Area Forecast Discussion
Issued by NWS Charleston, SC

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FXUS62 KCHS 221130

Area Forecast Discussion
National Weather Service Charleston SC
730 AM EDT Sun Apr 22 2018

High pressure will begin to lose its influence today as a low
pressure system approaches from the west. The low will bring
much needed rain to the area tonight into Monday before slowly
lifting up the coast through the middle of the week. Weak high
pressure will prevail before a cold front crosses the area late


Since temps are already starting the day about 10-15F warmer
than yesterday morning, and satellite is showing enough breaks
in the cloud cover, we have raised temps about 1-2F from the
earlier forecast.

Previous discussion...
Today: Surface high pressure centered over the NE portion of
the country will gradually relax its grip on the local area
today, as a deep cyclone across Oklahoma and Arkansas drifts
east ans begins to absorb surface low initially near the lower
Mississippi valley into its circulation. Dry air remains in
place this morning, but as the surface high and large scale
subsidence weakens, plus there is a continued influx of moisture
from off the Atlantic combined with increasing isentropic
ascent and low level convergence, we anticipate at least a
slight chance/chance of showers for the afternoon. However,
since the NAM and GFS show no measurable precip today, our
confidence is moderate at best.

Cloud cover will increase from off the ocean in the lower
levels and from the west aloft, leading to skies becoming mostly
cloudy or overcast by mid to late afternoon. While this does
limit how warm we can achieve, since we are starting out quite a
bit warmer than yesterday and there is weak low level warm
advection, we are forecasting highs of 70-74F most places inland
from the cooler immediate coast.

There is a tightening of the pressure gradient throughout the
day, and that along with decent mixing profiles will generate
breezy E-SE winds as high as 15-20 mph far inland and at least
20-25 mph along the coastal counties.

Tonight: The surface and low aloft become vertically stacked as
it heads into the lower-middle Mississippi valley. The surface
high is reluctant to move from its position over the NE states,
while a warm front slowly advances across northern Florida and
into Georgia in association with the upstream low. The results
in a deep and strong onshore low-middle level flow and
strengthening Theta-E advection. PWat climbs to near the 90th
percentile of around 1.5 inches between 06Z and 12Z Monday.
This occurs in sync with deep forcing for ascent created by
plentiful upper difluence with the left front quad of the upper
level jet, considerable low level convergence and strong
isentropic lift. This will allow for the development of numerous
to widespread showers, especially during the late evening and
overnight. We have categorical PoP just about everywhere, except
maybe near the Santee River where likely PoP is in place.

The ingredients for a widespread heavy rainfall are certainly
in place with a deep S-SE flow between a blocking high to the N
and the low to the W. But even at this late time there is just
enough model inconsistency to create some forecast uncertainty.
We did add mention of heavy rainfall into the forecast after
midnight, and our QPF tonight is generally a blend of the
previous forecast and the latest WPC guidance. This supports
amounts of 1/10 to 1/4 inch in the Charleston tri-county
district, trending upward to 3/4 to 1 inch over SE Georgia.
However, if training is able to develop up and/or there are
some convergent bands that set up, higher amounts would occur.
Even so, since we`re in a Severe Drought (D2), the area should
be able to handle the rainfall. The exceptions would be along
coastal areas from Edisto Beach south to Sapelo and Darien, and
in downtown Savannah and Beaufort during the 230-400 am high
tide which could well be occurring in tandem with heavy rains.
This would result in flooding of low lying and urban areas.

A little thunder and lightning could occur in a few spots late
south of I-16 in Georgia where there is limited elevated CAPE,
but not enough to add mention to the forecast.

It`ll be a warm night with elevated dewpoints, wet conditions
and a 35-45 kt low level jet moving in late to keep winds
elevated through the night. Highest winds from off the ocean will
be along the coastal zones, where 15-25 mph and higher gusts
will occur. This leads to lows about 5-8F above normal for this
time of year.


Unsettled weather will continue through Monday as a closed upper low
and associated surface low spin over the Tennessee Valley. Strong
isentropic ascent and lift from upper diffluence will combine with
precipitable water values of 1.6 inches (near climatological
maximums for mid-late April) to drive widespread coverage of
showers. This system should bring decent rainfall with it, but
extremely dry antecedent conditions will keep the probabilities for
exceeding flash flood guidance numbers fairly low. Rainfall totals
Monday into Monday night are forecast to be 1-3 inches on average,
highest across the Charleston tri-county area. Some instability is
present so mention of thunder will remain in the forecast, however
no severe weather is expected at this time.

