Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 201941
AFDGSP

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
241 PM EST Fri Jan 20 2017

.SYNOPSIS...
Numerous waves of energy will bring rounds of rain and
thunderstorms to the area on Saturday. A vigorous low pressure
system will sweep towards the Carolinas Sunday and Monday, bringing
abundant moisture and the potential for storms. Mild high pressure
will be in control Tuesday and Wednesday until a cold front crosses
the region Wednesday night.

&&

.NEAR TERM /THROUGH SATURDAY/...
As of 230 PM EST: the lull in-between systems continues.  Radar
clear at this time, so dry fcst for the balance of the afternoon
looks good. The satellite imagery is the real bummer...with low
stratus deck seen over the western Piedmont and Foothills of NC.
This will be very difficult to dislodge before we get past peak
heating, so have decided to lower the max temp fcst over the nrn
Foothills and nw Piedmont as it is increasingly likely that we will
not get out of the mid 50s. Meanwhile, satellite imagery shows
plenty of breaks in between cloud streets over the mtns/western
Upstate/ northeast GA. Have bumped highs upward there.

Should be quiet over the next 12 hours or so...through about
daybreak...as the upper ridge axis moves to the coast and we remain
without any meaningful mid/upper forcing and the stronger flow from
the Gulf has yet to arrive. Weak high pressure should keep
light/variable or calm wind overnight. Low level moisture should
remain plentiful while mid-levels remain relatively dry into the
early morning hours. This should be a good set-up for low stratus
and fog. The model guidance suggests widespread dense fog developing
in the early morning hours on Saturday outside the mtns and fcst
soundings support that scenario. Have bad luck at issuing dense fog
advisories before the fog begins to develop, so one will not be
issued yet, but will expect to wake up to dense fog in the morning
assuming the next round of precip remains to the SW.

The model guidance is in fairly good agreement with lifting the next
in the series of short waves out of the upper trof to the west...and
across the western Carolinas during the daytime on Saturday. At the
same time, weakening upper divergence moves across the region and
isentropic upglide improves. Add the arrival of more deep moisture
and we should expect another wave of precip to cross the region.
Arrival will probably not be until after daybreak if the model
timing is correct, with the rain spreading northeast across the
region through the late afternoon. Will ramp precip probability up
into the likely range accordingly. Precip amts do not appear that
impressive as the flow remains relatively weak. The big question is
whether any strong thunderstorms that will probably develop to our
southwest and near the Gulf Coast can reach this far northeast
during the day. The current thinking is probably not, and the
expected low stratus/dense fog might have a lot to do with it. If
the morning is as murky as expected, we would have a significant
delay to any warmup, which would ruin our chances for destabilizing.
That would mean any stronger storms that form to our SW would not
make it before running out of gas. Will not entirely turn our backs
on the possibility, given the strong shear that is expected, but for
now we think any Marginal Risk will not be a threat until after
sunset. The forecast will include some thunder possibilities because
of elevated instability over the srn half of the fcst area in the
afternoon. Temps will remain on the order of 15 degrees above
normal, but there is a significant bust potential.

&&

.SHORT TERM /SATURDAY NIGHT THROUGH MONDAY/...
As of 230 PM Fri: Upper heights continue to fall Saturday night into
early Sunday as shortwave cuts off over the southern Plains, all the
while moving eastward. In the lower levels, WAA and moisture
advection continue overnight. The combination leads to increasing
lapse rates aloft and therefore increasing buoyancy from atop the
inversion. Effective shear from the NAM and GFS exceeds 30 kt by
daybreak Sunday, closer to 50 kt in our srn NE GA and Upstate zones.
Lift will be enhanced by upper divergence and possibly MCVs.  This
could lead to strong thunderstorms, perhaps producing marginally
severe hail; damaging wind threat will be limited by the moist
profiles.

Models agree reasonably well on a compact, open shortwave ejecting
from the Gulf Coast up into the Carolinas Sunday morning. GFS and EC
have this entering our area around daybreak whereas the NAM is a
little slower, closer to 15z. This feature should supply a more
coherent shot of forcing. It also is depicted as backing low-level
flow and increasing shear and helicity. These parameters may peak
before SBCAPE increases in the afternoon. NAM/GFS would suggest some
surface-based instability in the morning, but not reaching
unseasonably high levels until afternoon. The midday overlap is when
we are currently most concerned about severe weather. Damaging winds
or even a tornado would be possible, but large hail might also be a
concern if the instability and shear are as great as the NAM depicts
(I`ll go so far as to say its prog soundings are a bit scary early
Sun aftn). It is important to note, however, that the EC features
much less CAPE than the others, though otherwise it is not all that
different from the GFS. 4km NAM, which now extends through 00z Mon,
does not develop much convection either Saturday night or Sunday.
These notwithstanding, given the primed environment and fairly good
agreement on dynamic lift mechanisms, we will continue to advertise
severe weather in the HWO more or less as the previous shift did.

