Area Forecast Discussion
Issued by NWS Greer, SC

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000
FXUS62 KGSP 280916
AFDGSP

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
416 AM EST Mon Nov 28 2016

.SYNOPSIS...
High pressure will weaken and move off the East Coast today,
with moisture sharply increasing ahead of an approaching cold
front. A much needed rainy pattern will take shape by tonight,
and persist across the region through Wednesday night. Cool and
dry high pressure returns Thursday and lingers through the weekend.

&&

.NEAR TERM /THROUGH TONIGHT/...
As of 300 AM Mon: Cirrus continue to stream in from the west,
with powerful jet streak upstream over the southern Plains. The
associated surface low over South Dakota has now dipped below 980
mb and will continue to strengthen as it moves slowly north over the
course of the day. A vort lobe rounding it will drive the cold front
east. Increasingly moist southerly to southeasterly flow preceding
the front will intercept the Blue Ridge, so it is there that precip
chances first reach mentionable values around midday today. From
there they gradually expand and increase into Tue morning. A look
at various WRFs, plus the 4km NAM, suggests chances will not rise
as fast as suggested by the global models. This seems reasonable
given the fact that the precip will be driven primarily by the
upslope/upglide during the day. I have kept accums on the low side
in this timeframe, with a strong focus on south-facing terrain.

Tonight is when things get a bit more challenging. For days the
expectation has been that remarkably strong moisture flux would
occur immediately preceding the front, within a 50-60 kt LLJ
and a Gulf connection, depicted as being unimpeded by convection
along the coast. Now that the event is in the range of the hi-res
guidance they make us question whether the abundant moisture will
play as much of a role as once thought, in generating significant
QPF. The WRFs are producing mainly just upslope-driven precip
until immediately prior to the front, and even once the upper
divergence associated with that jet streak arrives tonight, they
primarily produce precip in upslope areas. Once again the wisest
choice seemed to be to concentrate PoPs more along the ridges and
to tone down QPF a bit. The biggest changes are over the Piedmont,
of course, given the relative lack of terrain. PoPs have generally
been capped in the high-chance to likely range below the foothills.

The strong midlevel winds do pose some concerns of their own. Though
we`ll be in a warm advective pattern, which will impede mixing
down into the valleys and more populated areas, it is likely that
ridges will be exposed to sustained winds of 30-40 mph and perhaps
an occasional gust above 40. The peak will be in the evening
and very early morning hours as the LLJ crosses the mountains,
but winds start to ramp up this afternoon. Hence we will hoist a
wind advisory for areas above 3500 feet, valid thru 12z Tue.

&&

.SHORT TERM /TUESDAY THROUGH WEDNESDAY NIGHT/...
As of 330 AM Monday...it looks as though we might actually be set up
fairly well for a rain event that continues into the middle part of
the week. There are several issues to deal with. The initial round
of precip, which should be ongoing at least across the mountains at
sunrise on Tuesday, looks like it will be forced mainly by upper
divergence/DPVA which is concentrated to our west during the day,
and also by low level isentropic upglide/S-SE upslope flow centered
mainly along the Blue Ridge. That suggests keeping the highest
precip probability over the mountains through the day, with less of
a chance E of the Blue Ridge. The first interesting wrinkle is seen
in the new GFS, which allows for some destabilization in a corridor
to the E of the Blue Ridge in the afternoon, to the tune of
sfc-based CAPE in the 300-500 J/kg range overlapping 30-40 kt of
shear, which suggests some severe weather threat E of the Day 2
marginal risk. Oddly enough, this is not seen to such an extent in
the normally more robust NAM. We will probably lack a trigger, but
that might not be the case farther to the SW. We shall see if
something can organize to the west and reach the fcst area late in
the day.

Next problem...some of the operational guidance, particularly the
GFS and ECMWF, suggest a bit of a lull in the precip during late
afternoon and early evening. I can agree on a lull in principle, but
this will not really be reflected in the forecast because of the
timing differences seen in the model solutions. In effect, the lull
of sorts gets smeared out when taking into account the model timing
differences. At any rate, precip chances should start to ramp back
up from the west overnight as the actual front approaches from the
west.

