Area Forecast Discussion
Issued by NWS Charleston, SC

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FXUS62 KCHS 240620

Area Forecast Discussion
National Weather Service Charleston SC
220 AM EDT Mon Apr 24 2017

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


Early this morning: Surface analysis shows that the surface low
is now entering our inland southeast Georgia counties, with an
area of convection progressing to the northeast now over
southeast South Carolina. So far, the convection has not
exhibited updrafts that are very strong, and so far wind gusts
sampled by AWOS/ASOS sites have been around 30 knots.
Mesoanalysis and satellite imagery shows that the main cog in
this event, the deepening mid/upper level low across the
Tennessee Valley and the Deep South, still needs to draw closer
to the forecast area for the region of strongest forcing to
arrive. Once this slot of upper level divergence and coupled low
level convergence does arrive, it will be acting on a stream of
deep moisture defined by precipitable water values around 1.6
(close to the observed max for late April). Based on current
radar imagery it appears that the focus for heaviest rainfall
will be primarily across southeast South Carolina through
sunrise, especially across Colleton County and the Tri-County
region. No changes have been made to the Flash Flood Watch or
expected rainfall amounts, though it is interesting to note that
the 00z GFS came in with the axis of heaviest rain shift more to
the northeast into the Grand Strand and the Pee Dee region. We
will just have to watch and see how things evolve over the next
few hours, but there is certainly no reason to make drastic
changes to the forecast.


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.


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.


KCHS: No significant changes to the forecast thinking. A band of
moderate to heavy rain is expected to develop through sunrise
and impact the terminal. Made some minor tweaks to timing, but
otherwise IFR conditions appear likely due to visibility within
the band of heavy rain through just after sunrise. The band of
heavy rain is then expected to shift just to the east and
northeast through the morning and gradual improvement is
expected thereafter. There are signs that low ceilings will
develop Monday night, but as of now it looks like the best
chances will be beyond the end of the TAF period.

KSAV: The line of thunderstorms has passed through the area and
as of now it looks like there will not be anything to impact the
terminal through the next few hours. MVFR ceilings are nearby
and could get into the site through sunrise, but left the
forecast VFR for now. Through the morning and afternoon,
confidence is low that anything more than temporary impacts from
showers will occur. As such, there is nothing included in the
TAF. Then late tonight low ceilings are progged to develop, but
the best chances are beyond the end of the current TAF period.

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.


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.


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


SC...Flash Flood Watch through this evening for SCZ044-045-050-052.
MARINE...Small Craft Advisory until midnight EDT tonight for AMZ350.
     Small Craft Advisory until noon EDT today for AMZ374.



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