Another concern for Monday is the winds. East/southeast winds will
become gusty as the area becomes positioned between the approaching
low and stubborn high pressure to the north. The gradient will be
tightest across the northern zones where some gusts 30-35 mph will
be possible. Although mixing profiles are not the most favorable
over Lake Moultrie, the gradient wind could still be enough to
support Lake Wind Advisory criteria. Will highlight this potential
within the HWO.

Models indicate another wave of low pressure will develop over
Georgia later Monday night into Tuesday, which will slowly begin to
lift north and kick the best moisture out of the area. Rain chances
will decrease from south to north. May even see some breaks in the
clouds during the day. Temperatures will return closer to normal.

The closed upper low will open to a wave by Wednesday while the
surface low continues its slow track up the East Coast. There
continues to be discrepancies between models, but it appears that a
trailing cold front could approach the area late in the day. Lack of
moisture will preclude mentionable PoPs.


Forecast confidence is fairly low in the extended period due to
large differences in model solutions. A weak cold front is progged
to move across the area early in the period, then a rather ill-
defined pressure pattern will be in place through early Friday.
Models seem to diverge thereafter, but in general it looks like
another front could cross through early in the weekend. Main rain
chances will be on Thursday as shortwave energy traverses overhead.
Temperatures will be near normal.


KCHS: VFR ceilings into tonight, before showers develop late
tonight in advance of a warm front that is approaching from the
SW. MVFR conditions are possible, but the better chances of any
flight restrictions would occur beyond 12Z Monday. E-SE winds
will become gusty late this morning into this evening, getting a
boost from the sea breeze, then remain elevated tonight as a
35-45 kt low level jet moves through.

KSAV: VFR ceilings into early tonight, with maybe a -SHRA nearby
this afternoon and evening. As a warm front approaches from the
SW tonight and lift and moisture become enhanced ahead of low
pressure to the west, moderate to perhaps heavy SHRA will
develop. Flight restrictions down into the MVFR are currently
forecast, but IFR is possible toward daybreak on Monday. Winds
will be similar to KCHS through the 12Z TAF period.

Extended Aviation Outlook: Flight restrictions in reduced ceilings
and visibilities are expected Monday into at least Monday night.
Gusty winds expected on Monday.


Today and tonight: The waters will be positioned between strong
high pressure near the mid-Atlantic region and Long Island,
while a deep low is located in the lower and middle Mississippi
valley. The E-SE gradient tightens to around 3-4 mb today, then
to around 4-5 mb tonight. This results in increasing winds and
building seas with the favorable onshore fetch, leading to SCA`s
developing on all waters. We already have a pre-existing
advisory on the AMZ374 waters, but the next advisory will be in
Charleston Harbor starting this afternoon, which gets a boost
from the sea breeze and the nearby heating of the surrounding
land. All nearshore waters will then experience SCA`s beginning
this evening. Highest wind speeds will be around 25 kt tonight
(this afternoon and tonight in Charleston Harbor), with seas
building to 6 or 7 ft within 20 nm and up to 9 or 10 ft on the
outer Georgia waters. Mariners should also be prepared for
moderate to heavy rains and maybe isolated thunder/lightning
overspreading the area tonight.

Monday through Friday: Hazardous marine conditions expected on
Monday as a tight pressure gradient remains in place due to an
approaching low pressure system. Small Craft Advisories will be
ongoing for all waters, including the Charleston Harbor. Models have
been consistent in showing some gusts to gale force across the
Charleston county waters where the gradient is tightest during the
afternoon and evening. A Gale Watch has been raised which currently
runs 15z Monday through 06z Tuesday. Winds will drastically improve
early Tuesday, however seas will take a bit longer to subside. All
waters should be advisory-free by Wednesday morning. No additional
marine concerns are expected thereafter.

Rip Currents: A continued moderate onshore wind, maybe a little
swell energy and small astronomical influences from the recent
perigee will lead to a Moderate Risk of Rip Currents at all
beaches today.

A prolonged stretch of moderate to perhaps strong onshore winds
and larger surf will lead to an enhanced risk of rip currents
at the beaches early this week.


MARINE...Small Craft Advisory from 2 PM this afternoon to 11 PM EDT
     Monday for AMZ330.
     Small Craft Advisory from 8 PM this evening to 2 PM EDT
     Tuesday for AMZ352-354.
     Gale Watch from Monday morning through late Monday night for
     Small Craft Advisory from 8 PM this evening to 11 AM EDT
     Monday for AMZ350.
     Small Craft Advisory until 8 AM EDT Wednesday for AMZ374.


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