Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 221138
AFDGSP

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
638 AM EST Mon Jan 22 2018

.SYNOPSIS...
High pressure will continue to weaken as a moist cold front advances
toward the area and crosses east on Tuesday. In the fronts wake, dry
high pressure will overspread the region and persist through the
work week. Another moist cold front will likely affect the forecast
area over the weekend.

&&

.NEAR TERM /THROUGH TUESDAY/...
As of 630 AM EST Monday: Grids look good for the sunrise update,
with a continued increase in high cirrus through the morning hours.
Afternoon highs still look good for now. Some guidance pointing to
some delay in onset of pops and have made some minor adjustments
there.

Otherwise, deep cutoff upper low over the Plains will continue to
push east today as ridging builds in aloft over the Appalachians.
Occluded surface low is pushing the attendant front toward the MS
Valley at AFD time. Southerly flow will increase today ahead of the
front and around the western periphery of the Atlantic surface high
with low level WAA increasing, leading to another day of
temperatures above seasonal normals, though with increasing cloud
cover it should not be quite as warm as it was yesterday. With
moisture advection into the region, the addition of orographic and
weak insentropic upglide will allow for some showery activity to
form in the favored upslope areas of the SW mountains by midday.
Expect this activity to really pick up this afternoon and evening,
spreading across the rest of the mountains and into the Piedmont
overnight. The fast-moving front will only allow for a quick shot of
rain, and for now storm totals barely approach 1" in the favored
upslope areas.

The strongly dynamic system will lift ENE today toward the Great
Lakes, with an impressive >130kt upper jet rounding the base of the
trough and pushing into the mountains later today and tonight. 850-
925mb winds pick up as well, with a 40+kt 925mb jet out of the south
and over 50kt at 850mb. As the front approaches and the gradient
increases, mountain wave activity may pick up and this combined with
the pressure gradient and upper jet will allow for some hefty winds
across the higher elevations of the mountains. Have opted to issue a
high-elevation Wind Advisory (>3500ft).

Other concern is potential for convection along and ahead of the
front. By its nature as a nocturnal frontal passage, instability
would be low anyway, but the strong deep-layer shear (70-90kt 0-6km)
is a concern. Both NAM and GFS have a tongue of 100-200J/kg sbCAPE
lifting N ahead of the front, spreading across the Blue Ridge and
Piedmonts late tonight. SHERB values approach 1 across the area as
the front pushes through. Hodographs are more impressively curved
this afternoon and evening and begin to straighten out a bit (as
surface winds shift to SW) when the instability arrives, but still
shear is quite high. We remain only in General Thunder for SPC`s
Day1 outlook and generally feel that is reasonable, but cells will
have to be monitored closely tonight. Luckily CAMs do not seem to be
all that excited about discrete convection, so for now the biggest
concern would be from isolated wind gusts that are able to descend
in the convective cells that develop.

Precip moves out quickly on Tuesday and CAA lags a bit, allowing
highs to climb again above seasonal normals across the Piedmont.
Will have to continue to reevaluate the wind threat late Tuesday and
into the short term period for the northern mountains. Northwest
flow snow may pick up slightly late in the period, but of the
operational models, GFS has the most moisture which isn`t all that
impressive.

&&

.SHORT TERM /TUESDAY NIGHT THROUGH THURSDAY/...
As of 145 AM EST Monday...Upper level forcing continues to weaken
early Wed as a broad trof lifts across New Eng. This will allow a
small scale ridge to build in ahead of a developing and potent h5
s/w progged to traverse the area by Wed afternoon. The ridge will
develop a subs inversion over the higher terrain and limit the depth
of upstream nw flow moisture. There looks to be an ample feed of
Glakes moisture however...so expect relatively weak mech-lift snow
to continue across the nrn NC mtns overnight mixing with rain before
the s/w axis crosses by 21z or so. No sigfnt additional accums are
expected with perhaps arnd and inch in the favored higher peaks such
as Beech Mtn. There wont be much impact with the upper wave
elsewhere in the FA as a srn stream sfc hipres quickly builds in
from the west. This will keep winds aligned nw/ly Wed and most of
Thu. Max temps will likely reach normal levels or few degrees abv
normal Wed/Thu outside of the nrn zones where modest llvl CAA is
maximized Wed.

