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Area forecast discussion 
National Weather Service Greenville-Spartanburg SC
246 PM EST Sat Nov 22 2014

Synopsis...
dry high pressure will give way to a very moist southerly flow on
Sunday. A cold front will move over the region on Monday with drier
and colder air behind it. A coastal low is then expected to develop
off the southeast coast and move up the Atlantic Seaboard on
Wednesday and Thursday. High pressure will spread back over the area
in the lows wake.

&&

Near term /through Sunday/...
as of 200 PM EST Saturday...1033 mb high pressure currently centered
over the eastern Carolinas will continue to slide offshore through
tonight as a developing surface low over the western Gulf tracks
northeastward. As a southern stream upper shortwave trough
approaches from the southwest...upper pattern becomes southwesterly
over the region with middle-high level cloudiness overspreading the
region in its wake. Latest short range models continue to agree that
the low will track north-northeast across the middle Mississippi Valley and
into the Great Lakes region tonight into Sunday...lifting an
associated warm front NE across the Southern Plains tonight and into
southern Georgia/SC by Sunday. Isentropic upglide will strengthen over
the region ahead of this feature tonight as S/south-southeasterly low level jet
increases atop the in-situ cad. As a result...precipitation should
overspread the area from the southwest by Sun morning. Heavier
precipitation should occur during the day on Sunday as the combination of
diffluent upper flow and impressive isentropic upglide promote
stronger lift as well as maximizing deep Atlantic moisture flux over
the region. Hence...probability of precipitation ramp up from SW to NE Sun morning and
peaking categorical range through Sun afternoon.

Hydro...total rainfall should range from 1.5-2 inches over the
Blue Ridge Escarpment to 1 to 1.5" elsewhere. Given prolonged dry
conditions and +3.5" of 6hr flash flood guidance over much of the
area...the threat for flash flooding appears minimal...though
isolated flooding can not be ruled out in low lying/poor drainage
urban areas.

Severe weather potential...although will be very strong wind shear
aloft and long curvy hodograph...the threat for severe weather
appears unlikely thanks to persistent in-situ cad and the lack of
deep upper forcing. Hower...have carried thunder mainly east of the
mountains zones as models depict weak elevated buoyancy regime extends
northward toward late Sun afternoon.

&&

Short term /Sunday night through Tuesday/...
as of 230 PM Saturday...the timing of the passage of a strong short
wave has slipped just a bit into Sunday evening...which delays the
departure of the main precipitation area...so a categorical pop was kept
into the evening hours over the eastern half of the forecast area...with
likely west of there. Our main concern continues to be the threat for
severe storms...primarily over the area S/east of I-85. The operational
models suggest that our in-situ wedge will break down/erode from the
southeast late in the day allowing for some weak bouyancy to creep into the
southeast fringe of the forecast area. Thus...the model trend looks to be
pointed toward a bit more instability...to go along with the very
strong shear. However...the expected degree of organization still
appears to be lacking. In typical events where we see severe weather
from linear convection in high shear Low Cape environments...we tend
to have a very strong advective short wave that tracks right across
the forecast area. This wave tracks past the mountains to the northwest Sunday evening.
Furthermore...an experimental index for high shear Low Cape severe
weather (called the sherb index)...has values below what is expected
for an environment that supports severe weather...when run for the
operational NAM and GFS. Will include a chance for thunderstorms
over the southeast zones in the evening. Bottom line...we cannot totally
rule out severe weather over the southeast zones...but unless we see more
instability or a better indication of storm organization...we will
not mention the non-zero severe weather chances in the severe weather potential statement. As for
heavy rain...cannot rule out some heavy rain near the Blue Ridge in
the evening as a strong low level jet remains directed at the
mountains...but the 850mb flow veers quickly SW in the evening as the wave
passes. Antecedent conditions are dry...which lowers our flood risk.
Temperatures are interesting Sunday evening/night with the destruction of the
weak wedge and continued warm advection. Expect that most places
will have their high for Sunday after sunset as temperatures continue to
rise. Temperatures should remain mild overnight as the main front remains
to the west. On Monday...there are some model discrepancies as to
the amount of shower activity...with the GFS/European model (ecmwf) showing less
precipitation than the NAM. Will keep a low chance first thing and
gradually taper it down from there...mainly because the front should
pass through during the day. Feel the NAM is way overdone because it
keeps surface dewpoint values in the lower to middle 60s east of the mountains
which results in too much instability which triggers the convective
precipitation scheme. The GFS looks more reasonable with middle/upper 50s
dewpoints...so quite a bit less convective precipitation.

