Area Forecast Discussion
Issued by NWS Greer, SC

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000
FXUS62 KGSP 160834
AFDGSP

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
334 AM EST Sat Feb 16 2019

.SYNOPSIS...
A moist cold front will continue to drift southeast across our
area today. Beginning on Sunday, a series low pressure systems
will track from southwest to northeast across the Southeast
states, resulting in a persistent moist pattern through most
of next week. The heaviest rainfall is expected during the
second half of the work week.

&&

.NEAR TERM /THROUGH TONIGHT/...
As of 230 AM EST Saturday: At AFD time, the surface cold front is
stretched across the area, with a surface wave lifting up along the
front across the TN Valley. Widespread rain across the forecast area
(almost 1/3" of rain here at GSP since midnight) this morning is
supported by plenty of jet-level dynamics and dPVA embedded in a
broad mid-level shortwave aloft, along with the mechanical lift of
the front and isentropic upglide. The surface low will be in our
area by around sunrise as the slug of attendant moisture moves east,
zipping on out of the area through the late morning hours as weak
shortwave ridging builds in temporarily. Meanwhile, a surface high
centered in south central Canada will be ridging east across the
Midwest, and as the surface low pushes through our area, the
ageostrophic adjustment will force the ridge down the eastern
slopes, and the damming onset will be felt keenly as a sharp wind
shift. Meanwhile a second wave will be pushing through the center of
the country, with the western half to 2/3 of the country dominated
by troughing, resulting in a moist SW flow across the area as the
weak shortwave ridging pushes east. Moisture will be returning to
the area by the end of the period, though with the damming in place,
it will have to saturate the surface layer before precip is able to
reach the ground.

Given the antecedent dry conditions, rain across the area this
morning shouldn`t cause any problems worse than ponding, and should
mainly be over by daybreak, though diminishing showers will continue
across eastern zones into the early afternoon hours. Since most of
the QPF will be done with by the time the period actually starts,
the updated storm-total QPF will not include today, but rather start
tonight as the second wave pushes through.

The timing of the front and incoming damming will be a huge factor
on afternoon highs today, and we have quite a bit of bust potential.
For now, expect that recovery across the Upper Savannah after the
precip has moved out but before the front should allow highs to rise
into the mid 60s, with the rest of the Upstate and the CLT area
pretty close to seasonal normals, but definitely on the cool side
across the NW NC Piedmont where highs for the day will be this
morning. Confidence not particularly high however.

Concern tonight as light moisture returns to the area will be across
the northern mountains, where lows at or just below freezing may
juxtapose briefly with the incoming moisture around daybreak. Of
course there is some uncertainty with how quickly the low levels
will saturate, but less concern across the mountains than the
Piedmont. Prominent warm nose would freeze any ice nuclei
introduced, but with the shallow moisture the column isn`t saturated
in the dendritic growth zone anyway, so ptype will be dependent on
the surface cold layer. Right now, just do not think it`ll be cold
enough to freeze any raindrops before reaching the surface, so
continued previous trend of freezing rain vs. sleet wording. Without
deeper moisture, could see a lot of drizzle across the area as well.
Lows will be 5 or so degrees above seasonal normals.

&&

.SHORT TERM /SUNDAY THROUGH MONDAY NIGHT/...
As of 220 AM EST Saturday: the short-term fcst picks up at 12z
on Sunday with upper ridging still in place over the Southeast
and very broad upper trofing spread across the Western and Central
CONUS. Upper ridging is expected to remain largely in place thru
the period and even amplify towards the end of the period early
Tues. At the sfc, a lingering in-situ wedge could produce some
possible mixed precip types across the northern zones Sunday
morning. As we move into the afternoon, another low pressure
system develops just to our west and lifts over the fcst area
late Sunday/into early Monday. As the low lifts farther NE, its
trailing cold front will move thru the CWFA by Monday afternoon
and then offshore by Monday night. In its wake, drying high
pressure will spread back over the region and linger thru the
rest of the period. As for the sensible wx, the potential for
a wintry mix across the Northern NC mtns Sunday morning still
remains, although the latest guidance is now limiting frz temps
to only the northernmost mtns and only thru 15z or so. Otherwise,
precip in the form of -ra/dz will become widespread during the
day on Sunday as moist SWLY air is lifted over the sfc wedge.
There is also a slight chance of elevated convection late Sunday
aftn/evening, however the low-lvl looks too stable to allow any
unstable layer aloft to produce anything. Temps will be tricky
on Sunday depending on the extent/longevity of the wedge front.
High temps will rebound considerably on Monday as the wedge is
history and cloud cover has diminished.