&&

.LONG TERM /THURSDAY NIGHT THROUGH SUNDAY/...
As of 230 AM Monday...Not too many changes were made to the going
ext fcst. Friday will remain dry as a strongly amplified ridge
develops over the Midwest and propagates toward the Atl coast thru
Sat morning. This will merge a srn stream sfc high with a cP high
early Fri as it ridges down the east coast. Winds will remain se/ly
and then begin to veer e/ly which will help keep max temps right
arnd normal or a bit abv normal even with very good insol.
Increasing moisture adv from the SW will begin at h85 Fri night and
by Sat morning there shud be enuf lift over the srn BR to create sct
-shra. The upper dynamics begin to increase during this time as well
as a 150+ kt upper jet core develops across the OH Valley into TN.
This will leave the area in a very good div lift while Atl moisture
adv enhances the GOM moisture feed. So...have PoPs quickly ramping
up to likely Sat evening thru the overnight. With the enhanced
lift...there could be some localized hydro issues mainly across the
srn BR.

Things get more interesting early Sun as sfc convg and deep layered
shear increase. Bulk shear will rise to arnd 60 kts...yet due to the
current timing and cloud cover...the amount muCAPE looks to be very
low for much of an organized svr threat. However...this will be a
situation to watch and even low-end CAPE events can produce isol line
segment TORs and damaging winds. A lot can change before Sun...but
right now the confidence is moderate in the overall synoptic
pattern given the latest ECMWF is now similar to the other op
models. Temps will drop below freezing behind the front and -snsh
are probable across the mtns with low-end accums expected as
moisture wanes quickly. Max temps Sat shud be right arnd normal and
likely a couple cats abv normal Sun as sw/ly flow continues within a
developing warm sector.

&&

.AVIATION /12Z MONDAY THROUGH FRIDAY/...
At KCLT and elsewhere: Southerly flow will continue to increase
through the period as moisture continues to move in from the west
ahead of a strong system. High cloud will slowly lower through the
morning and early afternoon, but widespread MVFR expected to move in
from S to N generally after 18z, lowering to IFR and possibly LIFR
as -SHRA moves in. Continued trend of PROB30 -TSRA in the predawn
hours Tuesday along and ahead of the front where instability may be
enough to get isolated thunderstorms going. Lgt/vrb winds favoring
south will continue this morning, increasing this afternoon. Low-end
gusts to 20kt expected just ahead of the front, generally after
midnight, and LLWS at KAVL and possibly KHKY would be an added
concern this evening into the early overnight period. Improvement to
VFR expected for most sites (except KHKY) by the end of the period
with winds swinging toward SW with continued low-end gusts.

Outlook: Behind the front, dry high pressure will dominate the area
through the remainder of the week. Another system will move in over
the weekend, bringing another round of -SHRA and restrictions.

Confidence Table...

            11-17Z        17-23Z        23-05Z        05-06Z
KCLT       High 100%     High 100%     High  80%     High  87%
KGSP       High 100%     High 100%     Med   78%     High 100%
KAVL       High 100%     High 100%     High 100%     High  81%
KHKY       High 100%     High 100%     Med   75%     High  94%
KGMU       High 100%     High 100%     High  95%     High 100%
KAND       High 100%     High 100%     High  95%     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:

www.weather.gov/gsp/aviation

&&

.GSP WATCHES/WARNINGS/ADVISORIES...
GA...None.
NC...Wind Advisory from 10 AM this morning to 4 AM EST Tuesday for
     NCZ033-048>052-058-059.
SC...None.

&&

$$
SYNOPSIS...SBK
NEAR TERM...TDP
SHORT TERM...SBK
LONG TERM...SBK
AVIATION...TDP



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