Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 220217

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
1017 PM EDT Sat Oct 21 2017

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.


As of 10pm EDT Saturday:  Main axis of 500mb continental ridge has
moved east of the area with front of next upstream system now
passing through the Plains states.  As ridge continues eastward and
front approaches the area on Sunday, winds will gradually increase
from the southeast and begin to increase moisture and PWAT levels.
Current thin high clouds are expected to be joined by lower-level
clouds Sunday afternoon.  By Sunday evening, front will have reached
Western TN and PWAT levels will be near an inch.  A few scattered
showers may occur Sunday evening over the mountains with upslope
assistance, but significant rainfall will hold off until Monday
morning when upper forcing and the front will be directly impacting
the region.

Winds tonight are mostly calm with surface ridge axis still just
east of the mountains, and dewpoints are up slightly over yesterday,
however, some cloud cover may limit fog potential to a few isolated
locations.  Clouds and passage of ridge are also expected to trim a
couple degrees off of Sunday`s high temperatures, but highs and lows
are still looking to be 10 degrees above normal.


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

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.


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.


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...

            02-08Z        08-14Z        14-20Z        20-00Z
KCLT       High 100%     High 100%     High  90%     High 100%
KGSP       High 100%     High 100%     High 100%     High 100%
KAVL       High 100%     High 100%     High 100%     High 100%
KHKY       High 100%     High  95%     High 100%     High 100%
KGMU       High 100%     High 100%     High 100%     High 100%
KAND       High 100%     High 100%     Med   66%     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:




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