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FXUS62 KCHS 130025
AFDCHS

Area Forecast Discussion
National Weather Service Charleston SC
725 PM EST Mon Nov 12 2018

.SYNOPSIS...
A series of storm systems will impact the area through mid week
with cool and dry high pressure returning by this weekend. A
mainly dry cold front could move through early next week.

&&

.NEAR TERM /UNTIL 6 AM TUESDAY MORNING/...
Early this evening: A complex situation remains in place across
the forecast area. Surface analysis shows a damming high
situated to the northeast with a sharp and well defined coastal
front now laying right along our coastal counties. The cold air
damming is quite shallow, and the KCHS RAOB data shows that the
low level inversion is only about 1 kft deep. Winds quickly
shift from northeasterly at the surface to southeasterly at 1
kft and above. The current radar presentation also suggests that
the front is just inland of the beaches with most of the current
shower development just on the cool side of the boundary. Most
models show the coastal front surging inland through the night
as a surface low develops near the Florida panhandle. Usually,
models are too quick to erode the low level wedge, but in this
case it seems reasonable given the shallow nature of the cool
and stable air. We will continue to see periods of showers
develop and lift northward due to upper divergence in the right
entrance region of a powerful 150+ knot jet core that stretches
from the Great Lakes to Nova Scotia. Overall, the best forcing
for additional rain appears to be favored along and east of
I-95, and that is where the highest rain chances have been
placed.

There are a couple of concerns tonight, including the potential
for thunderstorms and severe weather. As the coastal front
progresses inland, much warmer air will arrive and could even
bring some surface based instability, though weak. Wind profiles
are concerning with the veering that takes place in the 0-3 km
layer, with accelerating flow and ~30 knots of shear.
Furthermore, hi-res models show some discrete cells developing,
and even some multi-cells. Given the background wind field, some
interesting storm structure is possible if enough instability
can develop. However, current thinking is that profiles are just
not supportive of enough instability and the overall severe
threat is low.

Finally, fog will be possible inland of where the coastal front
is able to progress. Some model guidance would indicate the
potential for dense fog, and it can't be ruled out given the
moist atmosphere and low level stable layer west of the front.
Added in some patchy fog for now, but could need to enhance the
fog wording later in some areas.

&&

.SHORT TERM /6 AM TUESDAY MORNING THROUGH THURSDAY/...
A surface cold front will manage to cross the forecast area by late
Tuesday with high pressure beginning to build back in behind. An
increasingly sharp upper trough located over the south-central
plains region will move east with many of the synoptic models
indicating formation of a closed upper low by late Tuesday or early
Wednesday. Interestingly, the frontal system could exhibit features
of both katabatic and anabatic structure. While persistent, low-
level southwest flow in advance of the front will provide the warmth
and moisture for precipitation production during and in advance of
front passage, the presence of the sharp trough and/or closed low to
the west will maintain southwest flow just aloft, even as surface-
level winds veer northeast behind the front, maintaining atmospheric
column moisture levels. While model soundings to a small degree show
very shallow drying, it is unclear at this point if the consequence
will be primarily expansive but dry cloud cover, or if elevated
precipitation chances will continue into early Wednesday.
Regardless, and despite the subtle dampening of the parent trough
through the day Wednesday as the cutoff low moves closer to the
southeast U.S., it appears the mid and upper level trough/low become
phase-locked and our next mid-latitude cyclone is off to the races.
Complicating matters for the Wednesday forecast include some degree
of cold air damming likely persisting east of the Appalachians,
which the low pressure system will have to ride over. Thursday into
Thursday night, the low pressure system will eject northeast away
from the area, with drier high pressure building in behind. While
some model variance exists with timing the end of precipitation on
Thursday, reasonable agreement suggests noticeably drier conditions
over much if not all of the forecast area by Thursday afternoon or
evening. While some thunder might be heard, and a nonzero chance of
damaging winds and/or tornadoes exists, the best potential for any
severe weather looks to remain well inland of the forecast area
through the short term period.

