Area Forecast Discussion
Issued by NWS Charleston, SC

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FXUS62 KCHS 230229

Area Forecast Discussion
National Weather Service Charleston SC
1029 PM EDT Sun Oct 22 2017

A cold front will push offshore Monday night and be followed by
a stronger, reinforcing cold front Tuesday afternoon. High
pressure will then prevail into late week before another cold
front possibly affects the area next weekend.


Late this evening: Made some changes to PoP`s for the next few
hours based on current radar trends. Went with likely PoP`s just
about everywhere as most locations should see rain, though
light. Otherwise, previous discussion continues below.

Early this evening: Rounds of showers continue to stream
onshore across much of the area, with brief periods of moderate
to even heavy rain possible. So far, no lightning strikes have
been noted and any instability that is present is waning with
time. As such, did not mention thunder in the forecast. The main
upper trough and closed low feature remain well to the west,
and the area remains largely devoid of any large scale forcing.
Instead the driver for the ongoing precipitation seems to be
isentropic ascent within strong low level flow just above the
surface. In fact, the 00z CHS RAOB revealed 25-27 knots of
southeasterly flow through the lowest 3 kft of the atmosphere.
The rounds of showers will likely persist through the night and
we have attempted to reflect this in the PoP forecast. By late
tonight, parts of southeast Georgia near the Altamaha should
start to feel the approach of the larger scale forcing from the
west and the focus for rain chances will shift to this region.
No changes to temperatures as it will be a very mild night. Lows
will remain above 70 for many locations.


Monday: The day will start out with a weak warm front
organizing near the Savannah River along the nose of 925-850 hPa
jet. Associated isentropic assent/warm air advection will
likely support scattered to numerous showers with a few embedded
tstms through the morning, lifting into the I-26 corridor by
early afternoon coincident with the warm front. There will
likely a several hours of rain-free conditions across much of
Southeast South Carolina and Southeast Georgia after warm FROPA
as the warm sector becomes more firmly established ahead of an
approaching cold front. Pops will range from 30-70% during the
morning hours with the highest pops shifting into the Charleston
Tri-county area by early afternoon.

Conditions will deteriorate Monday afternoon into Monday
evening as a large swath of rain moves west-east across the
region. The 22/12z GFS remains a bit more progressive than the
22/12z ECMWF and NAM, mainly due a weaker amplification of the
southern steam shortwave currently moving out of eastern
Oklahoma and Texas. This results in a quicker onset and
progression of rain across Southeast South Carolina and
Southeast Georgia with lower QPF. Generally favored a slower,
stronger and wetter ECMWF-NAM blend for timing and the
construction of hourly pops with some minor contribution from
the GFS given its continued more progressive run-to-run
consistency. It should be noted that both the NAM and ECMWF
initialized better with 12z 500 hPa heights and vorticity across
Texas into the Central Plains. Pops will range from 70-90%,
highest interior Southeast Georgia, Monday afternoon, 80-90%
region-wide Monday evening with rain chances ending from west-
east early Tuesday morning as a cold front pushes offshore.

Warm sector instability is expected to very slowly increase
through the day. Considerable cloudiness and some lingering
shower activity will likely limit the degree of instability
somewhat, but the combination of weak instability and increasing
deep-layered quasi-geostrophic forcing should be enough to
support at least scattered tstms. Shear will increase quickly
ahead of the front, but the limited instability should curtail
the overall risk for severe weather. If there are some
unexpected breaks in the cloud canopy and surface
temperatures/instability correspondingly rise, then the risk for
severe tstms could increase, especially right along the front
itself. An organized high shear/low CAPE severe weather event is
not currently expected, but an isolated severe thunderstorm
with damaging winds or even a tornado can not be completely
ruled out.

Highs will warm into the upper 70s well inland to the lower 80s
just inland from the coast. Lows will range from the upper 50s
well inland to the mid-upper 60s at the beaches.

