Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 170234
AFDGSP

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
934 PM EST Fri Feb 16 2018

.SYNOPSIS...
A cold from will bring much cooler air in to start the weekend. A
wave of low pressure will develop along the front over the weekend,
keeping rain in the forecast Saturday and Saturday night. Another
system will arrive by the middle of next week, bringing chances for
thunderstorms.

&&

.NEAR TERM /THROUGH SATURDAY/...
930 PM Update...The fcst is in good shape. The sfc bndry has made it
to the srn Upstate and has slowed. Some -dz is persisting across the
CLT region which will work south over the next few hrs. No need to
adj min temps as recent hr/ly trends are well within the fcst curve
most locales.

645 PM Update...The n/ly wind shift associated with a cold front has
pushed into the Upstate and will continue south and west thru the
evening. No good chance of precip outside of lingering spotty -ra/dz
and along the TN/NC border over the next couple hrs as hipres
continues to build in from the north. Made some downward temp tweaks
across the NC mtn valleys where llvl CAA has been the strongest. May
have to adj min temps down in these areas by the next update.

As of 200 pm Friday: The leading edge of a frontal zone and
accompanying area of weakening/mainly light showers has pushed into
extreme western-through-northern NC this afternoon. This will
continue to move steadily SE with the weakening/dissipating precip
trend expected to continue, as it moves increasingly away from the
better upper-level support, and succumbs to westerly downslope flow
downstream of the Blue Ridge. The highest pops this afternoon/early
evening will continue to be advertised across the mtns, particularly
the northern mtns. However, guidance remains consistent in depicting
resurgence of low level convergence and a narrow shower band along
the boundary (as well as some very weak instability) across the NC
Piedmont late this afternoon/early this evening, so will allow solid
chance pops to continue roughly along/east of I-77 during that time.
Precip chances taper off to less than slight chance by mid/late
evening.

Winds will shift to the NW behind the front through this evening.
Other than a brief surge of gustiness during the initial cold
advection stage, winds will gradually weaken while turning to the NE
tonight, as surface high oozes into the area from the north. Temps
will steadily fall north of the front such that min temps tomorrow
morning are expected to be roughly 15 degrees cooler than this
morning`s readings, but still 10-15 degrees above climo.

As transient 1025-ish surface high pressure moves into the northern
Mid-Atlantic early Saturday, hybrid cold air damming will become
established across the forecast area. Meanwhile, the next in a
series of short wave troughs passing through the fast/ low-amplitude
flow regime north of our area will allow for weak surface
development along the baroclinic zone across the TN Valley, with the
surface wave expected to lift west of the southern Appalachians on
Saturday. For our area, these features will translate to increasing
SSW flow/isentropic lift developing above the cool pool, with
chances for mainly light rain/perhaps drizzle increasing throughout
the morning and maximizing during the afternoon. QPF is light, with
significant precip amounts expected to remain confined to areas west
of the Appalachians. Max temps will be below normal in the CAD
regime, with highs likely occurring around sunrise, then remaining
steady or slowly falling throughout the day. Considering the hybrid
nature of the CAD, near-surface advection of cool/dry air may be
sufficient to create a low-end freezing rain/drizzle concern across
the northern NC  Blue Ridge (mainly Avery County) tomorrow. This
hasn`t been included in the forecast, and even if it does occur,
temps hovering right around freezing combined with the recent very
warm weather would yield minimal travel concerns.

&&

.SHORT TERM /SATURDAY NIGHT THROUGH MONDAY/...
As of 230 PM EST Friday: The short term period tips off on Saturday
night amidst subtropical H5 ridging while a broad H5 trof consumes
much of the central/eastern CONUS, with a potent shortwave sliding
east through the OH valley into the central Apps.  At the surface,
high pressure off the New England coast will dominate across the
eastern seaboard, albeit in retreat as a cold front moves east
out of the TN valley region.  Thus, pops will remain elevated
through the evening hours with chance levels featured regionwide,
concentrated mostly along the high terrain.  The front is progged
to move quickly through fcst area into early Sunday morning
yielding drier conditions as high pressure builds from the west.
Therefore by daybreak Sunday all pops are removed.  Overall Sunday
looks like a rather nice day as temperatures actually warm post
fropa given the erosion of aforementioned high pressure wedge,
thus am expecting highs on Sunday to top out a few degrees above
normal amidst mostly sunny skies.