The rest of the short range is quiet. We stay in a SW flow aloft
with no middle/upper forcing and high pressure building in from the
west. Some lingering light precipitation over the Tennessee border zones should
end Monday evening and after that the forecast was kept dry. Any
development closer to the old stalled front off the East Coast should
stay to our east. Temperatures Tuesday will be 10-15 degrees cooler than
Monday.

&&

Long term /Tuesday night through Friday/...
as of 2 PM Saturday...the medium range forecast picks up at 00z on
Wednesday with broad and deep upper troffing centered just to our
west. Over the next 24 to 36 hours...the trough axis is expected to lift
NE of the forecast area with some degree of deamplification in its wake.
The long range models differ with respect to exactly how long the deeper troffing
lasts over the County warning forecast area with the European model (ecmwf) maintaining a considerably
steeper trough through the first half or so of Friday while the GFS and
Canadian increase heights faster and flatten the upper pattern
sooner. Either way...all of the guidance suggests that from late Friday
Onward...the region will remain under a relatively zonal pattern
with upper heights relatively constant.

At the surface...the latest 12z run of the GFS is trending more towards
an European model (ecmwf)-like scenario with respect to the development of a southern coastal low
on Wednesday. By 18z Wednesday...both models now have a well defined low centered
just off the Carolina coast with deep layer moisture spreading over
the majority of the County warning forecast area. The European model (ecmwf) still maintains a larger...deep
moisture plume over our area compared to the GFS...but not by much.
Both models agree that by early Thursday...things should dry out pretty
quickly as the low tracks up the East Coast. A weak secondary cold
front is expected to pass just to our north as the main low lifts NE
and some lingering precipitation is possible over the northern and western
zones...however it should not amount to much. The rest of the medium
range should be dry with high pressure regaining control over the
region. As highlighted in the previous discussion...minor amounts of
frozen precipitation are possible with the coastal low on Wednesday. Model
soundings still support snow and/or snow/sleet over the higher
elevations early Wednesday...however beyond the late morning it looks like
temperatures will be too warm to support anything but rain. In
addition...at present the bulk of the quantitative precipitation forecast is expected over the
eastern half of the County warning forecast area and not the higher terrain. Yet...the NC
mts could see some additional lingering northwest flow snow Wednesday night into
Thursday...but it likely wont be much. Otherwise...temperatures start out
below climatology and remain there through day 7.

&&

Aviation /20z Saturday through Thursday/...
at kclt...expect VFR conditions to continue at least through early
Sunday morning as cp high pressure centered over the eastern
Carolinas continues to shift east. As moist southwesterly flow aloft develops
in response to an approaching upper shortwave trough from the
SW...expect increasing middle-high level cloudiness this evening into
tonight. Light southeast winds this afternoon will become northeasterly by
around 03z sun and stay that way through the end of the taf period as
cold air damming sets up over the area. As a southern stream upper
wave approaches the region tonight...isentropic upglide will
strengthen and ceilings will be lowered to MVFR level by around 14z sun
with precipitation overspreading the region from the SW. Latest guidance
suggests that IFR ceilings in moderate rain will affect the terminal by
16z sun as deep moisture/better upper forcing move through.

Elsewhere...expect VFR conditions to persist at least through late
tonight/early Sunday morning as cp high pressure centered over the
eastern Carolinas continues to slide east. As moist southwesterly flow aloft
develops in response to an approaching upper shortwave trough from
the SW...expect increasing middle-high level cloudiness through tonight.
Light east-southeast/southeast winds this afternoon will become northeasterly by this
evening and stay that way through the end of the taf period as cold air
damming sets up over the area. The exception will be kavl where
southerly winds will persist through the period. As a southern
stream upper wave associated with a warm front approaches the
region...isentropic upglide will strengthen and MVFR clouds with
precipitation will overspread the region from the SW late tonight into
early Sun morning. Latest guidance suggests that IFR ceilings...moderate
rain and MVFR visibility will affect much of the region by late Sun
morning as deep moisture/better upper forcing move through.

Outlook...restrictive ceilings and precipitation will be widespread
Sunday as a warm front lifts through the region from the south. Brief
drying follows the front Mon-Tue...with precipitation possibly returning
Wednesday.

Confidence table...

19-01z 01-07z 07-13z 13-18z
kclt high 100% high 100% high 94% medium 78%
kgsp high 100% high 100% high 88% high 94%
kavl high 100% high 100% high 96% high 82%
khky high 100% high 100% high 93% high 83%
kgmu high 100% high 100% high 86% high 94%
kand high 100% high 100% high 88% high 90%

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: (must be lower case)

Www.Weather.Gov/gsp/aviation_tables

&&

Gsp watches/warnings/advisories...
Georgia...none.
NC...none.
SC...none.

&&

$$
Synopsis...jpt
near term...joh
short term...PM
long term...jpt
aviation...joh

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