Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 260009

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
709 PM EST Sun Feb 25 2018

A cold front will pass through the region today, however with a
moist airmass remaining in place through Monday.  Cooler and drier
high pressure will slide south into the southern Appalachians on
Tuesday, before another wave of moisture moves into the region by
midweek, lasting through weeks end.  Drier and cooler conditions
will return next weekend behind another passing cold front.


of 7 pm Sunday: Although a large area of radar returns currently
can be seen covering much of the southern Appalachians and
surrounding areas, precipitation reaching the ground seems to mostly
be confined to northern portions of AL and GA early this evening.
Precip should begin filling in west and southeast of our area by
mid/late evening, as the next short wave trough ejects from the
Great Plains, allowing for weak wave development/increasing
frontogenesis along the quasi-stationary baroclinic zone in place
across the region.  This activity is expected to overspread our
forecast area late tonight, warranting likely/categorical pops
during that time and into Monday morning. In the interim, mostly
token slight/low chance pops will be featured for patchy areas of
-RA falling from the extensive mid-level cloud deck.

Showers are expected to bring around a half inch to an inch of total
rain through Monday morning.  With at best neutral convective
instability, any significant thunderstorms are not expected, and
little, if any, lightning.  Min temps tonight will again be well
above normal. Temperatures will be dropping behind the front with
highs on Monday down to only 5 to 10 degrees above normal.


As of 200 PM EST Sunday: The short term period will be lengthened a
little to account for the passage of the midweek system. We`ll begin
in the wake of the near-term cold front, with a shortwave trough
lifting out of the region and upper ridging beginning to build in
from the west. High pressure crossing the Ohio Valley at the
beginning of the period with moderate CAA behind the front will lead
to noticeably cooler lows Tuesday morning, maybe only 5 or so
degrees above seasonal normal but will likely feel downright cold
since we`ve gotten used to these warm temperatures. The surface high
will cross the mountains Tuesday and will dominate our sensible
weather, as the upper ridge axis moves over the Appalachians and
toward the Eastern Seaboard.

Meanwhile a cutoff midlevel low over southern CA will lead to quite
a bit of downstream diffluence aloft over the Southern Plains, with
low-level/surface WAA and return flow moving back north from the
Gulf around the western periphery of the surface high, allowing a
warm front to lift north. While the surface high (just off the NC/VA
coast by Tuesday night) won`t exactly be damming, moistening aloft
and virga falling into the dry surface layer Tuesday night and
Wednesday morning will likely lead to a bit of insitu damming,
keeping highs on Wednesday very close to seasonal normals (finally!)
and keeping the warm front more of a wedge front. Precipitation will
overspread the area from the west early Wednesday. The cutoff low
will kick out of CA and toward the Plains on Wednesday, with another
surge of moisture associated with the strengthening surface low and
developing cold front working its way east. Expect Wednesday into
Thursday to be fairly wet, with highest precip amounts in two
rounds: first Wednesday morning and then again Wednesday night with
the stronger lift associated with the cold front. Still though,
storm total QPF not terribly concerning, with 2-3" in western zones
and typical upslope areas, and generally 1-1.5" elsewhere.

Cannot totally rule out a thunderstorm threat on Thursday with some
minimal surface instability advecting north into the SC/NC Piedmonts
ahead of the front, and temperatures rising (again) to 10ish degrees
above seasonal normals with the erosion of the insitu damming. More
significantly the deep-layer shear will be quite impressive at 70-
80kt. But for now with the widespread rain/cloud cover it does not
look like much to be concerned about; will continue to monitor. The
cold front will be pushing through the area by the end of the
period, with a rapid decrease in pops expected shortly thereafter.


As of 215 PM EST Sunday: In the wake of the cold frontal passage
Thursday night, we will finally see a period of significant CAA
impacting our area. Moisture associated with the parent upper low
will impact the northern and likely central mountains Thursday night
into Friday night (diminishing rapidly Friday night). Depending on
how far south the upper low travels (currently operational models
keep it just north of the Ohio Valley), we could see more NW flow
moisture than the current forecast has. In any case, with the CAA,
could see temps dropping to freezing in the northern mountains and
higher elevations of the central/southern mountains Thursday night,
so some snow showers certainly not out of the question, and
naturally a few degrees cooler Friday night, so any lingering
moisture would likely be snow showers by that point; however
operational models are speeding up the exit of the upper low so it`s
looking less and less likely for NW flow showers (rain or snow) to
linger past midday Friday.

More significantly, along with the CAA that will allow seasonal
temperatures to return, the surface low will strengthen quite a bit
off the Mid-Atlantic to New England coasts (ECMWF is farther north
than the GFS), and with surface high across the Plains pushing east,
the pressure gradient will increase markedly across the area with a
corresponding increase in winds Thursday night into Friday. For now
forecast winds are kept below wind advisory criteria but 50-60kt
northwesterly 850mb jet and 40-50kt winds forecast at 925mb will be
something to watch as we get closer.


At KCLT and Elsewhere:  -RA could fall out of extensive mid-level
cloud shield just about anywhere at just about anytime. However, the
greater chance for heavier rain causing flt restrictions is expected
to hold off for most terminals until after midnight. Generally MVFR
visby is forecast in -RA/BR at pretty much every terminal by
daybreak-ish. The rain will also allow cigs to begin gradually
lowering, with MVFR expected at most terminals by sunrise-ish, with
low MVFR/IFR becoming likely by late morning. Cigs will then be
stubborn to lift, probably not becoming VFR until late afternoon/
evening at most terminals. S/SW winds at 5-10 kts at the start of
the period should begin turning the dial this evening, becoming
light NW btw 06-09Z, before settling at light NE at most terminals
by Monday afternoon.

Outlook: Improvement in flt conditions is expected to continue into
Monday night, with VFR appearing likely at most terminals Tue into
Tue night. Another wet frontal system will affect the area Wed and
Thu, with cool/dry/breezy conditions expected to develop by the end
of the work-week.

Confidence Table...

            00-06Z        06-12Z        12-18Z        18-00Z
KCLT       High 100%     High 100%     Med   79%     High  80%
KGSP       High 100%     High  90%     Med   77%     High  92%
KAVL       High 100%     High  87%     Med   78%     High  92%
KHKY       High 100%     High  97%     High  85%     High  89%
KGMU       High 100%     High  90%     Med   78%     High  92%
KAND       High 100%     High  88%     High  81%     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:




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