Area Forecast Discussion
Issued by NWS Charleston, SC

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FXUS62 KCHS 210229
AFDCHS

Area Forecast Discussion
National Weather Service Charleston SC
929 PM EST Sat Jan 20 2018

.SYNOPSIS...
High pressure will prevail into Monday. A cold front will move
through the region Tuesday morning, followed by more high
pressure for most of next week.

&&

.NEAR TERM /UNTIL 6 AM SUNDAY MORNING/...
A mid/upper level trough axis will shift over the forecast area
late tonight. Moisture will marginally increase over north Florida,
then shift toward the Atlantic waters to the south of our region
overnight. We maintained a dry forecast as surface high pressure
prevails. We have seen high level clouds increase over the region
this evening and will they will continue in various forms of
transparency overnight. Temps are tricky with these clouds along
with surface dew points generally running a bit lower than
guidance from early in the afternoon. We did not make much in
the way of change overall, mostly tweaking hourly trends for the
next several hours. If clouds do tend to thicken, some areas
may remain above 40 for lows with upper 30s at most areas still
the better bet given recent trends.

&&

.SHORT TERM /6 AM SUNDAY MORNING THROUGH TUESDAY/...
Sunday: A weak mid and upper level low across SE AL, SW GA and the
FL Panhandle will open into a wave as it lifts through the area and
into the Atlantic during the afternoon. This is followed by short
wave ridging that builds overhead late in the day and at night.
Meanwhile at the surface high pressure across the SE will slip
toward the coast the second half of the day, then pulls off into the
ocean after dark, as a subtle coastal trough forms out near the Gulf
Stream. Soundings and cross sections show little moisture, and any
forcing will stay over the Atlantic near the trough. Thus rainfree
conditions are anticipated. We stayed close to the 1000-850 mb
thickness forecast, supporting max temps in the mid and upper 60s
away from the cooler coast where weak onshore flow will limit highs
to the upper 50s and lower 60s. With the increase in heights aloft
and onshore low level flow, temps at night will be warmer than
recently, generally lower-middle 40s inland, upper 40s or near 50F
in downtown Charleston and along the barrier islands. There are
hints of fog late at night, but have disregarded this for now given
that we don`t reach our cross-over temps, there is too much mixing
just off the deck and condensation pressure deficits are no lower
than about 20-30 mb. Warm advection will boost temps into the upper
60s and lower 70s away from the cooler beaches, with lows only down
into the middle 50s in advance of the front Monday night. Elevated
dew points and the onshore flow will allow for at least some areas
of stratus and/or fog at night, mainly east of I-95.

Monday: A deep and occluding low pressure system in the central
Plains will head toward the mid-MS valley and upper Midwest through
the period. A trailing cold front will press east through the TN
valley, reaching into our forecast area by 12Z Tuesday. The western
extension of the surface high will pull east, as a subtle coastal
trough just offshore lifts north and dissipates. Although isentropic
ascent will increase, the forecast area will generally lie between
the trough over the ocean and the front further upstream on Monday.
So we have nothing more than slight chance PoP over Charleston and
eastern Berkeley counties during the daylight hours, then ramp PoP
up into the 40-50% range at night as the front approaches, upper
difluence develops with the 130-140 kt upper jet near the central
Appalachians and heights begin a slow decline. QPF is light, and the
risk of thunder is close to zero with no surface based instability.

Tuesday: The mature low pressure system will move through the
eastern Great Lakes, as the associated cold front slides through in
the morning and into the Atlantic. We still have slight
chance/chance PoP of morning showers, then a rainfree afternoon
behind the front with considerable drier air working its way in with
the zonal flow aloft and rising surface pressures. Deep mixing and
increasing sunshine will allow for gusty westerly breezes to
develop, maybe approaching Lake Wind Advisory criteria on Lake
Moultrie in the afternoon for gusts of 20-25 kt along the lake
shore. There isn`t much cold advection initially behind the front,
so would should still get well into the 60s for high temps.

&&

.LONG TERM /TUESDAY NIGHT THROUGH SATURDAY/...
High pressure will prevail for the long term, bringing dry
conditions and seasonal temperatures.

&&

.AVIATION /02Z SUNDAY THROUGH THURSDAY/...
VFR through 00Z Monday.

Extended Aviation Outlook: Flight restrictions are likely with
a cold front Monday night into Tuesday morning.

&&

.MARINE...
Overnight: South to southwest winds around 10 kt will become
light offshore after midnight as high pressure continues over
the region. Seas 1-2 ft near shore and 2-3 ft well offshore,
closer to the Gulf Stream.

Sunday through Tuesday: The waters will be under the influence of
high pressure early in the week, eventually pulling far enough
offshore where a weak trough will form near near or just east of the
area, before dissipating and giving way to a cold front early
Tuesday. There are no concerns regarding winds and seas Sunday into
early Monday. However, stronger low level wind fields (tempered by
warm advection) will develop later Monday and Monday night, but
probably not enough to cause the issuance of any Small Craft
Advisories. A cold front will clear the area Tuesday morning,
followed by stronger high pressure building into the central U.S.
There is some potential for sea fog Monday night and early Tuesday.

Wednesday and Thursday: Continental high pressure will dominate, and
with bouts of cold advection, steady pressure climbs and occasional
pinching of the gradient we could flirt with SCA`s across parts of
the coastal waters.

&&

.EQUIPMENT...
The KCLX radar is out of service until further notice. Repairs
are ongoing. Adjacent radars include: KLTX, KCAE, KJGX, KVAX and
KJAX.

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

&&

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

&&

$$
NEAR TERM...
SHORT TERM...33
LONG TERM...
AVIATION...
MARINE...
EQUIPMENT...



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