Area Forecast Discussion
Issued by NWS Charleston, SC

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FXUS62 KCHS 200307

Area Forecast Discussion
National Weather Service Charleston SC
1007 PM EST Sat Jan 19 2019

A powerful storm system will impact the area tonight into
Sunday, followed by cold, dry high pressure early next week. A
strong cold front will affect the area Wednesday night into


Late this evening: Still no real significant changes needed to
the going forecast. Radar imagery shows the outer edges of the
frontal zone convection is getting into the western portions of
the forecast area. As expected, instability is beginning to wane
and the intensity of the convective line is correspondingly
decreasing. There are currently no convective warnings upstream
of the area, other than a marine warning along the Florida
panhandle. Winds have increased ahead of the line and gusts will
continue through sunrise regardless of convection. The severe
threat remains low through the night, but still can`t be
completely ruled out. Previous discussion continues below.

Early this evening: Radar imagery shows the first showers just
starting to scrape the far inland southeast Georgia zones ahead
of the approaching line of convection associated with a strong
cold front. The main portion of the convection is now pushing
into far western Georgia, and so far the HRRR seems to be
handling the timing pretty well. In fact, the forecast looks to
be in great shape as far as rain chances and timing go. Current
thinking remains that the line of convection will push into the
forecast area around or just after midnight. This would put it
into the Savannah area in the 1-3 am time period, and the
Charleston area roughly 3-5 am. The background wind field with
this system is quite impressive, as evidenced by the degree of
severe weather back to the west across the Deep South this
afternoon. That same wind field will arrive into the forecast
area after midnight, the main difference will be the available
instability. Models continue to show weak instability with the
front overnight, with CAPE values in the 200-400 J/kg range.
This certainly warrants thunder in the forecast. The severe
weather potential is essentially unchanged, damaging wind gusts
will be possible, as it won`t take much to mix down some
impressive winds. In fact the 00z KCHS RAOB had 41 knots at ~2
kft this evening. Also, can`t rule out an isolated tornado,
though the shear is mainly unidirectional. Some line segments
with a broken-S threat are possible. The good news is the line
will move through quick and the active weather should be gone by


Sunday: Guidance is similar in pushing the cold front off the coast
just before daybreak Sunday with breezy conditions developing in its
wake. All the HREF members have the main rain band offshore by
20/12z with only a few post frontal sprinkles possible, mainly
across coastal Charleston County. Although daily highs will occur at
midnight, "daytime" highs will only reach lower-mid 50s for
most locations with upper 50s/near 60 possible in the Charleston
Metro Area just after sunrise with temperatures falling back into
the lower 50s for all areas during the afternoon. Clearing will be
slow to occur during the morning with cold mid-level temperatures
associated with the passing upper trough likely to support steep
lapse rates and lingering stratocumulus. While a sprinkle or two
could redevelop over mainly Southeast South Carolina within the mid-
level cold pocket, a rapid deepening of the post-frontal dry layer
suggests measurable rainfall is unlikely. Clouds will quickly
dissipate as sunset approaches with clear prevailing through the
overnight. Lows will range from the upper 20s well inland to the mid
30s along the Georgia beaches. A few mid 20s could occur in some
sheltered locations well inland. Wind chills just before sunrise
Monday will drop into the upper teens to mid 20s, coldest roughly
along/north of the I-26 corridor.

Monday and Tuesday: Quiet and dry conditions will prevail through
the period as heights build aloft and surface high pressure extends
into the Southeast States. Monday will start off quite chilly with
highs only topping out in lower 40s to around 50, coldest across the
Charleston Tri-County area and warmest along the Altamaha River.
Temperatures will moderate through Tuesday with highs topping out in
the lower 50s across northern portions of Dorchester and Berkeley
Counties to around 60 in the Darien-Ludowici corridor. Lows Tuesday
morning will range from the upper 20s well inland to the upper 30s
along the Georgia coast where low-level winds from off the Atlantic
will help moderate conditions there in response to the formation of
a weak coastal trough just offshore.


High pressure will move offshore Tuesday night and Wednesday,
then a low pressure system and associated cold front will sweep
through Wednesday night and Thursday. Numerous showers are
expected to accompany the front before dry weather returns by
late Thursday. Temperatures will be above normal Wednesday and
Thursday, then a few degrees below normal Friday into Saturday.


MVFR conditions are expected to arrive just ahead of the front
with a line of showers and thunderstorms. Added in a TEMPO group
at both sites for TSRA with winds gusting into the 35 knot
range. Included IFR visibility with the TEMPO group. Looks like
the main threat at KSAV will be 07-10z and 08-11z at KCHS based
on current timing. Also of note, the background wind field will
be quite strong and wind shear is carried at both sites
beginning around 06z. Once the front moves through and
convection ends, MVFR ceilings will hang on through roughly mid
morning before improving to VFR. Clouds should scatter and clear
out quickly in the early afternoon. Winds will turn westerly and
remain quite gusty through the day with frequent gusts into the
25-28 knot range expected.

Extended Aviation Outlook: There are no concerns with a greater
than 30% probability of occurrence.


This Evening and Tonight: South winds are expected to gradually
increase across all coastal waters as the pressure gradient enhances
well ahead of a cold front shifting over the Deep South. Warm-air
advection over cooler waters ahead of the approaching cold front
will limit low-lvl mixing potential during the evening, but
stronger low-lvl wind fields of approximately 50-60 kt are expected
to spread over the coastal waters beginning late evening into
early overnight hours ahead of a cold front that will likely
shift offshore near daybreak Sunday. Given the setup, south winds
of 10-15 kt with gusts to 20 kt are expected to increase to
20-25 kt with gusts as high as 35-40 kt at times overnight, greatest
over offshore Georgia waters and northern South Carolina waters
after midnight. Gale Warnings remain in place for all nearshore
and offshore waters overnight, with the exception of the CHS
Harbor where wind gusts should fall just short of gale force.
Given the possibility of thunderstorms after midnight, special
marine warnings could be needed in the CHS harbor. Seas will
build overnight until the front shifts offshore. In general,
seas will peak between 5-8 ft in nearshore waters and 7-10 ft in
offshore Georgia waters.

Sunday: Winds will peak just after daybreak as post-frontal cold air
advection will be the most intense during this time. Frequent gusts
to gales are likely over the Georgia offshore waters where mixing
will be the strongest, but a few gusts to gales can not be ruled out
across the chillier nearshore waters. Per coordination with neighboring
offices, the Gale Warning for the nearshore waters will be extended
until noon Sunday, although it will be a bit marginal after 15z
or so. A late afternoon ending time for the Georgia offshore waters
Gale Warning looks good. The Gale Warnings will likely be replaced
with Small Craft Advisories due to lingering winds 25 kt and seas
6ft that are expected linger into Sunday Night.

Monday through Wednesday: High pressure will bring improving conditions
to the marine area Monday through Wednesday. The approach of a cold
front will likely yield Small Craft Advisory level conditions over
most of the waters Wednesday night into Thursday.


The combination of astronomical factors and wind forcing could
result in minor coastal flooding with the morning high tide on
Tuesday when wind directions will become NE.

Blow out tides are also possible with low tide Sunday and
Sunday night/Monday morning with levels dropping to -1 to
-2 ft MLLW.


SC...Lake Wind Advisory until 6 PM EST Sunday for SCZ045.
MARINE...Gale Warning until noon EST Sunday for AMZ350-352-354.
     Gale Warning until 6 PM EST Sunday for AMZ374.
     Small Craft Advisory until 6 PM EST Sunday for AMZ330.



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