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000
FXUS62 KCHS 272041
AFDCHS

Area Forecast Discussion
National Weather Service Charleston SC
441 PM EDT Tue Oct 27 2020

.SYNOPSIS...
High pressure will linger through the middle of the week. A
storm system will bring impacts to the region Thursday, which
will help push a cold front across the area Thursday night.
High pressure will then return for the weekend.

&&

.NEAR TERM /UNTIL 6 AM WEDNESDAY MORNING/...
As of 4 PM: Latest SPC mesoanalysis indicated that MLCAPE was
limited to portions of the Deep South, including southern GA. This
northern edge of the MLCAPE is the likely position of a stationary
front. The front should slide north as a warm front tonight as TC
Zeta moves north over the Gulf of Mexico and sfc high pressure
pivots over the western Atlantic. By daybreak Tuesday, the warm
front is expected to push north of the SE US I-20 corridor, leaving
a wavy trough across the forecast area. At H85, GFS indicates that
temperatures will warm from 14C across the inland SC/GA counties
this evening to 16C by daybreak. Forecast soundings show a strong
inversion between 850-750 mb tonight. The combination of the
increasing moisture and weak isentropic lift between 300-310k should
yield increasing cloud cover across the CWA late tonight. However,
partly to mostly clear conditions and light winds may allow patchy
fog to develop late tonight inland, lingering as clouds arrive
during the pre-dawn hours. Min temperatures should range around 5
degrees warmer than values observed this morning, from the low 60s
across CHS Tri-county to near 70 along the Altamaha River.

&&

.SHORT TERM /6 AM WEDNESDAY MORNING THROUGH FRIDAY/...
Wednesday: Aloft, ridging persists over the area while a deep low
resides over the Southern Plains. At the surface, the region is
sandwiched between high pressure offshore to the northeast and
Hurricane Zeta to the southwest. Zeta is expected to make landfall
along the LS/MS coast where it will then track northeast. Its
mid/upper-level counterpart looks to merge with the stronger upper-
level low, which will cause Zeta to undergo an extra-tropical
transition. Surface winds shift primarily to the southwest in the
afternoon advecting offshore moisture; however, most of the deeper,
substantial moisture associated with the remnants of Zeta is
expected remain to our west. Isolated showers are possible in
the far inland SC/GA counties, with PoPs capped at slight
chance/low-end chance in those areas. Max and min temps are
forecast above-normal for this time of year; low to mid 80s
during the day with overnight temps in the upper 60s and lower
70s across the area.

By Thursday, the low pressure system will be situated just to our
west and shall continue to track to our northeast throughout the
day. The strong upper-level system tilting to the west with height
will provide an abundance of DPVA/forcing, but its target area
maintains just west of the forecast area. Again, isolated showers
are possible in the far inland SC/GA counties during the day. By
Thursday night, a cold front trailing the system will approach the
region. Models are struggling to analyze the amount of rainfall
that will be accompanied by the front. While surface forcing
and a decent amount of moisture trailing along the eastern
seaboard (PWATS ranging from 1.5" to 2.5" across model guidance)
are present, substantial upper level forcing and instability
are lacking. A slight chance of thunder is possible with any
isolated showers that form over the forecast area just prior to
the arrival of the front. QPF is minimal at this point, yet some
models show the possibility for a half-inch of rain. On the
other hand, the bigger concern is for gusty winds ahead of,
with, and just behind the front. If there is intense mixing, we
could need a Lake Wind Advisory for Moultrie. Beyond that, drier
air and clearing conditions are expected to move in from the
west after midnight. This will be the last day of 80 degree
temperatures for our area this week, which could put some record
high temperatures at risk of being broken.

Friday: A mid-level trough over the East Coast in the morning will
shift offshore with time. Meanwhile the cold front will have
moved offshore by the morning. High pressure will build to the
north and a significantly drier air mass will move in.
Additionally, subsidence will result in clear skies. Cold air
advection within northwest to north surface winds will lead to
much colder temperatures. Highs will return to near-normal;
only reaching the low to mid 70s.

&&

.LONG TERM /FRIDAY NIGHT THROUGH TUESDAY/...
Zonal flow is expected to prevail in the mid-levels Saturday,
followed by a long-wave trough over the East Coast Sunday into
Monday. At the surface, high pressure passing to our north should
bring dry conditions and below normal temperatures.

&&

.AVIATION /20Z TUESDAY THROUGH SUNDAY/...
VFR conditions at KCHS and KSAV to start the 18Z TAF period. Skies
are forecast to clear overnight at KCHS leading to prime conditions
for radiational cooling to occur, resulting in the formation of
ground fog. This could result in MVFR or even IFR conditions,
however no restrictions have been introduced as of this TAF
issuance. Broken to overcast skies along and ahead of a warm front
across northern Florida will remain overnight at KSAV. This will
limit the amount of radiational cooling and lower the chances for
fog formation. Overnight this warm front will move northward into
southern GA and eventually southern SC. After 14Z prevailing VFR at
both KCHS and KSAV, however cannot rule out a brief moment of MVFR
at KSAV associated with some lingering lower clouds ahead of the
northward moving warm front.

Extended Aviation Outlook: A storm system will bring flight
restrictions Thursday and Thursday night. Additionally, gusty winds
are expected Thursday into Friday.

&&

.MARINE...
Tonight: The center of broad high pressure will shift over the
western Atlantic late tonight. Steady ENE winds should continue
across the marine zones overnight, speeds between 5-10 kts. Seas
are forecast to range between 2-3 ft tonight.

Extended Marine: Weak high pressure located to our northeast
Wednesday morning will eventually shift into the Central Atlantic.
Additionally, Hurricane Zeta is expected to make landfall along
the Lower MS Valley Wednesday night, then combine with a
separate storm system. This new storm system will pass to our
north on Thursday, with its trailing front moving offshore
late Thursday night. The pressure gradient will rapidly
increase Thursday and will result in increasing winds and
building seas. Small Craft Advisories will likely be needed for
all of the waters, including the Charleston Harbor, with gusts
increasing to 25 kt and seas building to 4-6 ft across the
majority of the marine zones. The worst conditions are expected
during the frontal passage Thursday evening. Gusts could reach
Gale force across portions of the waters during that time
period, along with steep, wind-driven seas building up to 7-8
ft in portions of the waters. The front is expected to move
offshore by Friday and high pressure builds in from the west,
which will lead to a brief window of lower winds and seas in the
afternoon hours. However, conditions should deteriorate again
Friday night as cold air advection and the pressure gradient
steepens again. These near gale conditions should persist
through the weekend.

&&

.CLIMATE...
Record warmth possible Thursday, October 29:

KCHS: 85/1946 and 72/1984

KCXM: 82/1984 and 72/1984

KSAV: 87/1996 and 71/2019

&&

.CHS WATCHES/WARNINGS/ADVISORIES...
GA...None.
SC...None.
MARINE...None.

&&

$$

NEAR TERM...NED/RJB
SHORT TERM...BRM
LONG TERM...
AVIATION...CPM/NED
MARINE...BRM
CLIMATE...