Area Forecast Discussion
Issued by NWS Charleston, SC

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FXUS62 KCHS 220630
AFDCHS

Area Forecast Discussion
National Weather Service Charleston SC
130 AM EST Sun Jan 22 2017

.SYNOPSIS...
Upper level disturbances will cross through the region tonight.
Then on Sunday, a strong area of low pressure will deepen across
north Georgia and the Upstate of South Carolina, pushing a strong
cold front through the area Sunday night and Monday. High pressure
will return through mid week and another cold front will cross
the area late next week.

&&

.NEAR TERM /UNTIL 6 AM THIS MORNING/...
Convection continues to develop and lifts N-NE in association
with a warm front over the area, a short wave aloft ands
strengthening low level jet. the associated deep moisture
convergence and low level warm advection will produce numerous
to widespread PoP`s over GA spreading into SC during the
overnight. Given the strength of the shear, showing 0-6 km bulk
shear 60-70 kt, plus TTI in excess of 50 there does remain a low
end chance of isolated severe t-storms. But there is only
minimal instability, so the risk is quite small.

Temps will hold steady or even climb a couple of degrees
overnight within the warm advection regime.

The wet grounds plus some build-down of stratus will contribute
to patchy fog, but with visibilities no worse than about 1-3
miles.

&&

.SHORT TERM /6 AM THIS MORNING THROUGH TUESDAY/...
Sunday and Sunday Night: This is the focus of the forecast for sure
and the area remains centered within a large Enhanced Risk area from
the Storm Prediction Center. The overall setup remains unchanged and
continues to center around a deepening upper low tracking across the
lower Mississippi Valley and to the southern Appalachians Sunday
evening. As this occurs aloft, surface cyclogenesis will occur and a
deepening cyclone will swing around the deep south and across north
Georgia and the Upstate of South Carolina. In fact, the deepening of
the surface low looks quite impressive with the potential for a 985-
990 mb central pressure. As the low deepens, impressive deep layer
shear will surge across the forecast area as the low level and mid
level jet strengthens. Overall, the setup is quite good with
impressive dynamics and deep shear, acting on a environment of
perhaps moderate instability. Models again hint at morning
convection lifting across the area before the best thunderstorm
activity impacts the area in the afternoon and evening.
Convective mode still seems to favor line segments, with
some embedded supercell structures possible as well as even discrete
supercells ahead of the main line. However, despite the overall
impressive setup, there is enough uncertainty regarding the impacts
of morning convective overturning and the exact track/strength of
the surface low that the precise placement and strength of severe
convection is still in doubt. It`s quite plausible that any morning
activity disrupts later destabilization and helps to reduce the
overall severe threat. Also, if the surface low were to be more to
the northwest, the best deepening/pressure falls and overall
dynamics could be displaced too far away from the area. Regardless,
will continue to highlight the possibility for significant severe
weather, mainly damaging wind gusts and a few tornadoes.

After the line of storms pushes offshore in the evening, the rest of
the overnight is expected to be quiet with improving conditions.

Monday through Tuesday: The deep upper low will gradually pull away
from the area, finally replaced by upper ridging on Tuesday. The
surface low will also pull away slowly, and strong west to northwest
flow will prevail on Monday High pressure will then return for
Tuesday. There could be a few light wrap around showers on Monday,
but overall it looks dry. Temperatures will be cooler than recent
days, with highs in the low to mid 60s and lows in the low to mid
40s.

Lake Winds: Winds will become gusty around and over Lake Moultrie
late Sunday night through Monday night. Winds could gust to 25 knots
and a Lake Wind Advisory may eventually be needed.

&&

.LONG TERM /TUESDAY NIGHT THROUGH SATURDAY/...
Surface high pressure centered south of the forecast area Tuesday
night will drift east into the Atlantic on Wednesday. A cold front
will then cross the area on Thursday. Global models are a bit
different regarding precipitation potential, with the Euro the drier
solution and GFS wetter. Forecast currently features 20-30% PoPs.
High pressure will return in wake of the front and persist into the
at least the first half of the weekend.

