Area Forecast Discussion
Issued by NWS Charleston, SC

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FXUS62 KCHS 222358

Area Forecast Discussion
National Weather Service Charleston SC
658 PM EST Mon Jan 22 2018

High pressure will prevail today as a weak coastal trough moves
inland. A cold front will move through the region Tuesday
morning, followed by high pressure through the end of the week.
Another cold front will likely impact the area early next week.


On our early evening update, initial temps near shore were
tweaked down a couple of degrees per latest observations. We
have started to see a few reports of low stratus near the
immediate coast, perhaps an indication that sea stratus and
fog will eventually impact some of our coastal areas later
this evening.

A mature surface low and its accompanied deep mid level cyclone
will lift NE through the upper Midwest toward the Great lakes
region. A trailing cold front from the triple point occlusion
will extend south to the Gulf and moves into our far W-NW tier
close to daybreak. This keeps us firmly embedded within the warm
sector through the night, with an increasingly moistening air
mass due to a southerly low level flow and deep SW flow above

Most of the forecast area will be rain-free early tonight.
However, forcing from the approaching front, a 130-140 kt upper
jet off to the NW and a 35-45 kt low level jet moving through
will lead to increasing convective rain chances during the late
evening and overnight. There are mixed signals regarding how
things pan out, with some indications that upstream activity
will do the proverbial split, with the best dynamics off to the
NW and the greatest moisture and instability to the S. For that
reason we have nothing more than 60-70% chances moving in during
the post- midnight period. The progressive nature to the
showers and the possibility that we will be robbed from some of
the moisture, QPF looks to be less than 1/4 inch.

Regarding any t-storm potential, there is limited and mostly
elevated instability and poor lapse rates. However, given strong
shear and modest upward vertical velocities supports at least a
low risk of a few t-storms. We will carry a slight chance of
t-storms all areas after midnight.

After an abnormally warm day, increasing low level wind fields
will prevent temps from dropping too much through the night,
with most places inland from the coast not falling below 60F
until close to sunrise, if at all. The record high minimum temp
at KCHS could be challenged (See CLIMATE section below).


Tuesday: A cold front will shift offshore during late morning hours
with dry high pressure building in its wake. Fropa will put an end
to precip generally from west to east by noon. Temps will remain
mild until fropa occurs, peaking in the upper 60s to around 70
degrees before going through a cooling trend during mid-late
afternoon. Cold air advection behind the front should also support
deep mixing in the low-lvls, creating gusty winds around 15-20 mph
for much of the day. A Lake Wind Advisory could be needed Tuesday
afternoon. Skies should clear out by the evening, setting up a fair
radiational cooling night as high pressure settles over the region.
In general, low temps should dip into the upper 30s away from the
immediate coast.

Wednesday and Thursday: Dry high pressure will dominate the pattern
during the second half of the week over Southeast South Carolina or
Southeast Georgia. Despite sunny skies, weak cold air advection to
the region will keep temps cooler than experienced early week. In
general, high temps will peak in the upper 50s/lower 60s on
Wednesday under a zonal flow aloft, then peak a degree or two cooler
on Thursday behind a h5 shortwave shifting offshore. Wednesday night
lows should dip in the mid 30s away from the immediate coast.


High pressure will prevail Thursday night into Friday, bringing dry
conditions and above normal temperatures. Models are in excellent
agreement showing a strong cold front approaching from the west
Saturday, then moving through the region on Sunday. Given the great
agreement, we have POPs gradually increasing on Saturday, then rain
highlighted on Sunday. Periods of heavy rain and thunderstorms are
not out of the question, but we`re still several days out.


VFR at both sites at 00Z and into this evening. However, as low level
winds increase tonight, lower ceilings will develop, trending down
at least into the MVFR range and continuing through most of
Tuesday morning. With the stratus looming along the coast and
prefrontal moisture below the boundary layer inversion, IFR cigs
will probably occur for a period later tonight or Tuesday
morning. Our confidence for timing and areal extent too low for
any TAF inclusions as yet. A cold front will bring light to
moderate SHRA to both sites overnight into Tuesday morning, with
a very low risk of isolated TSRA, maybe knocking visibilities
down to 5nm. Conditions will improve Tuesday afternoon with
decreasing clouds in the afternoon.

Although presently not shown in the TAFs, there is a chance of
LLWS overnight as a 35-45 kt low level jet moves through. Since
it is marginal due to surface winds of 10-15 kt, we will defer
to later TAF issuances to see if it should be included.

Extended Aviation Outlook: VFR conditions are expected at both CHS
and SAV terminals through late week. Gusty winds around 15-20 mph
are possible at both terminals Tuesday afternoon behind a cold


Tonight: The sub-tropical Atlantic ridge will pull further east
as a cold front travels quickly through the TN valley and
northern Gulf coast region into the SE states before daybreak.
Warm advection will negate a good chunk of the 35-40 kt low
level winds from mixing down, but we still expect conditions to
reach marginal SCA`s on the AMZ350 and AMZ374 waters prior to
sunrise. Thus we have raised the advisory flags for these bodies
of water starting at 5 am Tuesday due to frequent gusts of 25 kt
or higher and some 6 foot seas (especially on the outer GA
waters). Isolated t-storms are also expected in advance of the
cold front after midnight.

Sea fog will be a cause for concern, but the overall favorable
conditions are low, so nothing more than "patchy" fog is
mentioned at this time. Initially and fog would be across the
GA nearshore waters this evening, spreading into the SC waters
thereafter as the flow turns S-SW. But as it does so speeds
increase substantially and then convection moves in. So the risk
for dense fog appears low, but since it is already mentioned in
the Hazardous Weather Outlook, we will leave it as is.

Tuesday and Tuesday Night: A cold front will push through the
coastal waters late morning with dry high pressure building in its
wake. A somewhat enhanced pressure gradient along fropa and cold air
advection behind it will support marginal Small Craft Advisory level
conditions in northern South Carolina waters and offshore Georgia
waters Tuesday morning into Tuesday afternoon. Thus, a Small Craft
Advisory will be ongoing until 5 PM Tuesday. A Small Craft Advisory
could also be needed for the CHS Harbor Tuesday morning, but
confidence remains lower. Conditions will improve over all waters
Tuesday night as the pressure gradient weakens while high pressure
settles over the region.

Wednesday through Friday: Sfc high pressure will dominate the
coastal waters during the second half of the week, but there could
be a period of enhanced winds Wednesday night when an h5 shortwave
shifts offshore. A Small Craft Advisory could eventually be needed
for some of the waters. Conditions are then expected to remain below
Small Craft Advisory levels late week with winds at or below 20 kt
and seas as high as 4-5 ft.


Record high minimums for 23 January:
KCHS: 62/1999
KCXM: 66/1937
KSAV: 65/1937


The KCLX radar remains out of service until further notice.
Repairs are ongoing. Adjacent radars include: KLTX, KCAE, KJGX,

The Downtown Charleston observation site (CHLS1/KCXM) remains
out of service until further notice.


MARINE...Small Craft Advisory from 5 AM to 5 PM EST Tuesday for AMZ350-


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