This new region of high pressure will continue it`s eastward track
across the Apps, setting up over the Delmarva region by Sunday
night.  With that, northeast flow will prevail overnight as a warm
front moves northward out of the I20 corridor.  Moisture advection
associated with this front will stream in from the south toward
sunrise Monday leading to increasing pops given improved upglide
potential along the southwest periphery of the high, as well as
upsloping along the southern escarpment.  From that point forward
the upward trend for pops will continue as the warm front passes
through the region, with pops topping out in the high chance to low
end likely range by periods end Monday evening.  Temperatures on
Monday will warm slightly from that of Sunday, thus generally 5-8
degrees above normal levels.  Given relatively stable profiles,
and lack of substantially deep moisture, not expecting any high
impact weather through the short term period.

&&

.LONG TERM /MONDAY NIGHT THROUGH FRIDAY/...
As of 2:30 PM EST Friday: A large sprawling area of high pressure
aloft remains situated off the southeast coast. Other than some very
slight movement of this feature, it looks to remain about anchored
in place, leaving our forecast area (FA) in a moist southwesterly
flow aloft.

In the beginning part of this forecast cycle Monday night and
Tuesday, we have kept POPS in the higher chance category over
northeast Georgia, western South Carolina mountains, and the western
upslope facing escarpment in North Carolina. This follows the
pattern of a moist H85 upslope component which the models have
latched onto. Elsewhere we kept small chance POPS with moisture
abundant and ripples of energy moving through the mean flow.

The middle and end part of the new work week is where we see a
divergence between the GFS and European. Judging by the flow aloft,
it is hard to see a big push from an approaching cold front, which
the GFS is advertising, while its upper level energy lifts into the
western Great Lakes area. We have sided with the European and
National Model Guide for frontal location, which allows the front to
make slow headway through the southwest flow.

Therefore from Wednesday through Thursday we ramp POPS into the high
chance, with likely values in our western FA. However, further
refinement will likely be needed to pin this down. Since CAPE values
increase between 500 and 1000 J/KG, especially southern parts of our
FA, we have thunder mentioned both Wednesday and Thursday afternoon.

For the end of the work week, considerable uncertainty will exist
depending on timing and ultimate location of the approaching front.
We have used a blend at that point to narrow the forecast.

Overall temperatures look above normal right through this period,
with the caveat if the GFS were correct late in the week, we could
have considerably cooler temperatures. But currently we are not
going that route.

&&

.AVIATION /03Z SATURDAY THROUGH WEDNESDAY/...
At KCLT and elsewhere: Gusty conds will continue to dampen over the
next few hrs except at KAVL where nw/ly channeling will persist thru
midnight or so. CIGs will continue to drop to MVFR in an increasingly
moist atmos west to east overnight. Generally deteriorating TAFs thru
the period all sites as a wedge of high pressure builds in from the
northeast and is reinforced with upper level s/w precip moving in
from the southwest btw 14z-16z. With steady state conds developing
and moisture lowering...expect a drop to IFR CIGs along with precip
enhanced IFR VSBY beginning shortly after noon and persisting thru
the TAF period all terminals.

Outlook: Gradual clearing is expected Sat night into Sunday, with
dry/VFR conditions possible on Sunday. Moisture returns early next
week, and the weather will remain periodically unsettled, with
occasional restrictions likely through much of next week.

Confidence Table...

            02-08Z        08-14Z        14-20Z        20-00Z
KCLT       High 100%     High 100%     High  89%     High 100%
KGSP       High 100%     High  98%     High  86%     Med   71%
KAVL       High  95%     High  89%     High  86%     High  85%
KHKY       High 100%     High 100%     High  98%     High  93%
KGMU       High 100%     High  97%     High  84%     Med   75%
KAND       High 100%     High  91%     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...None.
SC...None.

&&

$$
SYNOPSIS...Carroll
NEAR TERM...JDL/SBK
SHORT TERM...CDG
LONG TERM...TS
AVIATION...SBK


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