&&

.AVIATION /06Z SUNDAY THROUGH THURSDAY/...
Convective rains will impact KSAV overnight, making its way into
the KCHS terminal before dawn. Conditions will lower at least
down into the MVFR range, although IFR is possible. In addition,
LLWS will dominate the first 9 hours or so of the 06Z TAF cycle,
with a 35-40 kt low level jet across the area, prior to daytime
mixing causing gusty S-SW surface winds at both sites. Speeds
will peak as high as 25 or 30 kt during Sunday into Sunday
Night.

On Sunday, there will definitely be at least a couple rounds
of showers and tstms, with potential of severe weather and
intense convection wind gusts. The problem to this point is
timing. At this point, prevailing convective rains in the
KCHS/KSAV TAFS was not an option just yet.

Extended Aviation Outlook: Periods of flight restrictions are
likely into Monday. VFR will return thereafter. Gusty winds
will impact the terminals Sunday and Monday.

&&

.MARINE...
Overnight: There is a further tightening of the gradient as
deep low pressure heads east from the southern Plains to the
lower-middle MS Valley. Warm advection will temper the amount
of mixing, but given a 35-40 kt low level jet and geostrophic
winds at 1000 mb 25-35 kt, winds across the local waters will
increase further. SW winds will reach 15 or 20 kt, but with
frequent gusts near or exceeding 25 kt on the outer GA waters
where the best mixing will reside. We have initialized with
the Small Craft Advisory in our outer GA waters where seas
should build to 5-6 ft late.

Sunday through Thursday: The most active period of the forecast
will be Sunday through Tuesday as a strong area of low pressure
system develops and then pulls away from the area. Southerly
winds will increase starting on Sunday and Small Craft
Advisories will be needed for all waters beginning in the
afternoon. The Charleston Harbor may also need an advisory, but
confidence wasn`t high enough to hoist one yet. Then in the
evening winds will increase over the outer Georgia waters and
frequent gale force gusts will be possible. A Gale Watch has
been issued that runs through Monday night. There will also be a
chance for gales across the nearshore waters, but confidence
wasn`t high enough to include in the watch with this package.
Conditions will improve on Tuesday as the low lifts northeast
and high pressure builds into the area. Winds mid-week will
generally be at or below 15 knots with seas less than 4 feet. A
cold front will then cross the waters on Thursday.

Sea fog: Sea fog may be ongoing Sunday morning, but is expected to
dissipate through the day due to the influences increasing winds and
thunderstorms impacting the area in the afternoon and evening
hours.

&&

.CLIMATE...
Record High Minimum Temperatures for Saturday January 21:
KCHS: 59 set in 1954
KCXM: 62 set in 1935
KSAV: 66 set in 1937

Record High Minimum Temperatures for Sunday January 22:
KCHS: 59 set in 1972
KCXM: 66 set in 1937
KSAV: 64 set in 1937

Record High Maximum Temperatures for Sunday January 22:
KCHS: 80 set in 1999
KCXM: 76 last set in 1937
KSAV: 80 last set in 1937

Record Rainfall for Sunday January 22:
KCHS: 1.77 inches set in 1999
KCXM: 1.67 inches set in 1973
KSAV: 1.33 inches set in 1966

&&

.CHS WATCHES/WARNINGS/ADVISORIES...
GA...None.
SC...None.
MARINE...Small Craft Advisory from 5 PM this afternoon to 6 AM EST
     Tuesday for AMZ352-354.
     Gale Watch from this afternoon through late Monday night for
     AMZ374.
     Small Craft Advisory until 5 PM EST this afternoon for AMZ374.
     Small Craft Advisory from 10 AM this morning to 7 AM EST
     Tuesday for AMZ350.

&&

$$
NEAR TERM...
SHORT TERM...
LONG TERM...ECT
AVIATION...
MARINE...
CLIMATE...


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