&&

.LONG TERM /THURSDAY NIGHT THROUGH MONDAY/...
Latest model guidance suggests improving conditions Thursday night
with dry weather prevailing into early next week, although another
reinforcing mostly dry cold front looks to move through probably
Sunday or Sunday night. Temperatures will mostly be near to below
normal.

&&

.AVIATION /00Z TUESDAY THROUGH SATURDAY/...
At KCHS: The coastal front is in the vicinity and ceilings have
become more variable from their earlier IFR level. While MVFR
conditions could settle in for a few hours, a lowering to IFR is
expected later tonight. As showers move through the terminal
the next few hours conditions could temporarily lower until a
more widespread lowering late this evening. IFR conditions could
improve around midday for a few hours before deteriorating
again late in the afternoon with the arrival of a front from the
west. Also, showers will likely accompany this front at the end
of the forecast period. Data from the KCHS weather balloon
showed surface winds out of the northeast around 5 knots, with
winds at 2 kft out of the southeast around 35 knots. This
results in about 35 knots of shear across the 2 kft layer, so
low-level wind shear has been included for a few hours this
evening.

At KSAV: IFR ceilings are expected to persist through tonight
and into much of Tuesday morning. Isolated to scattered showers
could move through the terminal this evening, but should be
short-lived. Fog development will be possible tonight, but is
mainly expected inland of the terminal. Conditions should
improve to MVFR sometime Tuesday morning, before falling back to
IFR in the afternoon with the arrival of a front from the west.
Also, showers will likely accompany this front at the end of
the forecast period. Data from the KCHS weather balloon showed
winds at 2 kft out of the southeast around 35 knots. This
combined with KSAV surface winds light and out of the northeast results
in about 35 knots of shear across the 2 kft layer, so low-level
wind shear has been included for a few hours this evening.

Extended Aviation Outlook: Flight restrictions are expected at both
terminals into Thursday due to a series of low pressure systems,
likely reducing primarily ceilings but also possibly surface
visibilities below IFR levels at times. In addition, gusty winds are
expected, mainly Wednesday into early Thursday.

&&

.MARINE...
Tonight: Winds and seas will build tonight as the coastal front
pushes inland. Winds should turn east and eventually southwest.
Winds look to be highest off the Charleston County coast where a
few gusts to gale force could occur. Recently noted 41004 has
been gusting in the upper 20s to around 30 kt for the past
several hours. Seas will build to 6-8ft Charleston and Georgia
offshore waters with 4-7 ft elsewhere. Small Craft Advisories
remain in effect for all marine legs with the exception of
Charleston Harbor.

Tuesday through Saturday: Unsettled conditions are expected for
marine zones through much of this period as a series of storm
systems impacts the region. Small Craft Advisories are likely
for most of the area through Thursday, or through Friday for
offshore Georgia waters. At this time, the worst conditions look
to occur Wednesday into Thursday morning as surface pressure
falls result in the tightest pressure gradient of the period. As
a result, gusts to gale force will be possible Wednesday
through Wednesday night, with seas reaching near 9-10 feet.
Conditions will then improve Thursday into Friday as the low
pulls away from the southeast and the pressure gradient relaxes.

&&

.CHS WATCHES/WARNINGS/ADVISORIES...
GA...None.
SC...None.
MARINE...Small Craft Advisory until 1 PM EST Tuesday for AMZ352.
Small Craft Advisory until midnight EST Friday night for
AMZ374.
Small Craft Advisory until 7 PM EST Tuesday for AMZ350.
Small Craft Advisory until 7 AM EST Tuesday for AMZ354.

&&

$$

NEAR TERM...BSH
SHORT TERM...JMC
LONG TERM...RJB
AVIATION...BSH/JMC
MARINE...ST/JMC
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