Area Forecast Discussion
Issued by NWS Charleston, SC

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579
FXUS62 KCHS 220309
AFDCHS

Area Forecast Discussion
National Weather Service Charleston SC
1009 PM EST Sat Jan 21 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 SUNDAY MORNING/...
A very complex mesoscale environment was noted at mid evening
across the forecast area. Various perturbations had developed
in the wake of the severe QLCS which impacted the region
through late afternoon. A few areas have seen gusty conditions
followed by lulls and weak rise/fall couplets have been
traversing the area in the wake of the earlier convection. Most
areas were dry for now under a shield of mid to high layered
clouds.

Big changes expected later tonight as strong low level warm
advection re-tools in associated with a progressive mid level
short wave ejecting from the Gulf Coast states. After midnight,
high resolution models continue to expand deep convection
northeast from southwest and southern GA into the forecast
area, expected to reach se South Carolina nearing daybreak if
not a little sooner. Robust low level wind fields and building
deep layered wind fields are forecast after 08Z with model
consensus 0-1KM helicity progs bulging values into the 200-300
unit range prior to 12Z. This coupled with steep mid level
lapse rates and deep layered instability adding concern for a
late night severe weather potential. Storm modes favor both
supercell and multi-cell modes. We maintained increasing pops
overnight which spread from inland se GA into se SC late.

It`s yet another warm night within the warm advection regime,
and overnight lows could again be near record high minimum
levels, which is about 20-25F above normal.

&&

.SHORT TERM /6 AM SUNDAY 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 /03Z SUNDAY THROUGH THURSDAY/...
Overnight, the main forecast challenge will be timing the next
round of convective rains moving through the region later
tonight. Prior to that uncertainties in cigs overnight as no
low cigs remain from the earlier convection this afternoon.
Through 08Z, perhaps we can maintain VFR conditions until then.
We also have marginal Low level wind shear overnight as the
strong winds persist atop the stable boundary layer.

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 Sunday 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.

Sea Fog: Given a favorable S-SW fetch and the elevated T/Td
atop the cooler waters, we continue to show at least patchy sea
fog in the forecast overnight.

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 Sunday to 6 AM EST Tuesday for
     AMZ352-354.
     Gale Watch from Sunday afternoon through late Monday night for
     AMZ374.
     Small Craft Advisory until 5 PM EST Sunday for AMZ374.
     Small Craft Advisory from 10 AM Sunday to 7 AM EST Tuesday for
     AMZ350.

&&

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



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