Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 220756

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

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.


As of 230 AM EST Monday: 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


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 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.


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 s/w 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.


At KCLT and elsewhere: Southerly flow will continue to increase
through the period as moisture moves in from the west ahead of a
strong system. High cloud bases will lower through the early morning
hours, and cannot rule out some brief fog or MVFR clouds around
daybreak for portions of the area, but widespread MVFR expected to
move in from S to N generally after 18z, lowering to IFR and
possibly LIFR as -SHRA moves in. Winds will continue to increase
late in the period with low-end gusts to 20kt expected, and LLWS at
KAVL and possibly KHKY would be an added concern. No improvement
until after the front passes just beyond the end of the period.

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...

            08-14Z        14-20Z        20-02Z        02-06Z
KCLT       High 100%     High 100%     High  87%     High  86%
KGSP       High 100%     High 100%     High  85%     High  91%
KAVL       High 100%     High 100%     High  89%     High  89%
KHKY       High 100%     High 100%     High  90%     High  95%
KGMU       High 100%     High 100%     High  86%     High  89%
KAND       High 100%     High  95%     High  83%     High  86%

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:


NC...Wind Advisory from 10 AM this morning to 4 AM EST Tuesday for


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