Next problem...the guidance is fairly consistent in bringing the
main frontal band across the fcst area Wednesday afternoon and
evening with strong low level forcing. Will increase precip chances
in the afternoon and drag the higher probability into the evening
over the east. The Day 3 convective outlook has the entire fcst area
in the Marginal Risk, which will not be argued. Can see a scenario
where a strong convective line develops to the west and moves in
late in the day along with a high shear-low CAPE type environment.
The probable orientation of the boundary does not look optimal at
this point so expectations are being managed.

Next problem...the precip amounts. Wish I could say confidence was
high, but it is not particularly good. Either way, we go into this
event very dry which suggests that we should be able to tolerate a
great deal of precip before flooding would occur, provided it does
not all come at once. No watches will be contemplated at this time.

The precip is expected to exit the forecast area to the east by
daybreak on Thursday and precip chances will reflect that. The
frontal passage will bring a return to cool temps for the rest of
the week.

&&

.LONG TERM /THURSDAY THROUGH SUNDAY/...
As of 345 AM EST Monday...the bulk of the forecast beyond Thursday
looks quiet and relatively cool as we are under a nearly zonal flow
aloft thru Friday followed by a weak upper ridge amplifying in the
srn stream into Saturday. No changes were made through Saturday. The
situation looks dynamic later in the weekend, though, particularly
in the new ECMWF. Both models have some isentropic lift developing
from the west toward daybreak Sunday, moreso in the ECMWF which has
a powerful- looking upper trof digging over the Srn Plains. It`s
hard to put too much stock into these developments out on Day 7, but
the guidance is in fairly good agreement that something will be
developing over the southeast, so a chance of precip was introduced
from the SW on Sunday. Depending on when it begins, it could present
a brief precip-type issue across the mtns early Sunday, but this
would be brief and generally minor as warm advection takes over. The
trends will have to be monitored as this could provide another good
shot at widespread precip.

&&

.AVIATION /09Z MONDAY THROUGH FRIDAY/...
09z KCLT update: Confidence on low MVFR arriving before 06z no
longer is sufficient to warrant a TAF mention. Model guidance has
delayed the front and precip also looks questionable before that
time. Hence the PROB30 and sub-020 deck have been removed.

At KCLT and elsewhere: Cirrus cigs will be seen over the area thru
the morning. Meanwhile a moist southerly flow will build between
the deep low pressure over the central CONUS and high pressure
over the western Atlantic. Low VFR stratus cigs will develop
from south to north beginning around daybreak, with areas along
the southern facing Blue Ridge looking likely to see restrictions
first as moisture flux increases. Hi-res guidance suggests a later
onset to precip than once thought, so precip mention has been
delayed. Interestingly, despite what would appear to be heavily
saturated low-level profiles from the NAM and GFS, most MOS products
delay restrictions until tonight (possibly picking up on the effects
of exceptionally dry soils). I was therefore a bit more optimistic
with cigs than what was reflected in the previous TAF set. I have
KAVL going MVFR in mid-afternoon, followed by KGSP/KGMU/KHKY/KAND
after sunset. Winds will remain mainly in the SE quad, beginning
to veer by the end of the period as cold front approaches from the
west.

Outlook: Chances for precip and restrictions will increase early
Tuesday, with widespread rainfall and associated restrictions
expected at least periodically from Monday night through
Wednesday. Drying is expected Thursday and Friday.

Confidence Table...

            09-15Z        15-21Z        21-03Z        03-06Z
KCLT       High 100%     High 100%     High 100%     High  82%
KGSP       High 100%     High 100%     High  91%     High  94%
KAVL       High  94%     High  95%     Med   75%     High  90%
KHKY       High 100%     High 100%     High 100%     High  94%
KGMU       High 100%     High  96%     High  91%     High  94%
KAND       High 100%     Med   66%     High  90%     Med   76%

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...Wind Advisory from 1 PM this afternoon to 7 AM EST Tuesday for
     NCZ033-048>053-058-059-062>064-505.
SC...None.

&&

$$
SYNOPSIS...Wimberley
NEAR TERM...Wimberley
SHORT TERM...PM
LONG TERM...PM
AVIATION...Wimberley



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