Area Forecast Discussion
Issued by NWS Charleston, SC

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340
FXUS62 KCHS 222034
AFDCHS

Area Forecast Discussion
National Weather Service Charleston SC
434 PM EDT Sun Apr 22 2018

.SYNOPSIS...
High pressure will remain northeast of the forecast area
tonight as a low pressure system approaches from the west. The
low will stay west of the area on Monday, then move up the east
coast through mid week. Weak high pressure will prevail before a
cold front crosses the area late week.

&&

.NEAR TERM /UNTIL 6 AM MONDAY MORNING/...
Satellite imagery indicates a mid-level cut-off low over
Arkansas late this afternoon. Models move this low slowly to the
east this evening and overnight, with it becoming located over
western TN by daybreak. At the surface, low pressure will follow
it`s mid-level parent in time and location. Fronts attached to
the low will approach our area from the west. Meanwhile, high
pressure over the Northeastern U.S. will gradually move away.
Deep moisture will keep increasing across our region overnight.
PWAT`s are forecasted to exceed 1.5" late, which is 2 standard
deviations above normal for this time of year per NAEFS. Even
the integrated WV transport is 2 standard deviations above
normal. Lift is in place with the approaching system and upper
level support. Widespread showers are expected to begin
overspreading our area late tonight, closer to daybreak. The
highest POPs are initially across GA, tapering into SC. The
synoptic models show the bulk of the rain beginning after
midnight, but the CAM`s are closer to daybreak. We tried to go
with a blend of the two in this forecast package. QPF has
trended downward a bit. As for thunder, there is some
instability in place across southeast GA. But not enough to
warrant mention in this forecast. Winds will increase,
especially near the coast. Lows be mild, generally in the 60s.

&&

.SHORT TERM /6 AM MONDAY MORNING THROUGH WEDNESDAY/...
Monday: Aloft, the deep upper low will sit nearly stationary
across west Tennessee through the day, with plenty of vort
energy passing over the forecast area. Similarly, the attending
surface low will be nearly stationary over west Tennessee with a
secondary weak low developing over central Georgia in the
afternoon. High pressure will continue to extend into the region
with a warm front progged to lift northward across the forecast
area in the afternoon. The last few model runs have started to
show the upper low and surface low making less eastward
progress, and that has implications for the heavy rain
potential. Rain chances start out around 100 for the southern
half of the area around sunrise, then the 100 percent steadily
spreads across the rest of the forecast area through the day.

Heavy rain potential: The trend in the models the last several
runs has been to focus the best rain potential further northeast
and seems to now be settling on an area including the Tri-
County. We will have a very moist airmass with precipitable
waters values near the climatological high for 4/23, with
increasing forcing for ascent through the day. The forecast now
features 2-3 inches across southeast South Carolina, with
highest amounts focused around Colleton County and the Tri-
County region. For southeast Georgia, amounts are more in the
1-2 inch range. Differences remain in the global/regional models
and the hi-res models, so we will have to see how the bands of
precipitation take shape overnight. Fortunately, since we are
4-7 inches below normal across the region, the flooding threat
is relatively low. We will have to watch the afternoon high tide
cycle as heavy rain could be ongoing at the same time causing
issues along the coast.

Severe thunderstorms: The overall thunderstorm and severe
weather potential is more interesting than in previous days. Low
level wind profiles should exhibit impressive veering with
elevated storm relative helicity values and very low LCL`s. In
fact, winds at 1km could be in the 40-50 knot range. However,
instability is virtually non-existent. However, there could be
enough mechanical lift to overcome the lack of thermodynamics,
and if any convective elements are able to become surface-based
it will certainly bear watching. The main feature will be the
northward lifting warm front, where any storms that are able to
develop could experience enhanced helicity in the vicinity of
the front. There will be the potential damaging wind gusts, and
even a tornado due to the strong low level flow, veering, and
low LCL`s.

Gradient winds/lake winds: Strong winds will funnel into the
Charleston Tri-County region by late morning and continue into
the afternoon. The current forecast has wind gusts topping out
in the 30-35 mph range, but there is a low end chance a Wind
Advisory could be needed (mainly for Charleston County).
Regardless, wind could impact travel on area bridges tomorrow.
Also, a Lake Wind Advisory could be needed for Lake Moultrie.