In addition to any severe threat, we are anticipating at least a
small possibility of flash flooding. Area streamflows are below
normal, so rainfall totals for the whole event are not expected to
bring them even to action stage. Late Saturday night and Sunday
morning we have good overlap of deep moisture and forcing, and this
occurs again Sunday evening (when strong sfc convergence may develop
near the deep sfc reflection of the upper low). Any t-storms that
develop in those periods might cause localized flash flooding, but
the threat of severe wind/hail outweighs the flood threat.

Late Sunday night into Monday our area will be on the back side of
the low, with winds becoming northwesterly in the morning. Colder air
will begin to return to the area as a result, and winds accordingly
will become quite gusty in the mountains. The NAM does suggest some
high elevations could see some snow mix in with rain if sfc temps
turn out cold enough; however, the best-performing guidance blend for
that period keeps mins too warm Monday morning for anything but rain.
Downsloping should offset the CAA and permit maxes to remain above
normal, though cooler than those over the weekend.

&&

.LONG TERM /MONDAY NIGHT THROUGH FRIDAY/...
AT 200 PM EST Friday...On Monday evening an upper trough will be
extend from the Great Lakes to the Bahamas, while an upper ridge
will be over the Eastern Great Plains. The pattern progresses such
that by Wednesday the ridge reaches the East Coast, while a
positively tilted upper trough extends from Central Canada to AZ. By
Friday the trough moves to Atlantic Canada and TX, with strong
shortwave energy reaching the Southern Appalachians.

At the surface, on Monday evening an upper low and associated
moisture will be departing Northeast GA and the Western Carolinas,
however some northwest flow moisture may linger over the Western NC
Mountains into Tuesday, with light rainfall resulting. A cold front
will approach from the west on Tuesday night, with precipitation
reaching the NC mountains before the front arrives on Wednesday
evening. The front crosses the area by Thursday morning, although
moisture lingering behind the boundary does not depart until late in
the day. There is some potential for a changeover to snow in the NC
Mountains before the front departs, if the cooler GFS solution if
right. There is better potential for snowfall in the NC mountains
from Thursday evening into Friday as a persistent round of northwest
flow moister arrives from the northwest. Temperatures will run above
normal until frontal passage, after which they will fall to near or
below normal.

&&

.AVIATION /20Z FRIDAY THROUGH WEDNESDAY/...
At KCLT...looks like a busy period of weather has begun. Satellite
imagery shows an extensive deck of IFR stratus covering the western
Piedmont. This will take some time to scatter/lift around the edges.
Conditional climo does not have a large enough database of previous
events to be of much help with confidence, so the idea put forth by
the LAMP guidance of that ceiling lifting to MVFR around 20Z seems
reasonable and was followed. Once that happens, expect a few hours
of SCT/BKN ceiling in the MVFR range before clouds scatter out for a
time and we go VFR for several hours this evening with the absence
of deeper moisture and forcing. Things get more tricky overnight.
Nearly all the guidance keeps plentiful low level moisture,
light/variable wind, and dry air aloft, which is a good setup for
widespread IFR low stratus/fog. Much of the guidance takes KCLT down
to VLIFR conditions, and that is very much within the realm of
possibilities. At this time, prefer to go with an LIFR fcst this far
out, but know to expect many locations to be VLIFR around daybreak.
It may take until the arrival of shower activity from the SW ahead
of the next upper wave to bring some improvement to flight category.
The arrival time is uncertain, but could be as early as 15Z, thus
the beginning time of a PROB30 group. Precip is much more likely
beyond the end of the TAF period. Wind will be light, but will favor
a southerly component during the period.

Elsewhere...seeing clearing from the west as drier air mixes in.
This should take care of lingering IFR at KGSP/KGMU in short order,
leaving most sites with SCT/BKN low end MVFR thru the rest of the
day. The exception is KHKY where low IFR stratus may hold on and not
burn off at all. The rest of the period should behave similar to
what was outlined for KCLT, with earlier timing on precip chance
arrival on Saturday morning.

Outlook: The very unsettled pattern will continue through Sunday,
bringing periods of rain and/or restrictions. Heavy rain showers and
perhaps thunderstorms are possible Sunday. Expect improvement early
next week.

Confidence Table...

            19-01Z        01-07Z        07-13Z        13-18Z
KCLT       High  83%     High  81%     Med   64%     Med   77%
KGSP       High  87%     High 100%     Med   65%     Med   72%
KAVL       Med   75%     Med   64%     Low   52%     Med   64%
KHKY       Low   50%     Med   65%     Med   66%     Med   65%
KGMU       High  90%     High  98%     Low   55%     Med   64%
KAND       High  86%     Med   72%     Low   53%     Low   58%

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...LEV
NEAR TERM...PM
SHORT TERM...Wimberley
LONG TERM...JAT
AVIATION...PM



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