Area Forecast Discussion
Issued by NWS Greer, SC

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197
FXUS62 KGSP 171528
AFDGSP

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
1028 AM EST Sat Feb 17 2018

.SYNOPSIS...
Cool high pressure to our north today combined with low pressure
approaching from the west will make for a cool, wet day.  Cool and
dry high pressure will give our region one dry day on Sunday. Expect
a warm up through mid week as another cold front arrives from
Tennessee by Thursday. This front is expected to move north as a
warm front at the end of the week as our temperatures remain well
above normal.

&&

.NEAR TERM /THROUGH TONIGHT/...
As of 1005 am: E/W-oriented cold front has sagged to almost the I-20
corridor, with NE winds and cool temps (i.e., 15-20 degrees cooler
than at this time yesterday)/hybrid cold air damming noted across
all non-mountain locations. Meanwhile, VAD wind profiles indicate a
quick veering to S/SE flow above 3kft, where speeds are also
gradually increasing with time, suggesting increasing isentropic
lift atop the cool wedge. Indeed, weak radar echoes have been
expanding across the forecast area over the past hour or so, with
increasing occurrence of -RA and/or DZ reports at the surface. This
trend should continue through the next few hours, with at least
likely pops warranted across much of the area. The highest pops will
be reserved across the far western mtns, as precip associated with
weak surface wave riding northeast along the baroclinic zone across
the TN Valley brushes the southern Appalachians. Rainfall totals
should be very light across the vast majority of the area, generally
less than .25 inch, although some localized amounts of around a half
inch will be possible across the southwest NC mtns this afternoon.
Other than the valleys across these southwest mtns, most locations
should have probably already experienced their high temps for the
day, as wet bulb effects associated with falling light rain/drizzle
will only serve to cool things down a little more through the day.
Some diurnal warming is possible across the southern SC Piedmont,
where the upglide will shut off first, but even there, temps will
struggle to reach the 60 degree mark.

Any lingering precip tapers off during the evening as the synoptic
forcing moves northeast out of the area, and the low level flow
turns west then northwest ending the isentropic lift. There will be
some lingering moisture across the mountains which could keep a
brief period of NW flow showers across the northern mountains. Even
with weak cold advection developing, temps do not fall very quickly
and precip ends before any wintry precip can develop. Low clouds may
diminish outside of the mountains after midnight as the strong
damming inversion erodes. Should this occur, patchy dense fog would
be possible toward morning. Lows will be 5 to 10 degrees above
normal.

&&

.SHORT TERM /SUNDAY THROUGH MONDAY NIGHT/...
As of 300 AM EST Saturday: The short term will feature an upper
ridge building over the western Atlantic, with deep-layer
southwesterly flow setting up atop the region. At the surface, a
transient high will cross the southern and central Appalachians
during the day on Sunday, exiting off the Mid-Atlantic coast Sunday
night. This will result in a mostly sunny day with pleasant, above
normal temps. Moisture returns quickly Sunday evening, however, as a
deep low in the lee of the Colorado Rockies activates a warm front
across the Gulf states. This front will lift north thru the area
Sunday night, bringing a round of clouds and light showers
overnight. The best chances will be in the southwesterly upslope
areas. Temps will remain elevated under the overcast skies and
low-level WAA flow.

Monday`s forecast looks a bit tricky, as the guidance hints at a
weak in-situ CAD holding on thru the day. This would need to be
anchored by precip, but forecast soundings show mid levels dry out
with a subsidence inversion. So precip should pretty much taper off
during the day, except for lingering chances in the upslope areas.
If low clouds can hold thru the day, temps should be held back, but
still above normal. If clouds break out, temps may bust by a couple
categories from the current forecast. Will go with mostly cloudy
skies thru the day and into Monday night. Temps mainly in the 60s
for highs, and 50s for lows (well above normal).

&&

.LONG TERM /TUESDAY THROUGH FRIDAY/...
As of 130 AM EST Saturday.  The medium range period begins Tuesday
morning with 500mb pattern having the center of high pressure just
south of Bermuda with the axis of a trough over the Rockies. The
cold front from Texas to the Great Lakes will approach our region
from the NW in mid week. The low level southerly flow ahead of this
front will result in upslope enhancement of showers from NE GA to the
Caesars Head area.  The GFS is faster in bringing the front and bulk
of the rain into our area while the ECMWF holds the front from east
Texas to KY Wednesday evening.  Instability increases and it appears
Wednesday afternoon has the best CAPE values of 600 to 900 from
Elberton to Rock Hill. After the front stalls out over east
Tennessee to VA Wed night into Thursday, the front will drift north
as a warm front Friday becoming stationary well north of our area
over the weekend. Models vary during this time on afternoon CAPE
values depending on smaller scale vort maxes and amount of moisture.
Interesting that the surface high over northern New England late
Friday may push the front south as a backdoor cold front. The
current official front display keeps the front near Maryland.
Regardless...still going with above normal temperatures through the
period.

&&

.AVIATION /16Z SATURDAY THROUGH WEDNESDAY/...
At KCLT and elsewhere: MVFR cigs spread in across the area this
morning with IFR developing by noon. Vsby will drop through the day
as precip develops. The bulk of the afternoon will be LIFR cigs and
IFR vsby. Conditions improve during the evening as precip ends and
drier air moves in. Expect some fog to redevelop toward morning. May
even be dense fog if skies can clear in time. VFR develops after
daybreak Sunday. NE winds this morning become ENE by noon with ESE
for late afternoon. Winds turn NW during the evening then N
overnight.


Outlook: VFR conditions expected for the bulk of the day Sunday.
Moisture returns early next week, and the weather will remain
periodically unsettled, with occasional restrictions likely through
much of next week.

Confidence Table...

            15-21Z        21-03Z        03-09Z        09-12Z
KCLT       High  87%     High  83%     Low   50%     High  89%
KGSP       High 100%     High  94%     Med   72%     High  95%
KAVL       Med   66%     High  90%     Med   64%     High  82%
KHKY       Med   74%     High  81%     Med   77%     Low   49%
KGMU       High  94%     High  94%     Low   50%     High  97%
KAND       High  90%     High  96%     Med   79%     High  88%

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...DEO
NEAR TERM...JDL/RWH
SHORT TERM...ARK
LONG TERM...DEO
AVIATION...RWH



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