Monday night: Deep moisture will begin to strip out and rain
chances steadily diminish through the night. Any lingering heavy
rain and isolated thunderstorms will focus up through the
Charleston area in the evening.

Tuesday through Wednesday: The upper low will finally lift
northeastward and eventually phase with a northern stream trough
by Wednesday, with a more zonal flow setting up across the
area. At the surface, the low will linger on Tuesday and with a
cold pocket within the trough aloft, we could certainly see
isolated to scattered showers and possibly a thunderstorm.
Models have minimal precipitation response, thus the low rain
chances. The forecast is then dry Tuesday night and Wednesday as
weak high pressure builds in. Temperatures will actually be on
the mild side with upper 70s progged for highs.

&&

.LONG TERM /WEDNESDAY NIGHT THROUGH SUNDAY/...
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.

&&

.AVIATION /21Z SUNDAY THROUGH FRIDAY/...
18Z TAFs:
KCHS: Gusty winds this evening, easing for a few hours
overnight. VFR for roughly the first half of the TAF period.
Showers will begin impacting the terminal for the second half of
the TAF period, initially bringing MVFR. As the showers become
heavy after daybreak Monday, IFR seem favorable. But due to some
uncertainty with timing, we`re opting to leave prevailing IFR
out of the TAF for now. Additionally, gusty winds will develop
late in the TAF, with gusts ~30 kt.

KSAV: Gusty winds for the entire TAF period, strongest during
the daylight hours. VFR for roughly the first half of the TAF
period. Showers will begin impacting the terminal for the second
half of the TAF period, initially bringing MVFR. As the showers
become heavy around daybreak Monday, IFR seem favorable. But
due to some uncertainty with timing, we`re opting to leave
prevailing IFR out of the TAF for now.

Extended Aviation Outlook: Flight restrictions in reduced
ceilings and visibilities are expected into Monday night, mainly
impacting KCHS. Gusty winds expected on Monday, with the
highest winds at KCHS.

&&

.MARINE...
Tonight: High pressure to our north will move away while low
pressure approaches from the west. Fronts attached to this low
will also be approaching. The interaction between all of these
features will lead to strengthening winds across the coastal
waters, especially after midnight. Small Craft Advisories are in
effect for all the waters and the Charleston Harbor. Afternoon
surface and buoy observations along with short term models
indicate the conditions may worsen a little later than what we
are currently forecasting. However, we didn`t see much benefit
to starting the advisories a few hours later given they will
still be needed later this evening and overnight. Additionally,
wind-driven seas will build through the night.

Monday through Friday: Strong southeast winds will setup on
Monday, bringing high end Small Craft Advisory conditions across
all zones. The strongest winds will be in the Charleston County
waters where a Gale Watch remains in effect. Wasn`t confident
enough at this point to upgrade to a Gale Warning, and the
threat remains. Briefly contemplated extending the Gale Watch
into the Charleston Harbor, but still think gusts will top out
around 30 knots there. Seas will increase, and could top out as
high around 10-11 feet out near 20 nm in the Charleston County
waters. Seas will be lower elsewhere, but still topping out
around 7-8 feet in the southern South Carolina waters and the
outer Georgia waters, and 6-7 feet in the nearshore Georgia
waters. Conditions will improve Monday night with the relaxing
of the gradient, but lingering seas will keep Small Craft
Advisories going. All waters should be advisory-free by
Wednesday morning. No additional marine concerns are expected
thereafter.

High Surf: With strong southeasterly winds setting up on
Monday, seas will increase significantly in the Charleston
County waters. Breakers in the surf zone could reach 6 feet for
a period of time, and a High Surf Advisory might be needed.

Rip Currents: Monday will bring a strong surge of onshore winds
and increased surf along the entire coast. Winds will be
strongest for the Charleston County beaches, and after
collaboration with NWS Wilmington, NC we decided on a High Risk
of rip currents. A Moderate Risk of rip currents is in place
everywhere else.

&&

.CHS WATCHES/WARNINGS/ADVISORIES...
GA...None.
SC...High Rip Current Risk from Monday morning through Monday
     evening for SCZ050.
MARINE...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
     AMZ350.
     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.
     Small Craft Advisory until 11 PM EDT Monday for AMZ330.

&&

$$

NEAR TERM...MS
SHORT TERM...BSH
LONG TERM...ECT
AVIATION...MS/BSH
MARINE...MS/BSH



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