Area Forecast Discussion
Issued by NWS Charleston, SC

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FXUS62 KCHS 190220

Area Forecast Discussion
National Weather Service Charleston SC
1020 PM EDT Thu Apr 18 2019

A strong cold front will approach from the west tonight, then
sweep through the region on Friday. Low pressure over the Ohio
Valley will prevail Saturday, followed by high pressure
building into the region Sunday and into early next week.


Atlantic moisture transport will continue to increase overnight
as southerly flow strengthens and deepens ahead of the powerful
upper trough. Initially only scattered low clouds exist,
however stratocumulus will thicken overnight with the increase
in low-level moisture. Most model guidance shows a surge in
low-level dewpoints across coastal SC later tonight, along with
cooling temperatures aloft. This will yield modest surface-
based instability. The high-res guidance continues to show a
slug of moisture convergence moving off the Atlantic into the
Tri-County area after 2 or 3 am, bringing scattered showers and
perhaps a few thunderstorms into the area. Surface winds will
persist overnight, strengthening a bit toward daybreak as the
low-level jet begins to strengthen. As a result of the robust
mixing overnight, temperatures will not drop more than a few
degrees from current values.


Friday: The focus of the short term forecast continues to be the
potential severe weather event Friday. Overall, there has been no
change to the overall dynamic setup. A deep trough is still forecast
to dig across the lower Mississippi Valley and close off into an
upper low. This anomalous setup will push impressive forcing into
the forecast area as a strong cold front moves west to east across
the forecast area. The ambient wind field is progged to be quite
impressive, producing strong shear profiles and notable low level
veering. The shear environment continues to look supportive of an
elevated damaging wind threat and isolated tornadoes. As is almost
always the case, the biggest question mark remains instability and
that will likely hinge on the timing of the frontal line of

History favors a faster timing as the convection tends
to race out ahead of the main front. The ECMWF and GFS continue to
be the faster solutions, while the NAM is a few hours slower. Also
of note, we are now getting into the window of time where the HRRR
and RAP extend into Friday morning. It`s a bit concerning that both
the HRRR and RAP are even slower than the NAM, with the RAP showing
the line of thunderstorms still west of our inland zones (Jenkins,
Candler, and Tattnall counties) at 15 UTC. The slower the line is in
getting into the area, the more the atmosphere can destabilize,
further enhancing the severe weather threat. The forecast does slow
arrival down a bit, but still depicts the line knocking on the
doorstep of southeast Georgia around 12 UTC. This would make the
main time period for severe weather for southeast Georgia roughly
from 8am-noon, then noon-4pm for southeast South Carolina. This
timing will likely need adjusting overnight as the line develops and
we can make better assessments of how various models are
initializing. As SPC notes in the Enhanced Risk severe weather
outlook, MLCAPE values of 1000-1500 J/kg are possible. This
instability combined with the ambient shear will favor a linear
convective mode with embedded supercellular elements. The line is
expected move east of the area by mid/late afternoon at the latest,
ending the severe weather threat.

Also of note, the strong wind field will produce gusty winds outside
of thunderstorms on Friday. Confidence has increased to the point
where we have issued a Wind Advisory for Beaufort, Coastal Colleton,
and the entire Tri-County region for frequent gusts to 40-45 mph.
Such winds will also be possible further to the southwest, primarily
along the Georgia coast, but confidence isn`t high enough at this
time to include in the advisory. The advisory will run from late
morning through late afternoon.

Friday night: The front will move well to the east and mid/upper
level moisture will strip out of the region. There doesn`t appear to
be much support for lingering shower activity so the bulk of the
overnight will be dry. Low temperatures should dip into the low to
mid 50s in most areas.

Saturday through Sunday: The large and deep upper low will encompass
much of the eastern CONUS on Saturday and then start to drift
eastward on Sunday. The surface low will spend much of Saturday
across the Ohio Valley, leading to a breezy and cool day with clouds
filling in from the west and northwest. Cold temperatures aloft,
around -20C at 500 mb, could help produce isolated showers during
the day, but virtually every model keeps shower activity upstream of
the forecast area. We still show a slight chance of showers but this
area has been trimmed to a smaller region. For Sunday, high pressure
will begin building in and skies will clear out. Look for Saturday
highs to range from the low 60s inland to the mid/upper 60s closer
to the coast. Then for Sunday, a nice warmup with highs rising into
the mid and upper 70s.