&&

.LONG TERM /TUESDAY THROUGH FRIDAY/...
As of 330 AM Saturday: The upper level pattern will feature a
longwave trough over the Rockies and a broad ridge along the East
Coast thru the entire medium range. This means a persistent deep-
layer SWLY flow will exist across the Southeast US, keeping an
unsettled pattern across the forecast area thru the period. The
biggest concern will be a gradual increase in hydro/flood issues, as
rounds of rain are expected from Tuesday thru Friday.

It is still looking like the period with the heaviest rain will be
Tuesday thru Tuesday night, as the right entrance region of the 300
mb jet sets up over the region. This will be coupled with a round of
strong isent lift and 850 mb WAA atop a strengthening hybrid CAD.
Southerly upslope flow will also enhance rainfall totals across the
mountains. Precip rates should come down somewhat on Wednesday into
Wed night, as deep forcing shifts slightly to the west. However,
rainfall totals from 12z Tue to 12z Thu could total in the 4-6"
range across the upslope areas of the western NC mountains. Another
concern for Tuesday night into early Wednesday is the strong CAD.
The latest guidance has trended a little colder, with some wintry
mix now possible across the Northern Mountains and adjacent
foothills. Snow, sleet, and freezing rain are all on the table, with
some accums possible. Confidence is still low that this would be an
advisory or warning-level event. But the forecast trends will
continue to be closely monitored. Temps Tue night and Wed were
lowered by blending in some of the CONSRaw. Overall, it will be
chilly, rainy, and dreary Tuesday and Wednesday.

For Thursday and Friday, forecast confidence decreases somewhat, as
the models generally have the upper ridge make a little nudge west
into the region and force the better jet dynamics to our west. This
may give us a lull in PoPs Thursday and a northward retreat of the
wedge front, allowing a slight rebound in temps (and some sbCAPE in
the southern zones). With that said, the forecast area will still be
in a moist SWLY low-level flow. Plus, the 00z GFS and ECMWF agree on
another wave developing across the Deep South and bringing another
round of rain Thursday night into Friday. So PoPs stay in the chc
range, and additional light amounts of QPF could cause rises in
stream levels. The wedge front will likely push back south with the
next round of rain, and result in cooler temps for Friday (but not
as cold as Wednesday).

Even though it is beyond our 7-day forecast period, I just wanted to
note that the medium range models don`t break this pattern until
late next weekend (Days 9-10). So concerns are increasing that some
areas, especially in the mountains, may experience significant
flooding before we can dry out.

&&

.AVIATION /09Z SATURDAY THROUGH WEDNESDAY/...
At KCLT and elsewhere: Cigs will continue to lower over the next few
hours (right now they range from VFR for KHKY/KCLT to LIFR in
portions of the Upstate), with little change through daybreak.
Occasional -SHRA/-RA will come with alternatively higher and lower
vsbys, but MVFR to IFR should predominate. -SHRA will push out by
sometime this afternoon with a slow lifting trend seen in cigs, and
expect at least some MVFR overnight in the lull before the next
system (though KAVL may remain IFR), with trends back downward
toward the end of the period. Winds on the S side to start the TAFs
and through a chunk of the overnight, but as the front slides south,
winds will swing around to the NE (timing at KCLT just after
daybreak), with some low-end gusts possible along with the wind
shift.

Outlook: A series of low pressure systems will move through or near
the region the latter half of the weekend and through much of next
week. This will produce precipitation and associated restrictions
for long periods of time during each wave.

Confidence Table...

            08-14Z        14-20Z        20-02Z        02-06Z
KCLT       Med   70%     Med   74%     High  92%     High  92%
KGSP       Med   77%     High  81%     High  97%     High  91%
KAVL       High  82%     High  87%     High  92%     High  80%
KHKY       Med   74%     High  93%     High  97%     High  96%
KGMU       High  95%     High  89%     High  98%     High  91%
KAND       Low   46%     High  87%     High  92%     High  87%

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...JPT
NEAR TERM...TDP
SHORT TERM...JPT
LONG TERM...ARK
AVIATION...TDP


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