Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 140338
AFDGSP

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
1038 PM EST Fri Jan 13 2017

.SYNOPSIS...
A slowly moving cold front will settle southward across the region
tonight through Saturday as cool high pressure ridges down from New
England. The resulting wedge front will oscillate northward and back
southward Sunday through Monday before returning solidly northward
on Tuesday. A cold front will approach from the west during the
middle of next week.

&&

.NEAR TERM /THROUGH SATURDAY/...
As of 1030PM EST Friday:  As expected, light upglide induced
precipitation is now starting to develop across portions of the
northern NC Piedmont, while a few residual light showers struggle to
survive along the frontal boundary which is now pushing south of the
I85 corridor.  Guidance favors weaker upglide for this southern tier
of the fcst area, while the aforementioned NC Piedmont zones will
likely see light showers all night.  Thus, did increase and spread
out likely/categorical pops along the I40 corridor, east of the NC
mtn zones.  Given increased low/mid stratus, latest minT guidance
has come in a bit warmer than previously progged, therefore did
blend in consshort which effectively increases minimums around a
degree or two beneath the intruding wedge.  Otherwise, no additional
adjustments were needed/made with this update.

Previous Discussion: Saturday will be cloudy and considerably
cooler as a back door cold front/wedge front with a cold pool and
dense low clouds behind it flows across the area from northeast to
southwest tonight and Saturday morning.  Some light, stratiform
precip is also nicely depicted by high-res models behind the
front through Saturday morning.  With clouds and cool-advection,
Saturday temps. will have a narrow range from the upper 40s for
lows in the mountains to upper 50s for highs in Piedmont areas.

Parent surface high that supports northeasterly cool flow behind
front Saturday morning gradually dissolves Saturday afternoon and
dense low clouds associated with cool pool behind the front may
begin to erode Saturday afternoon or evening, with Sunday seeing
a warm-up back to above normal temps.

The balance of Friday will continue with increasing clouds and
light winds, and a few isolated light showers, with denser low
clouds coming with the back door front early Saturday morning.
Some showers aided by westerly 850mb flow along the Southern
Appalachians will continue this evening, being joined by some light
showers behind the back door front later.  Upper flow remains zonal,
with a large upper high over FL and the Gulf Coast.

Some marginal convective instability has developed over western
parts of the CWA with MUCAPE to 250 j/kg.  Current radar and
satellite do not show any convective activity so far, beyond some
low-top stratocu.  However, some of the widely scattered showers
could become stronger over the next few hours before diurnal
cooling rapidly eliminates this marginal instability.

&&

.SHORT TERM /SATURDAY NIGHT THROUGH MONDAY/...
As of 200 PM EST Friday: A flat upper level ridge will be in place
across the southern tier of the central and eastern CONUS the latter
half of the weekend as a deep, closed low lifts from western Mexico
across West Texas. Deeper moisture feeding east of this system will
spill over the northern periphery of the ridge and stretch out close
to western North Carolina Saturday night into Sunday before
retreating northward Sunday night. Meanwhile, at the surface, weak
cold air damming may still be in place Saturday evening. The wedge
boundary will begin a period of oscillation, with some northward
movement through Sunday and then a possible backdoor retreat into
the area once again Sunday night as surface high pressure slides
east from the Great Lakes to the East Coast. Forcing over the
oscillating wedge appears quite limited during the period, but any
chance of weak moist upglide will be best over western NC.

A highly amplified pattern will remain in place on Monday with the
reinforcing trough axis gathering over the plains states while a
ridge builds over the southeast. Transient northern tier upglide may
continue into Monday and this could help pull the wedge boundary
back southward across our region. Any QPF through the period will be
light. Temps should see a smaller diurnal range across the more
wedge-impacted northern tier and more warming and less precip threat
across the southern tier.

&&

.LONG TERM /MONDAY NIGHT THROUGH FRIDAY/...
As of 240 PM Friday, confidence has taken a hit for the medium range
as models are in flux with the potential weather systems moving
through the area. The ECMWF keeps the upper ridge over the eastern
CONUS in place on Tuesday while the GFS has it weakening. This
allows an upper low to cross the Great Lakes Tuesday night with its
trailing trough to cross our area late Tuesday night and early
Wednesday. The ECMWF brings the trough through late Wednesday and
early Wednesday night. The ECMWF keeps the flow zonal for Thursday
with a dampening short wave crossing the area. The GFS shows flat
ridging with a weak short wave. Both models show a progressive upper
low crossing the area Friday, but the ECMWF shows it weakening while
the GFS is stronger.

At the surface, the damming high over the area weakens Tuesday as a
cold front approaches from the west. The ECMWF brings the front into
the area Wednesday while the GFS moves it through Tuesday night. The
ECMWF stalls the front just to our south through Friday with some
weak moisture return and precip over the front. The GFS is dry for
Wednesday and Thursday with moisture and precip returning late in
the day on Thursday and continuing into Friday in developing low
level southerly flow. Given all these differences, have gone with a
mix of guidance blend and national center guidance. This means
slowly increasing PoP Tuesday and Wednesday. PoP drops off a little
Thursday into Friday but remains in the chance range. There is also
little to no agreement on whether any thunderstorms can develop, let
alone severe storms.

Highs 5 to 10 degrees above normal Tuesday rise to 10 to 15 above
normal for Thursday and Friday. Lows Monday night 15 to 20 degrees
above normal rise to around 20 above normal for the rest of the
period

&&

.AVIATION /04Z SATURDAY THROUGH WEDNESDAY/...
At KCLT/KHKY:  Initially VFR, lowering to MVFR/IFR by morning,
possibly LIFR by midday as swly llv flow advects moisture atop
an intruding wedge front.  Said wedge front is currently moving
across the NC Piedmont into the I85 corridor region.  With that,
flow behind the front has backed nely, and will remain there
through the entire taf period.  As with most wedges, cigs/visb
will deteriorate overnight into Saturday morning amidst iso/sct
shra/dz, which was featured in both tafs via tempos/prob30s.
Not expecting any recovery until late in the day Saturday as
the parent surface high across New England advects out to sea.
That said, if enough light precip occurs, wouldn`t be suprised if
the wedge locks in thru/beyond this taf cycle.

Elsewhere:  Trends similar to the sites above, however with less
confidence in restrictive conditions for the duration of the period
as any wedge erosion will relieve KAND and the remaining Upstate
sites first on Saturday afternoon.  Nevertheless, current swly
winds will back nely very soon, with mid lvl cigs to prevail.
Cigs/visb will lower to MVFR/IFR and possibly LIFR at all sites
overnight, including at KAVL where ample BL moisture will lead
to restrictive stratus/fog.  Winds at KAVL will remain nwly for
the first quarter of the taf cycle, backing sely as ne flow is
channeled up the French Broad valley through the period.

Outlook: Cool high pressure ridges down the East Coast over the
weekend, bringing chances for precipitation and restrictions.

Confidence Table...

            03-09Z        09-15Z        15-21Z        21-00Z
KCLT       Med   77%     High 100%     Med   70%     Med   79%
KGSP       High 100%     High  89%     Med   79%     Med   72%
KAVL       Med   72%     High  89%     High  81%     High  98%
KHKY       Med   64%     High  85%     Med   76%     High  82%
KGMU       High 100%     Med   68%     High  81%     Med   68%
KAND       High 100%     High  81%     High  83%     High  90%

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...HG
NEAR TERM...CDG/WJM
SHORT TERM...HG
LONG TERM...RWH
AVIATION...CDG



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