temperatures will remain well below normal. More Cold Weather
Advisories are expected along with possibly an Extreme Cold Warning
Wednesday night.

&&

.AVIATION /18Z SUNDAY THROUGH FRIDAY/...
19/18z TAF Discussion:
KCHS/KJZI/KSAV: IFR cigs will quickly lift to MVFR then VFR over
the next few hours as a cold front pushes through the terminals.
A brief shower could impact both KCHS and KJZI around 18z as a
line of showers just ahead of the front pushes through. VFR will
then prevail through 18z Monday. Gusty winds will persist
through the afternoon and well into the overnight.

Extended Aviation Outlook: Flight restrictions are expected later
Tuesday through Wednesday morning due to winter weather. Additional
flight restrictions are possible Thursday night due to another round
of winter weather.

&&

.MARINE...
Webcams out of Tybee show patches of locally dense sea fog have
developed. The latest RAP and CONShort data have picked up on
this and keep it in place through FROPA, possibly expanding it
to areas as far north as Edisto Beach. The forecast was recently
updated to reflect patchy fog wording and a Marine Weather
Statement has been issued.

Today and Tonight: Strong low-lvl wind fields shifting across local
waters early along with a strengthening pressure gradient in advance
of a cold front arriving late morning/early afternoon supports
deteriorating conditions across all waters and Small Craft
Advisories for all areas by mid morning. Early this morning, light
rains and/showers have been able to tap into the stronger low-lvl
wind fields, producing gusts around 25 kt for a period of time
supportive of a Small Craft in the Charleston Harbor and nearby
South Carolina waters off the Charleston County Coast. Although
there could be a temporary lull in stronger wind gusts between
rain/showers early this morning, heating ahead of the front should
place gusty winds across the Charleston Harbor late morning and
afternoon, supporting a Small Craft Advisory for much of the day.
Elsewhere, Small Craft Advisories will prevail today and tonight
with southwest winds generally gusting to 20-25 kt, turning west and
gusting upwards to 25-30 kt at times as strong cold air advection
occurs with high pressure entering the region. Seas will also build
throughout the day and night, peaking between 4-6 ft this afternoon
and tonight (largest across outer Georgia waters and northern South
Carolina waters).

Monday: Strong High pressure will be located over the Middle MS
Valley in the morning. It`ll shift eastward, becoming centered over
the Mid-Atlantic region overnight. For the second half of the night,
troughing develops just south of our coast and ushers moisture to
the north. This synoptic pattern will cause gusty NW winds early in
the morning to quickly ease, then gradually turn to the NW and
increase late at night.

Tuesday and Wednesday: A storm system will impact the Southeast U.S
late Tuesday into early Wednesday, with the primary Low passing well
offshore. At the same time, High pressure will be located well to
our north. The pressure gradient between these synoptic features
will be very elevated. Expect strong and gusty winds rapidly ramping
up Tuesday morning and persisting into Wednesday morning. There
could be a brief period of gale conditions Tuesday night into early
Wednesday morning, mainly for the Charleston waters and GA waters
beyond 20 nm. At least Small Craft Advisories will be needed for all
of the marine zones, including the Charleston Harbor, with a
possibility for Gale Watches/Warnings for the two aforementioned
zones. Both winds and seas start to trend lower Wednesday afternoon,
continuing into the overnight hours. Seas look to peak early
Wednesday morning, 5-8 ft for the waters within 20 nm and 8-12 ft
for the GA waters beyond 20 nm. Though, they`ll be slower to subside
into Wednesday night. Finally, mariners should be alert that some
winter precipitation could fall over the coastal waters Tuesday
night into Wednesday morning, reducing visibilities to 1 nm or less
at times.

Thursday: Winds will stay somewhat elevated, while seas slowly
subside.

&&

.CLIMATE...
Record Low Maximum Temperatures:

January 22:
KCHS: 31/1970
KCXM: 31/1970
KSAV: 34/1970

January 23:
KCHS: 34/1970
KCXM: 38/2016
KSAV: 40/2016

Record Low Temperatures:

Thursday, January 23:
KCHS: 19/1985
KCXM: 24/2003
KSAV: 19/1960

Record Snowfall:

Tuesday, January 21:
KCHS: T/1984*
KSAV: 0.2/1910

Wednesday, January 22:
KCHS: T/2022*
KSAV: T/2011*

* A trace is not considered a record in terms of the daily
  climate. Trace amounts shown above are more for information of
  when the last instance of snow occurred on the listed date.

&&

.CHS WATCHES/WARNINGS/ADVISORIES...
GA...None.
SC...Lake Wind Advisory until 11 PM EST this evening for SCZ045.
MARINE...Small Craft Advisory until 5 PM EST this afternoon for AMZ330.
     Small Craft Advisory until 10 AM EST Monday for AMZ350-352-354-
     374.

&&

$$