Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 220000
AFDGSP

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
800 PM EDT Sat Oct 21 2017

.SYNOPSIS...
Dry high pressure will remain across the region through Sunday.
Meanwhile, a low pressure system will develop across the deep south
and bring moist air to the Carolinas early Monday. A cold front
associated with this system will cross the region Tuesday with
cooler air moving in behind the front through mid-week.

&&

.NEAR TERM /THROUGH SUNDAY/...
As of 5pm EDT Saturday:  A little moisture above 25kft is providing
some thin overcast this afternoon, but conditions are fair overall
with no radar echoes currently, and none expected through Sunday
afternoon.  Current near-term forecast looks in good shape and will
only make minor tweaks to temperature and sky cover.

A highly amplified upper trough will cross the Plains tonight and
approach the MS Valley Sunday. This will result in height falls
encroaching on the region toward the end of the near term. At the
sfc and low levels, high pressure will shift east off the
Mid-Atlantic coast and bring flow around more out of the SE late
tonight thru Sunday. This flow will begin to advect some low-level
moisture into the Carolinas and GA. Guidance is mixed, but seem to
be trending toward some stratocu development, at least in the SELY
upslope areas late. The combination of increasing cirrus aloft and
potential low stratocu early Sunday morning should limit radiational
cooling somewhat. So min temps are expected to be 2-3 categories
above normal. Some fog may develop in the Little TN Valley, but
little elsewhere.

Sunday, the early morning stratocu that does develop will likely
just expand in coverage, but should lift slightly in altitude by
early AFTN. So expecting more cloud cover around and this should cap
temps a few degrees where it persists, namely near the eastern Blue
Ridge escarpment. The lower piedmont and lower valleys near the TN
border should see similar temps to today, maybe a deg or two cooler.
As for PoPs, forecast soundings show the moist layer to be shallow
with lingering inversion around 800 mb. So I am going with a dry
fcst. Cannot rule out a few sprinkles late AFTN in the western
Upstate and adjacent mountains just to the north, as moisture depth
starts to increase.

&&

.SHORT TERM /SUNDAY NIGHT THROUGH TUESDAY/...
As of 230 PM EDT Saturday: We have made some significant changes to
the forecast based on the latest guidance from the GFS/NAM and
European models. The culprit is the upper low near the state lines
of Arkansas, Missouri and west Tennessee late Sunday night and early
Monday. Earlier guidance held onto the closed low scenario, which
allowed for a slower movement of the upper features, and also their
accompanying frontal low and cold front.

It appears the models have latched onto the evolution in the
southern stream, where the upper lows are embedded, to open up into
a rather robust trough axis. This trough axis, and surface cold
front, will then race quickly east.

Initially isentropic lift, and an increasingly strong H85 inflow
Sunday night, will allow for an expansion of precipitation as the
column of moisture saturates. Elevated instability suggests that a
small chance of thunder will be possible near daybreak in our south
FA.

The modeling of the upper system and cold front Monday into Monday
night will be dialed into the overall speed. The NAM and GFS were
the fastest, the European a tad slower - but not by much. A 50+ knot
H85 jet will continue to pump moisture into the region, while very
strong DPVA, a coupled jet structure and converge along the front,
all spell a hefty increase in POPS from Monday into Monday night.

Shear values will be very impressive, although the buoyancy factor
is a bit of a wildcard. However, we have noted a trend up with the
latest CAPE values. Another question will they line-up together in
our area? We also will be watching closely how the hybrid wedge
reacts. Our thinking it will ease, but still could be a player.

Precipitable water values surge higher as this unfolds, especially
late Sunday night and Monday. We are still carrying some hefty
rainfall amounts, and although it has been dry, we will need to also
watch high "short fused" rainfall rates.

Following later Monday night and Tuesday, we have cut way back on
POPS, but left some smaller values in the forecast for a 2ndry wind
shift and some elevated CAPE.

Temperatures may become a problem because of a number of factors:
rain cooled air, colder air arriving quickly, breaks in the clouds.
The best course of action we have decided was to trend temperatures
cooler from northwest to southeast across our FA ... right through
this part of the forecast cycle.

&&

.LONG TERM /TUESDAY NIGHT THROUGH SATURDAY/...
As of 130 PM EDT Saturday: The region is progged to be near the axis
of a deep upper l/wv trough at the start of the period. The post-
frontal northerly pressure gradient lingers, resulting in a well-
mixed, brisk and cool Wednesday featuring max temperatures 10 to 12
deg f below climo. Channeled vort energy rounding the base of upper
trough in concert with NW flow moisture may also be able to generate
a few NC mountian showers and perhaps high elevation snow showers
into Wed nite. Advective freezing temperatures are also possible
across the northern NC mountains early Thursday morning.

Rising upper heights and the development of weak llvl ridging is
still on tap for Thursday followed by a more prounced sw flow
through a deep layer on Friday. Sensible weather for this period
will feature sunshine, but still well below normal maximum
temperatures on Thursday, followed by a milder Friday, although sun
should becoming increasingly filtered by the influx of clouds within
the aforementioned SW flow. Shower chances should increase at some
point next weekend as moistening is onoing ahead of the slow
encroachment of elongated frontal zone.

&&

.AVIATION /00Z SUNDAY THROUGH THURSDAY/...
At KCLT and elsewhere:  Another 24 hours of VFR weather, with the
possible exception of some fog for a brief period at KAVL early in
the morning. High pressure moves eastward throughout the day which
will increase southeasterly winds on Sunday slightly to 5 to 10kts.
Late in the day, some mid-level moisture will move into the area
from the southeast ahead of the next system which is expected to
bring CIGs down to FL035 or so.

Outlook:  Increasing low clouds possible Sunday night, as a cold
front approaches from the west. The cold front will cross the region
on Monday, bringing a round of widespread showers and possible
thunderstorms.  Drier and cooler conditions return for the rest of
the workweek.

Confidence Table...

            00-06Z        06-12Z        12-18Z        18-00Z
KCLT       High 100%     High 100%     High  93%     High 100%
KGSP       High 100%     High 100%     High 100%     High 100%
KAVL       High 100%     High 100%     High  97%     High 100%
KHKY       High 100%     High  97%     High  94%     High 100%
KGMU       High 100%     High 100%     High 100%     High 100%
KAND       High 100%     High 100%     Med   77%     High 100%

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...SBK
NEAR TERM...ARK/WJM
SHORT TERM...TS
LONG TERM...CSH
AVIATION...ARK/WJM



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