Area Forecast Discussion
Issued by NWS Greer, SC

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147
FXUS62 KGSP 042001
AFDGSP

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
301 PM EST Sun Dec 4 2016

.SYNOPSIS...
A moist wedge of high pressure will remain across the area through
the overnight before a weak cold front crosses from the northwest.
High pressure quickly builds back in while moist air returns late
Monday associated with a low pressure system moving up the Tennessee
Valley. Dry high pressure returns Wednesday and will remain until
another cold front approaches early Thursday.

&&

.NEAR TERM /THROUGH MONDAY/...
As of 145 PM EST Sunday:  A deep/closed upper low atop the Gulf
Of California continues to advect Pacific moisture across Mexico
into the central/southern CONUS.  Meanwhile, a northern stream H5
shortwave is diving through the plains states, while a broad area
of surface high pressure is entrenched along the east coast.  With
that, hybrid cold air damming is underway with the aforementioned
moisture lifting isentropically atop the CAD wedge periphery.
This upglide regime is enhanced by the northern steam shortwave,
which once it passes, will yield a lull in the widespread light
to moderate rain that is underway across northeast GA and the
western Carolinas.  Fortunately, llv warm advection has allowed for
temperatures to moderate to levels not supportive of any wintry
ptypes, thus this event looks to be all rain from here on out.
As stated above, the continued upglide will warrant categorical pops
across the region tonight, amidst a non diurnal temperature trend.
Once the H5 shortwave passes, along with the H85 front, the llv
flow will veer w/nw into mid/late morning leading to eventual
dissipation of widespread rain as the parent CAD surface high
erodes/ejects, leaving only weak upsloping along the TN line as
the primary forcing mechanism.  Therefore, by daybreak Monday pops
will be lowered to slight chances north of I85, with chances south.

By that point, model pattern evolution indicates movement of the
deep upper low into south TX, while another transient surface
ridge slides through the OH valley and begins wedging down the lee
side of the Appalachians.  Given that the moisture stream from the
southwest will still exist, am expecting light upglide induced rain
to move in from the southwest by Monday afternoon, thus pops will
comply with likely levels west of I26 by periods end.  As already
stated, temperatures will favor a non diurnal trend given continued
southerly llv flow tonight, then trending more diurnal into the
later part of the fcst period.  All in all, expecting lows to be
at or just above normal, with highs at or just below normal.

&&

.SHORT TERM /MONDAY NIGHT THROUGH WEDNESDAY/...
As of 255 PM EST Sunday: All set for a wet Monday night and Tuesday.
An upper level low over Texas opens up and crosses the area Tuesday.
The surface low along the Texas Gulf Coast takes on a Miller B
pattern and drags a cold front across the area Tuesday as well. High
pressure will be wedged into the area in a cold air damming pattern
Monday night and remain in place until late Tuesday when the cold
front moves through.

PW values increase to 2 standard deviations above normal as the
system moves in. A coupled upper jet will lead to enhanced
divergence and strong synoptic scale forcing. Isentropic lift will
also be impressive as relatively strong low level winds develop as
well. Despite these features, widespread excessive rainfall does not
appear likely given the system`s quick movement. Still QPF values
will range from nearly 1 inch along the TN border to nearly 2 inches
south of the I-85 corridor. This combined with the previous days
rainfall could result in isolated flooding across portions of the
area. With the damming high firmly in place, instability is
virtually non-existent, so any thunderstorms should remain south of
the area. May see breezy conditions across the mountains with the
increasing low level jet, and outside of the mountains in the
stronger ageostrophic NE flow. Lows Monday night will be 10 degrees
above normal, with highs Tuesday near to a little below normal over
the Mountains and 5 to 10 degrees below normal elsewhere.

Precipitation quickly outside of the mountains late Tuesday or early
Tuesday evening as the low pressure system moves east. Lingering low
level moisture and NW flow will keep scattered showers along the TN
border Tuesday night. However, temps will be warm enough for the
precip to be all rain. Dry conditions with some lingering clouds
expected for Wednesday. Lows Tuesday night will be around 5 degrees
above normal, with highs Wednesday near to slightly above normal.