Tuesday and Wednesday: Some shower activity could linger along
the Charleston County coast right around sunrise, but will
quickly translate offshore. A stronger, secondary cold front
will push offshore Tuesday afternoon, delineated by a sharp 850
hPa thermal gradient and onset of considerably stronger cold air
advection. Temperatures should rise to near 80 at the coast
prior to onset of stronger cold air advection, while far inland
areas may only warm into the mid 70s. Considerably cooler
conditions will occur Tuesday night into Wednesday with lows
dropping into the mid 40s well inland to the mid 50s at the
beaches. Temperatures will only recover to the mid 60s Wednesday
afternoon as cool, Canadian high pressure dominates the region.


Dry/unseasonably cool conditions will prevail through Thursday
before some moderation occurs late in the week as high pressure
quickly shifts offshore allowing a milder return flow to set up.
Some uncertainty in the longer range model guidance regarding
timing of the next cold front implies lower forecast confidence
starting Friday night. For now we stayed pretty close to the
latest WPC guidance which has a cold frontal passage Saturday
night into Sunday. Temperatures will be below normal through
Thursday before likely getting back to or even above normal.
Many inland areas should get into the 40s each morning from
Wednesday through Friday.


A complex forecast on tap through 00z Tuesday. Through the rest
of the night, showers will continue to stream onshore and could
bring short periods of MVFR. Conditions should be VFR through
most of the night outside of showers. Late tonight, showers
should again become more organized across southeast Georgia and
draw closer to KSAV through sunrise. Introduced prevailing MVFR
conditions starting at 10z there. This area of showers, and
possibly thunderstorms, will arrive in the mid morning time
frame at KCHS. The main cold front and line of thunderstorms
along it will likely not arrive until late in the day.
Thunderstorms will be possible, but cloud cover earlier in the
day could inhibit destabilization. As such, not enough
confidence to introduce thunder at this time.

Extended Aviation Outlook: MVFR with pockets of IFR or LIFR
(mainly in vsbys) will occur Monday afternoon and evening as
showers/tstms associated with a cold front push through. Tstm
impacts are possible.


This Evening and tonight: A weak coastal trough will lift north
of the area this evening well ahead of a cold front advancing
into the Eastern Conus. Aloft, a mid/upper lvl ridge of high
pressure centered over the Western Atlantic will gradually shift
further offshore as a longwave trough advances toward the
region late. Expect chances of showers and the pressure gradient
to slowly increase overnight ahead of the system. East winds
will become south late, increasing to 20-25 kts over much of the
coastal waters outside the CHS Harbor. Seas will also build
from 3-5 ft to 4-6 ft in nearshore South Carolina waters while
approaching 5-7 ft in offshore Georgia waters. For this reason,
Small Craft Advisories will begin for nearshore South Carolina
waters starting at 6AM Monday and will be ongoing in offshore
Georgia waters tonight. Models insist on a fairly unstable
marine environment late tonight with SBCAPE approaching
1000-1500 J/kg. Given the approach of mid-lvl energy aloft, have
maintained a slight chance of thunderstorms over Georgia waters
after midnight.

Monday through Friday: Southerly winds will help build seas to
6 ft across mainly the South Carolina nearshore waters after
sunrise Monday which will linger until Tuesday morning before
subsiding as winds turn offshore behind a cold front. A Small
Craft Advisory will be issued for these zones to reflect this.
Seas will remain above 6 ft over the Georgia offshore waters
until Tuesday morning as well and an advisory remains in effect
there. There is a brief window where seas could breach 6 ft over
the Georgia nearshore waters, mainly late Monday afternoon into
Tuesday morning, but will defer the need for an advisory there
to the midnight shift given the event looks marginal. Expect
improved conditions Wednesday into Friday as high pressure


MARINE...Small Craft Advisory from 6 AM Monday to 5 AM EDT Tuesday for
     Small Craft Advisory until 11 AM EDT Tuesday for AMZ374.
     Small Craft Advisory from 6 AM Monday to 11 AM EDT Tuesday for



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