Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 210014

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
714 PM EST Sat Jan 20 2018

High pressure will gradually move farther offshore tonight and
tomorrow bringing warm southerly flow to the region. A wet cold
front will then cross the region late Monday into early Tuesday.
In the front`s wake, dry high pressure will overspread the region
again and persist into next weekend.


As of 710 PM EST Saturday: Main update for 00Z TAF issuance, as the
near term forecast remains on track.

After temperatures soared into the low to mid 60s in most
locations this afternoon, latest obs show temperatures have
already dipped into the mid to upper 50s (cooler across the
mountains). Expect this downward trend to continue as
temperatures drop quickly with overnight lows settling into the
low to mid 30s, near 40 across the Upstate.

Otherwise, quiet conditions will persist through tonight with weak
ridging in place as winds remain light and thin cirrus pass
overhead (with the exception of stratus across the TN/NC
border). Latest fcst soundings continue to suggest the potential
for patchy fog towards daybreak with ideal radiational cooling
in place and limited low level moisture. Would not entirely rule
out reduced visibilities for an hour or two especially across
portions of the NW Piedmont where soil moistures are fairly high
with the recent snow melt. By mid morning, expect any fog to
quickly disperse giving way to another day filled with plenty of
sunshine and warming temperatures. As the weak sfc ridge shifts
to the Carolina coast, expect afternoon highs to once again
climb into the upper 50s to low to mid 60s across the area with
light winds prevailing.


As of 225 PM EST Saturday: A deep, closed low pressure system will
emerge over the central/southern plains Sunday night, while ridging
persists downstream over the southeast. The closed system will lift
across the midwest on Monday with a steadily intensifying, deep-
layer southwesterly flow developing across the western Carolinas and
northeast Georgia. An upper jet streak ahead of this feature will
strengthen to around 130+ kt over the southern Appalachians by
Monday night to produce a narrow window of stronger upper-level
divergence. We still expect the best dynamics to pass north of the
region Monday night into Tuesday, but with channeled vorticity and
the upper jet support providing decent deep-layer omega atop the
strong frontal zone.

Meanwhile, the deep frontal band moisture will likely reach the
southern Appalachians late Monday, cross the forecast area Monday
night, with dry slotting rapidly wrapping in from the southwest by
daybreak Tuesday. The low-level jet should peak around 50 to 55 kt
Monday evening just ahead of the 850 mb front. Lapse rates should
steepen to 6 to 6.5 deg C/km under the passing trough Monday night
and there may be a narrow sliver of some 100 to 200 J/kg sbCAPE,
with slightly better elevated CAPE. Thus, thunderstorms will be
mentioned along with the likely to widespread shower chances, but
the chance of severe convection still looks too low for any HWO
mention at this point. Any severe thunderstorms would be of the high
shear/low cape variety. In addition, QPF looks too limited (and of
limited duration) for any significant hydro problems with the fropa,
but a few southern mountain upslope areas could see an inch of
rainfall - with associated river/stream rises through Tuesday.

Finally, snow levels will crash across the mountains by daybreak
Tuesday with westerly upslope moisture continuing in the cold
advection through the day on Tuesday. Scattered upslope flow snow
showers are likely on Tuesday along the western NC mountains, but
any accumulations should be very light and confined to the higher
peaks and immediate Tennessee border.


As of 210 PM Sat: Northwest flow will continue across the mountains
Tuesday evening as a shortwave lifts out of the region. Enough
moisture persists along with resultant brisk boundary-layer winds
to warrant a small PoP near the Tenn border, mainly as snow. Mean
troughing remains in place over the East through Thursday, with dry
high pressure expanding beneath the resultant confluent upper flow
and keeping PoPs too low to mention through the end of the work
week. Despite insolation, the cold nature of the airmass suggests
max temps only slightly above normal, and mins at or slightly below
normal. Models continue to feature a pronounced ridge moving over
the CWFA Friday. This sets up WAA and begins to ramp up rain chances
ahead of the next fropa, most likely sometime over the weekend.


At KCLT and Elsewhere: Potential for patchy areas of marginal MVFR
fog towards daybreak on Sunday. Otherwise, expect VFR through the
valid TAF period.

Conditions remain quiet across the area this evening with latest
satellite imagery depicting FEW/SCT cirrus moving across the area,
though some lower cigs (~4kft) persist along the TN/NC border,
not affecting any of the TAF sites. Light southwest winds
(northwest at KAVL) will become calm across much of the area
overnight with sfc high pressure in place. Towards morning, with
ideal radiational cooling and limited low level moisture (along
with suggestion from latest guidance), anticipate patchy areas
of marginal MVFR to develop around daybreak, thus have 5/6SM
prevailing at all terminals through 14/15Z. Otherwise, VFR will
prevail through the end of the valid TAF period with passing
thin cirrus and light southwesterly winds (becoming southeast at
KAVL in the afternoon).

Outlook: Potential for MVFR Monday into Tuesday with an approaching
cold front. Otherwise expect VFR.

Confidence Table...

            00-06Z        06-12Z        12-18Z        18-00Z
KCLT       High 100%     High 100%     High 100%     High 100%
KGSP       High 100%     High 100%     High 100%     High 100%
KAVL       High 100%     High  97%     High  99%     High 100%
KHKY       High 100%     High 100%     High 100%     High 100%
KGMU       High 100%     High 100%     High 100%     High 100%
KAND       High 100%     High 100%     High 100%     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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