&&

.LONG TERM /WEDNESDAY NIGHT THROUGH SUNDAY/...
of 255 PM EST Sunday: The surge of colder air, tied into a strong
cold front, should arrive in our western forecast area Thursday
morning. The front will then quickly cross the region during the day
Thursday. Note, the GFS remains the fastest of the models, while the
European model continues to show a wave on the front, and somewhat
slower progression. We have taken a compromise on the timing, similar
to WPC, with the cold air arrival.

Scattered shower activity will begin to impinge on the mountains of
western North Carolina late Wednesday night and cross the CWA on
Thursday. Fairly good jet structure and just enough moisture/forcing
should justify some POPS during this time frame.

Meanwhile, considering temperature profile, the highest elevations
in the  western North Carolina mountains could mix with snow by
Thursday morning. Frontal showers will move quickly zip across the
region Thursday, while northwest upslope precipitation develops in
the mountains. Our POP structure by the end of Thursday will be back
in the higher terrain to the west.

Upslope snow showers should then persist Thursday night into Friday
morning in the higher terrain. Snow amounts of an inch or so will
certainly be possible near the Tennessee state line and west facing
slopes.

Elsewhere quiet weather conditions are expected Thursday night and
Friday.

We considered going with a hard non-diurnal temperature trend for
Thursday, and went that route but not as aggressively as the GFS
would have. This is taking into account a percentage of the European
model (slower movement) in a blended package.

Coldest air of the season Thursday night through Saturday with
temperatures some 10 to 20 degrees below normal. Below zero wind
chills continue to be advertised in the western mountains Thursday
night and Friday night, with single digits and teens common
elsewhere.

The next problem will be the very end of the weekend potential
precipitation issue. The GFS is on the dry side, while the European
model want to spin up a potential wintry mess Sunday into Monday.
At this juncture we have followed our neighbors, and WPC, with
chance POPS Sunday into Monday. Precipitation types could be a
troubling, but this far out we kept it rain or snow.

&&

.AVIATION /20Z SUNDAY THROUGH FRIDAY/...
At KCLT and Elsewhere: A mixture of MVFR/IFR and LIFR conditions
expected at all sites through this taf cycle as moist upglide atop
a CAD wedge prevails.  Initialized all tafs with either IFR/LIFR
cigs/visb per obs and near term guidance, with further deterioration
expected over the next few hours as the next wave of light/moderate
ra moves into the region from the southwest.  Therefore, late this
afternoon into tonight conditions are lowered to LIFR at all sites,
remaining at those levels through the overnight.  Expecting ra
to taper through early morning as the northern stream upper wave
passes and the 850mb front moves east.  Thus all sites indicate
such by way of no prevailing ra however with prob30s for ra, timing
based upon location.  Lastly, expecting cigs to recover to at
least mvfr by late in the period at all sites, with the exception
of KAND where impacts of the next round of precip being to occur.
Winds through the period will remain light northerly at all sites,
except for at KAVL where flow prevails southeasterly down the French
Broad valley, but should eventually veer northwesterly by daybreak.

Outlook: Another round of heavier rain will move into the region
Monday night into Tuesday leading to widespread restrictions
. Brief dry weather is expected on Wednesday, with another front
approaching from the west.

Confidence Table...

            20-02Z        02-08Z        08-14Z        14-18Z
KCLT       High  82%     Med   67%     Med   69%     Med   66%
KGSP       High  83%     Med   75%     Med   68%     Low   55%
KAVL       Med   78%     High  83%     Med   71%     Low   44%
KHKY       High  90%     Med   66%     Med   69%     Med   66%
KGMU       High  86%     Med   75%     Med   69%     Med   61%
KAND       Med   79%     Med   66%     Med   64%     Med   60%

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...CDG
SHORT TERM...RWH
LONG TERM...TS
AVIATION...CDG



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