Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 160604
AFDGSP

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
104 AM EST Mon Jan 16 2017

.SYNOPSIS...
A cooler airmass will push into the region on Monday, but
temperatures rebound through mid-week as a cold front approaches
from the west with light rainfall.  Behind the front, temperatures
will cool slightly but remain above normal. A warm front will move
through the Southeast Thursday and Friday with another round of
showers likely while a more significant storm system moves into
the Southeast during the weekend.

&&

.NEAR TERM /THROUGH TODAY/...
As of 1 AM, earlier shower activity north of the frontal boundary
has all but dissipated across western NC, but will retain a small
pop along the I-40 corridor through the morning. Otherwise,
expanding stratus within a moist isentropic upglide regime is
expected, esp east of the mtns, while plenty of low stratus and fog
will persist in the mtn valleys until after sunrise.

Otherwise, a negatively tilted upper ridge will progress across the
MS River Valley to the Southern Appalachians today and tonight,
while an upper low moves from West TX to Northwest MO. At the
surface, a ridge will nose down the Easter Seaboard tonight and
Monday. A front aloft will lift north over this boundary, while
moisture from the Gulf of Mexico spreads east across the Gulf States
as a cold front reaches the lower MS River Valley.

The upshot of all this is that the best chance of precipitation
initially moves north of our area tonight and early Monday, then the
effects of Gulf Moisture reach the area later on Monday. Liquid
precipitation amounts are expected to be quite light. Instability
and shear will be insufficient for convection. Temperatures will
remain above normal under the upper ridge, but with a reduced
diurnal range due to cloud cover,

&&

.SHORT TERM /TONIGHT THROUGH WEDNESDAY/...
As of 220 PM Sunday: The upper flow pattern will become rather
complicated over the CONUS through the short range. A wave ejecting
across the Great Lakes Monday night should give way to a split-flow
pattern with a northern stream system digging across the Midwest
while a southern stream closed low churns over northwest Mexico
through Tuesday. East of these features, southwesterly low level
flow will increase across the southern Appalachians Monday night
through Tuesday with deeper moisture gradually pooling along an
approaching cold front. Will maintain a strong west to east gradient
in PoP for showers Tuesday afternoon - with warm max temperatures
throughout despite the clouds.

The northern stream system gets especially out of phase among the
model solutions Tuesday night through Wednesday, with the ECWMF
leading the faster camp, the ensemble means in the middle, and the
operational GFS slower and deeper through Wednesday. Sensible
differences, however, are not tremendous with the associated frontal
zone likely sliding southeast through the region sometime Tuesday
night through Wednesday, with some measure of NW flow downslope
drying east of the mountains later Wednesday afternoon. Any warm
sector thunder currently appears possible mainly in the very far SW
NC mountains before daybreak Wed. Will maintain very mild
temperatures with morning mins in the 50s and highs in the 60s all
but the higher terrain.

&&

.LONG TERM /WEDNESDAY NIGHT THROUGH SUNDAY/...
As of 230 PM EST Sunday:  The medium range fcst period kicks off on
Wednesday evening amidst the passage of a potent H5 shortwave, with
a favored position generally centered atop the Western Carolinas
at fcst initialization per the GFS/CMC, while the ECMWF is slightly
more progressive.  Elsewhere,  the persistent GOM upper anticyclone
is compressed/flattened thanks to said shortwave, as another closed
upper low strengthens over the Desert Southwest.  At the surface,
guidance does agree that a cold frontal passage will be underway,
with the front favored along/south of I85 by 00z Thursday, while
broad yet weak ridging moves in behind across the TN valley.

Pattern evolution through the remainder of the period is highlighted
by upper amplification of a ridge axis across the southeast, while
the old Desert Southwest upper cyclone migrates northeast into the
Central/Southern Plains Friday, before opening up across the Upper
Midwest on Saturday.  As a result, surface cyclogenesis over the
MidSouth is favored, with said low pressure system to move into
the OH Valley by Saturday.  The old weak/dry surface anticyclone,
which by this time will have setup along the east coast, will move
out to sea as the upper ridge axis does the same thanks to height
falls from the west.  With that, moisture looks to advect into the
region from the south by way of a H85 warm fropa, likely yielding
a period of isentropic upglide Friday night into Saturday.

Moving along, another brief period of shortwave H5 ridging to move
overhead early on Saturday will be quickly followed by additional
height falls Saturday night into Sunday while broad surface high
pressure anchored over the northeast wedges down the seaboard in
the lee of the Appalachians.  Additional H5 waves passing through
the southern stream across the Plains will lead to another round
of surface cyclogenesis in the MS River Valley, and thus additional
moisture advection and precipitation across the deep south to round
out the weekend.  At this point, although with some slight timing
differences, guidance seems to converge on a solution featuring a
final and much deeper H5 low diving through the Southern Plains
into the MS Valley late Sunday.  By that point the southeastern
CONUS looks to be rather moist and thus increasingly unstable
ahead of a deepening surface wave and cold front for Sunday night
into Monday.  The ECMWF is a bit faster with the evolution of this
late period system relative to the GFS, however both favor a pattern
for active weather, possibly yielding strong/severe convection and
heavy rainfall.  Said heavy rainfall threat looks to be maximized
along the southern escarpment where upsloping will be favored.
All said, at this range the details are a bit too muddy for any
mention in the outlook given lack of confidence on timing, as
well as the potential for coastal convection to block significant
convection northward into northeast GA and the Western Carolinas.

&&

.AVIATION /06Z MONDAY THROUGH FRIDAY/...
At KCLT and Elsewhere: Increasing moist lift above a back door cold
front is expected to result in expanding/lowering cigs across the
region through the morning hours. Meanwhile, fog/low stratus will
continue across the mtn valleys through at least sunrise, with at
least brief periods of LIFR, and perhaps VLIFR expected at KAVL.
At the other terminals, cigs are expected to lower to IFR at some
point this morning, prior to sunrise at KCLT, around sunrise at
KHKY/KGSP/ KGMU, and by around late morning at KAND. Once the IFR
cigs settle in, they are expected to remain, possibly lowering to
LIFR at times, through the end of the period, as weak cold air
damming becomes established.

For the most part, surface temp/dewpoint spreads appear to be too
high, esp in the face of increasing cloud cover to support much in
the way of fog outside of the mtns. However, MVFR visby will be
featured in most tempos later this morning. Guidance actually
maintains reduced visby through the day, although it`s not at all
clear that this is plausible, so will allow visby to improve to at
least 6SM at most terminals by afternoon.

Outlook: A moist airmass and sfc boundary will remain close to the
fcst area through Thu, with flight restrictions possible each
day.

Confidence Table...

            06-12Z        12-18Z        18-24Z        00-06Z
KCLT       High  87%     High  80%     Med   63%     Med   72%
KGSP       High  80%     High  88%     Low   55%     Med   76%
KAVL       Med   69%     Low   58%     High  83%     High  82%
KHKY       High  99%     High  95%     Low   58%     High  90%
KGMU       High  83%     High  87%     Low   50%     Med   72%
KAND       High  95%     Med   76%     Med   76%     Med   78%

The percentage reflects the number of guidance members agreeing
with the scheduled TAF issuance flight rule category. Complete hourly
experimental aviation forecast consistency tables and ensemble forecasts
are available at the following link:

www.weather.gov/gsp/aviation

&&

.GSP WATCHES/WARNINGS/ADVISORIES...
GA...None.
NC...None.
SC...None.

&&

$$
SYNOPSIS...JMP
NEAR TERM...JAT/JDL/RWH
SHORT TERM...HG
LONG TERM...CDG
AVIATION...JDL


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