Area Forecast Discussion
Issued by NWS Charleston, SC

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FXUS62 KCHS 232238
AFDCHS

Area Forecast Discussion
National Weather Service Charleston SC
638 PM EDT Sun Apr 23 2017

...RECORD RAINFALL LIKELY MONDAY...

.SYNOPSIS...
A low pressure system will affect the area tonight through
Tuesday. High pressure will then build into the area through the
weekend.

&&

.NEAR TERM /UNTIL 6 AM MONDAY MORNING/...
23/22Z mesoanalysis placed 1005 hPa low pressure near KOKZ with
a stationary front extending east along a Waynesboro-Savannah
River Site-Bowman-Saint Stephen line and a cold front extending
south across south-central Georgia through the central Florida
Panhandle and into the northern Gulf of Mexico. Much of
Southeast South Carolina and Southeast Georgia will remain in
the warm sector overnight as low pressure moves east into
southern South Carolina by sunrise Monday. Upper forcing/strong
mid-level height falls associated with the powerful upper low
spinning across south-central Tennessee will gradually
overspread the region from the west this evening and continue
through the night. Increasing moisture from off the Atlantic
coupled with this increased deep layered forcing will support
numerous to widespread showers/tstms working from west- east
through the night.

There are signals that a band of strongly forced convection
will move across the CSRA, southern Midlands into interior
Southeast South Carolina ahead of the surface low by late
evening and interact with the stationary front. There will be a
risk for severe weather with this line as it moves quickly east
with damaging winds and possibly some large hail a possibility.
An isolated tornado or two could also occur closer to the front
itself where low-level wind fields will become locally backed,
thus inducing a pocket of enhanced low-level helicity (noted RAP
SigTor values >1). The risk for severe weather looks to be the
greatest roughly along and north of a Millhaven-Aleda-Ashepoo-
Hollywood-Folly Beach line, including the Charleston Metro Area,
but the tornado risk looks the highest across northern parts of
Colleton, Dorchester and Berkeley Counties. Coordination with
the SPC continues this evening.

The risk for heavy rains will focus across the Charleston Tri-
county area early Monday as the corridor of strongest upper
difluence with the upper low itself begins to cross the area. A
Flash Flood Watch becomes valid around 6 am for Berkeley and
Charleston Counties, but if current mesoscale model trends
persist, this may be moved up by several hours pending
additional analysis. An expansion into Dorchester and possibly
Colleton is also being considered. The risk for flooding will be
highest around high tide Monday morning. High tide in Charleston
Harbor will occur just before 7 am EDT.

Overall, the forecast is in great shape, but some adjustments
were made to various hourly parameters.

&&

.SHORT TERM /6 AM MONDAY MORNING THROUGH WEDNESDAY/...
Monday: The deep mid-level cut-off low pressure area will gradually
end a digging phase and bottom out near east central GA during the
morning and then move east toward the CSRA by late day. Surface low
pressure will also translate from northeast GA into coastal SC by
later in the day. While models are in relatively good agreement on
major synoptic features, there remain subtle timing differences
which could make or break some forecast elements, especially
rainfall amounts. We have a Flash Flood Watch for Charleston and
Berkeley Counties for Monday as deep moisture interacts with strong
upper forcing. The main concerns for the watch were obviously the
heavy rain likelihood in Downtown Charleston along with
portions of upper Charleston County west to Moncks Corner and
Jamestown where models continue to hone in on the axis of
highest amounts. Temps aloft will be cooling during the day
adding to the elevated instability pool to the north of the
Savannah River. Severe weather potential Monday is strictly
limited to the morning and early afternoon hours across the
Charleston 4-county region and highly dependent on breaks in
the clouds during the period where the most potent upper forcing
is expected. We think the risk for isolated storms with large
hail and damaging winds is isolated while the heavy rain threat
remains paramount. Over southeast GA, rain chances will be lower
and despite some colder temps aloft, tstm potential does not
look too impressive either to the south of I-16.

Monday Night: The upper low will begin to move away from the upper
SC coast late while the conveyor of deep moisture pulls well to the
north. Clouds will be variable at times as deep cyclonic flow
prevails. Lows will be cooler in the 55-60 degree range.

Tuesday: Upper heights continue to rise and mostly dry weather
is expected. There could be a few instability afternoon showers
across eastern SC, but coverage and amounts look sparse.

Wednesday: A warm day is on tap as upper ridging dominates. Highs
in the mid 80s.

&&

.LONG TERM /WEDNESDAY NIGHT THROUGH SUNDAY/...
Deep layered ridging will result in dry weather and highs in
the mid to upper 80s. A weak upper shortwave will approach early
Friday but is expected to be deflected to the north.

&&

.AVIATION /22Z SUNDAY THROUGH FRIDAY/...
KCHS: Main concerns are low vsbys/cigs within TSRA. Corridor of
greatest impacts looks to occur roughly 09-15z as a strong line
of convection moves through. Opted to show prevailing MVFR with
a tempo group with vsbys as low as 1 1/2SM, or just below
Alternate Minimums. Briefly lower conditions will be possible as
the main line moves through. The main line should move east of
the terminal by 14-15z with improving conditions thereafter,
although a secondary line could move through the cold front
itself.

KSAV: Right now, it appears the risk for tstms will remain just
north of the terminal, but close enough for possible impacts.
Will show TSRA from roughly 04-07z given latest model trends,
but limited conditions to MVFR for now. Lower conditions could
occur if tstms move farther south than expected. A secondary
line of showers with the cold front itself will cross the
terminal roughly 11-14z. Opted for VFR for now with this line,
but lower conditions could occur depending on how much lingering
instability is realized.

Extended Aviation Outlook: Periods of flight restrictions, mainly
low ceilings, possible through Tuesday morning as a low pressure
system moves through. VFR conditions will return Tuesday afternoon
into late week.

&&

.MARINE...
Tonight: A low pressure system will move into southeast SC/GA
tonight with winds and seas expected to increase. Small Craft
Advisory is in effect for the Charleston county waters and outer
Georgia waters beginning at 02z. Elsewhere, conditions are
expected to remain below advisory levels, although certainly
could not rule out a few gusts nearing 25 kt. Seas on average
will 3-5 ft nearshore, 4-6 beyond 20 nm.

A wind shift from south/southeast to southwest will occur across SC
waters on Monday as the strong upper low digs into GA. While the
onshore flow will subside, a SCA will linger through the day into
the early evening on Tuesday over AMZ350. Over the outer GA waters,
seas will subside during the day but are expected to ramp up again
Tuesday Night in jetting offshore flow.

Modest offshore winds expected Tuesday behind the low pressure
system with seas over the offshore GA waters possibly rising back
above 6 ft for a brief period with a chance for periodic gusts to
25 KT from Grays Reef seaward.

Southerly winds will return mid to late week as Atlantic high
pressure prevails.

&&

.CLIMATE...
Record rainfall amounts for 24 April:
KCHS: 0.81" set in 1943,
KCXM: 1.12" set in 1937,
KSAV: 1.55" set in 1941,

&&

.CHS WATCHES/WARNINGS/ADVISORIES...
GA...None.
SC...Flash Flood Watch from 6 AM EDT Monday through Monday evening
     for SCZ045-050-052.
MARINE...Small Craft Advisory from 10 PM this evening to noon EDT
     Monday for AMZ374.
     Small Craft Advisory from 10 PM this evening to midnight EDT
     Monday night for AMZ350.

&&

$$



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