Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 220637

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
237 AM EDT Sun Oct 22 2017

High pressure over the region will retreat offshore today as a low
pressure system and associated cold front approaches from the
Mississippi Valley. The cold front, and associated abundant
moisture, will sweep east through the region Monday through Monday
night, with drier and cooler conditions then developing through mid-
week. Another cold front will approach from the west toward next


As of 230 AM EDT Sunday:  The synoptic pattern continues to amplify
and become more dynamic as the upper ridge and associated surface
anticyclone migrate east along the Atl coastline, while the nearly
full latitude upper trof slides east across the Plains and a surface
cold front moves into the mid/upper MS river valley.  Closer to
home, light/calm nely flow prevails amidst the weakening ridge
with only high cirrus streaming aloft.  With that, radiational
cooling trends have been stunted to some degree as temperatures
across the piedmont remain in the upper 50s to lower 60s, while
low/mid 50s prevail across the high terrain.  Dewpoint depressions
on the order of 1-3 degrees could present some issues relative to
patchy fog toward daybreak, however confidence is low given the
aforementioned sky cover.

Moving on, guidance favors dry and partly sunny conditions
through the first half of the day, ahead of sly veered flow and
thus improving moisture advection.  Am expecting sky coverage to
gradually increase into the afternoon/evening, with bases lowering
as well. Cam/Op guidance suggests some light shra moving into the
region after sunset, therefore pops will slowly increase from the
south for areas west of I26.  From that point forward pops will
ramp up sharply focused heavily on the central/southern mtns,
with the highest pops/qpf fcst along the upslope regions of the
southern escarpment thanks to modest/moist sly H85 flow.  Thus, by
periods end Monday morning the fcst will feature likely pops across
much of the region west of I77, with categorical pops highlighted
over the mtns.  Pwats will increase by way of the moist advective
pattern as fcst profiles favor nearly 2", therefore think some of
the deeper convection could be quite efficient yielding rainfall
totals upward of a half inch overnight.  Fortunately these same
profiles exhibit poor low/mid lvl lapse rates ahead of sunrise,
therefore think thunder chances are low through the end of the
period, albeit sharply increasing thereafter.  Temperatures on
Sunday will remain above normal by around 4-5 degrees, with lows
Monday morning around 10 degrees above normal thanks to cloudy


As of 230 AM EDT Sunday: A southern stream cutoff low will begin
opening up across the mid and lower MS River Valley early Monday and
then gradually phase back into the northern stream as it lifts over
TN through the day. The associated surface frontal system, with
waves of low pressure riding northward along the cold front, will
approach from the west through Monday as a very weak in situ cold
air damming layer east of the southern Appalachians retreats
northward and precipitable water values and 60s dewpoints surge
quickly northward across our region. The deepest moisture associated
with the frontal system, upper wave, and jetlet dynamics will likely
cross the region quickly Monday evening. A 40 to 50 kt 850 mb jet
will traverse the area, most likely from 21Z to 00Z. Instability
continues to trend a bit higher, and this seems reasonable as
entrenched cold air damming is not expected given the unfavorable
offshore parent high position and the scouring low level south to
southeast flow. Clouds and rain should keep temps in the 60s to near
70 for much of the area Monday, but still expect some 500 to 1000
J/kg of best parcel CAPE to be possible by 21Z Monday, especially
along and southast of I-85 in the region of highest dewpoints. The
latest SPC Day 2 Marginal Risk appears well-placed, and damaging
wind gusts with isolated tornadoes will be added to the HWO for the
southeast half.

Surging precipitable water along with an upslope enhancement should
focus the heavy rainfall threat in the southern mountains Monday
through Monday evening. Isolated 4 to 5 inch rainfall totals will be
possible there through Monday night, but dry antecedent conditions
and the rapid movement of the system should preclude much of any
flood issues. Also, expect marginal advisory criteria gusts along
the highest ridge tops of the southern mountains Monday afternoon,
but not widespread enough for a wind hazard product.

A mid level dry lot will start to wrap into the forecast area from
the southwest after 00Z Tuesday, and all of the deep forcing should
lift northeast of the area by 06Z Tuesday. PoPs will thus ramp down
quickly from the southwest overnight.

A closed 500 mb low center developing over the Great Lakes on
Tuesday will produce a high amplitude trough over the central CONUS,
with the trough axis migrating over the Appalachian mountain chain
by 12Z Wednesday. Despite the steepening lapse rates aloft with the
amplifying/approaching trough, precipitation chances will be quite
limited east of the mountains Tuesday/Tuesday night due to westerly
downslope flow. The main impact of the reinforcing cold air arriving
in the mountains Tuesday night could be isolated/scattered high
mountain snow showers mixing in at times.


As of 230 AM EDT Sunday: The amplified upstream trough axis will
cross the region on Wednesday, with a period of deep layer NW flow
on the backside of the trough developing Wednesday night into
Thursday. Temperatures will flirt with freezing Thursday morning
across much of the mountains and another round of brief upslope
moisture could wring out isolated rain or snow showers along the
ridge tops and near the TN border. Will continue the freeze outlook
in the HWO for Thursday morning.

A shallow ridge will cross the area quickly on Thursday night before
another deep trough develops back to the west over the plains states
on Friday. A warm front will lift northward over the southeast
Thursday night through Friday. The system to the west will deepen
into a full latitude trough over the MS River Valley into next
weekend. The deeper moisture along the associated frontal system may
remain west of the mountains through Saturday, but improving
southerly flow will allow shower chances to gradually ramp up next


At KCLT and elsewhere:  VFR conditions expected to prevail through
the first 3/4 of the taf cycle as high pressure gradually retreats
thanks to an approaching upper trof and associated surface
cold front.  With that, expecting llv flow, which is currently
light/nely, to increase and veer ely/sely.  Moisture advection
will prevail amidst this newly veered flow allowing for gradually
increasing low VFR cloudiness through the day, all beneath increased
mid/high cloud cover.  Into the overnight hours of Monday morning,
guidance favors developing shra across the western tier of the cwfa
to affect all sites aside for KCLT/KHKY, thus such is included
via shra prob30s with MVFR/IFR cigs.  Also favored MVFR cigs at
KCLT/KHKY to round out the period.

Outlook:  A cold front will move through the region on Monday
increasing chances for restrictions via low cigs and reduced visbs,
as well as showers and possible thunderstorms.  Drier and cooler
conditions return for the rest of the work week.

Confidence Table...

            06-12Z        12-18Z        18-24Z        00-06Z
KCLT       High 100%     High 100%     High 100%     High  96%
KGSP       High 100%     High 100%     High 100%     High  94%
KAVL       High  94%     High  94%     High 100%     High  97%
KHKY       High  97%     High  94%     High 100%     High  97%
KGMU       High 100%     High 100%     High 100%     High  94%
KAND       High  97%     High  94%     High 100%     High  98%

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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