Dry sfc high pressure will prevail across the region Sunday night
through the middle of next week with temps warming each day as a
mid/upper lvl ridge axis shifts over the Southeast United States. In
general, high temps in the the low 80s Monday will warm into the low
80s Tuesday, then low/mid 80s Wednesday. Overnight lows should range
in the low mid 50s away from the coast Sunday night, then mid/upper
50s inland to lower 60s near the coast Monday night, followed by
low/mid 60s Tuesday night.


Stratocumulus will thicken overnight, particularly southern SC
as convergence increases late. Fairly good chance for MVFR
ceilings moving in off the Atlantic at KCHS a few hours prior to
daybreak, along with scattered showers.

Winds will rapidly increase during the morning at both
terminals, peaking in the late morning through early afternoon
hours. The latest timing on the line of strong to severe
thunderstorms has it pushing toward KSAV after 18Z and KCHS
after 20Z. Damaging straight line winds will be the primary
threat with this line, though isolated tornadoes are also
possible. The convection should be off the Atlantic coast by 00Z
Saturday. Flight restrictions are a near certainty with the main
band of storms but given the timing uncertainty this far out we
have yet to include specific mention of restricted vsby/cigs.

Although winds at the 1 kft and 2 kft levels will be very strong
on Friday, ample mixing down to the surface should preclude
low-level wind shear concerns.

Extended Aviation Outlook: VFR conditions should return
overnight Friday.


Tonight: Conditions will deteriorate with time tonight. As a
strong cold front approaches from the west and south winds
strengthen, Small Craft Advisory winds/seas will spread across
the coastal waters, including Charleston Harbor overnight, and
gale force gusts are expected to spread into AMZ374 overnight.
Seas 2-3 feet nearshore and 3-4 feet beyond 20 nm to start will
build to as high as 4-8 feet nearshore waters and 7-9 feet
beyond 20 nm by late tonight.

Friday through Tuesday: A strong cold front and associated line
of thunderstorms is expected to move across the local waters on
Friday. Southerly winds will increase significantly Friday
morning ahead of the cold front and confidence has increased
such that Gale Warnings are now in effect for all waters
including Charleston Harbor. Expect frequent gusts into the
35-40 knot range beginning in the morning and continuing into
the afternoon. The likelihood of gales will end from west to
east as the cold front moves through and the warnings will come
down at various times. However, all the warnings should be down
by late Saturday night. Seas will increase significantly, and
could rise into the 6-10 ft range out to 20 nm on Friday, and
9-12 ft beyond. Conditions will improve Friday night, but we
will certainly need Small Craft Advisories through Saturday
evening to account for elevated west-southwesterly winds and
seas in an enhanced pressure gradient around low pressure well
to the north. Much more tranquil conditions can be expected
starting Sunday and continuing into early next week as high
pressure builds in across the Southeast.

As thunderstorms move across the waters on Friday, strong
damaging winds and isolated waterspouts will be possible through
the day, first in the Georgia waters and then in the South
Carolina waters.

Surf Zone Hazards: Strong winds could produce high surf and a
High Surf Advisory could be needed for breakers of 5 ft or
greater. A Moderate Risk of Rip Currents is in effect Friday at
all beaches.


Although the tides are not expected to reach advisory criteria
with the high tide around 9 am Friday, there is a chance that
some heavy rainfall will occur around Charleston close to the
high tide which could cause some minor flooding issues.

Blow-out tides are possible along the SC/GA coast with Saturday
morning`s low tide.


SC...Wind Advisory from 10 AM to 5 PM EDT Friday for SCZ044-045-
MARINE...Small Craft Advisory until 10 AM EDT Friday for AMZ352-354.
     Gale Warning from 10 AM to 8 PM EDT Friday for AMZ330-352.
     Small Craft Advisory until 7 AM EDT Friday for AMZ350.
     Gale Warning from 7 AM to 11 PM EDT Friday for AMZ350.
     Gale Warning from 10 AM to 5 PM EDT Friday for AMZ354.
     Small Craft Advisory until 4 AM EDT Friday for AMZ374.
     Gale Warning from 4 AM to 11 PM EDT Friday for AMZ374.
     Small Craft Advisory from 2 AM to 10 AM EDT Friday for AMZ330.


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