Area Forecast Discussion
Issued by NWS Charleston, SC

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451
FXUS62 KCHS 230139
AFDCHS

Area Forecast Discussion
National Weather Service Charleston SC
939 PM EDT Sat Apr 22 2017

.SYNOPSIS...
Atlantic high pressure will prevail through tonight. A storm
system will bring unsettled weather to the region Sunday into
Monday night. High pressure will build over the region mid to
late week.

&&

.NEAR TERM /UNTIL 6 AM SUNDAY MORNING/...
The backdoor cold front is moving south across North Carolina
this evening and shows up as a well-defined fine line on KRAX
reflectivity data. No major changes were made for the late
evening update. The latest HYSPLIT output on the West Mims
Wildfire burning in the Okefenokee National Wildlife Refuge
shows smoke reaching the southern portions of Long and McIntosh
counties early Sunday. Will include "patchy" smoke in these
areas as the bulk of the smoke looks to remain south of the
Altamaha River.


Dry conditions will prevail overnight as Atlantic high pressure
holds firm. A backdoor cold front will sink south into the
Midlands and Pee Dee early Sunday as weak low pressure moves
into northeast Georgia and high pressure begins to wedge down
east of the southern Appalachians. Nocturnal jetting and
associated speed convergence could support a few showers over
the coastal waters, but any meaningful activity should remain
confined to the western wall of the Gulf Stream where marine
instability will be the greatest. Some of the higher resolution
models are generating some light sprinkles across the CSRA and
southern Midlands early Sunday along the eastern flanks of an
initial push of weak mid-level warm air advection/isentropic
assent, but confidence in measurable rainfall falling across
interior Southeast South Carolina is not high. It will be
another fairly warm night across the area with lows ranging from
the lower 60s well inland to the lower 70s along the beaches
and in Downtown Charleston.

&&

.SHORT TERM /6 AM SUNDAY MORNING THROUGH TUESDAY/...
Models remain persistent with a closed upper low moving across the
Tennessee Valley on Sunday. A strengthening mid/upper level jet on
the low`s backside is then forecast to dig the upper low from north-
east Alabama during Sunday night, across Southern GA on Monday. The
low will lift out by Tuesday as ridging from the lower Gulf Coast
builds north and east. At the surface, a rather disorganized area of
low pressure across north GA will translate east-southeast reaching
the SC coast Sunday Night into Monday morning. The low may brush far
eastern NC on it`s way out the picture during Tuesday. The forecast
for the Sunday night through Tonight night time fame appears quite
complex both on the synoptic and mesoscale. While potential impacts
do not look widespread, a few areas in southeast SC may see some
heavy rains along with an isolated severe weather potential across
the forecast area late Sunday and Sunday evening and perhaps again
Monday across southeast SC.

Sunday: A backdoor surface cold front is expected to get hung up
across the northern half of SC with consensus model solutions
showing it just north of the Santee Cooper Lakes in SC. The
region will remain in the warm sector and temps should warm back
into the mid 80s by afternoon to the south of the front. Models show
the best increases in PWATs and greater mid level height falls will
occur across inland zones along and west of I-95 after 21Z and
especially after 00Z Monday. This region would appear to have the
best potential for severe weather if storms can congeal prior to the
evening decreasing trend in diurnal instability. Zones bordering the
CSRA and southeast midlands to the south of the surface may see a
few storms with damaging winds and large hail late in the day into
the early evening. Across coastal zones, there is evidence a
subsidence cap will keep deep convection at bay for much of the day
but we maintained persistence 20-30 POPs to account for sea breeze
convergence and 850 MB theta-e advection through the afternoon.
Significant upper forcing will remain well off to the west.

Sunday night through Monday night: While the complex cut-off upper
low traverses through GA and off the coastal Carolinas, periods of
unsettled weather are expected across the region. Convective rains
will increase Sunday night and persist Monday and Monday night,
mainly impacting our SC zones with a risk for locally heavy rainfall
and perhaps isolated severe weather again on Monday. The exact path
of the mid/upper low and it`s inherent strong upper forcing will
impact the timing and location of the surface low. Thus, confidence
leaning toward any single model solution at this time remains low.
We have trended much higher on POPs across our northern SC zones
including the Charleston area during much of these periods. Moisture
transport progs suggest that this low will tap some of the deep
tropical moisture over the southwest Atlc however much of the
conveyor seems to be aimed at eastern NC. We have bumped up storm
total QPFs to 1-2 inches across the Charleston Tri-county region to
blend between the ECMWF and GFS/GEM solutions. If the surface low is
more onshore and slower during Monday, there may be enough air mass
destabilization to support a marginal risk of severe weather along
coastal SC with large hail a most probably threat. Across southeast
GA Monday, scattered rains and tstms are less likely on the back
side of the upper low. Cooler temps aloft will keep temps down below
80 degrees with mid 79s across inland southeast GA.

The resulting wrap around moisture will taper off late Monday night
with Tuesday a drier day with the storm system moving well away to
the northeast. Temps will warm to 80-81 degrees along and south of I-
16 with mid to upper 70s across coastal SC as skies gradually to
clear out.

&&

.LONG TERM /TUESDAY NIGHT THROUGH SATURDAY/...
Deep layered ridging will result in dry weather and highs
warming back into the mid to upper 80s.

&&

.AVIATION /02Z SUNDAY THROUGH THURSDAY/...
VFR. Isolated to scattered showers/tstms will likely be lurking
near KCHS and KSAV Sunday, but the bulk of this activity is
expected to remain west of both the terminals through 24/00z.
The risk for rainfall and possible impacts will increase just
beyond the 00z TAF period.

Extended Aviation Outlook: Periods of flight restrictions, mainly
low ceilings, possible Sunday night through Tuesday morning as a low
pressure system moves through. Heavier rains around KCHS are
possible Sunday night into Monday resulting in potential vsbys
restrictions at times as well. VFR conditions will return Tuesday
through mid week.

&&

.MARINE...
Tonight: The coastal waters will remain on the western periphery
of Atlantic high pressure tonight. South winds will stay 15 kt
to 15-20 kt through the night with seas 2-4 ft nearshore waters
and 3-5 ft over the Georgia offshore waters.

Sunday through Thursday: Southerly winds will steadily increase on
Sunday, peaking early in the evening as a surface low approaches and
eventually moves through the waters. We could see sustained winds
near 20 kt along the coast at times with some higher gusts at times.
Seas will build to 3-5 FT by late Sunday night. SCA conditions are a
small possibility, but the situation looks marginal for most of our
waters.

A more significant surge is expected Monday night into the first
half of Tuesday as the surface low begins to depart and the flow
becomes offshore. SCA conditions are possible with highest chances
at this time beyond 20 NM off the GA coast. Winds/seas diminishing
late Tuesday and thereafter as high pressure builds back in.

&&

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

&&

$$



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