168
FXUS62 KGSP 230840
AFDGSP

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
340 AM EST Sat Feb 23 2019

.SYNOPSIS...
The wet pattern will continue today into tonight before a robust
cold front finally moves through the region late tonight through
Sunday morning. The early part of next week should be dominated by
dry high pressure. A cold front may move through the region from the
northwest on Wednesday, but moisture with this front remains highly
uncertain. Better moisture could arrive toward next weekend.

&&

.NEAR TERM /THROUGH TONIGHT/...
As of 300 AM: High pressure centered over New England will continue
to ridge down the East Coast today, maintaining cold-air damming. A
robust upper ridge is centered over the eastern Caribbean. In
between these features, a broad warm frontal zone extends eastward
out of developing low pressure in the Plains. Widespread rainfall
is occurring across Tennessee as a result, with lighter and more
spotty rainfall extending across the Carolinas where it sits atop
the CAD wedge. Given WSW to W flow around the upper ridge, some
of the Tennessee rain will advect into NW NC and our northern
zones over the course of the morning, and categorical PoPs are
fcst there. Further south there will be frequent opportunities
for brief showers and drizzle, warranting high chance to likely
PoPs. The continued northward shift of the warm front suggests
chances lessen for a time later today, though we will retain
at least weak upglide over the wedge so we can`t let the precip
mention drop entirely. Temps will continue to reflect the CAD,
remaining more or less steady in the 40s over most of the CWFA;
the un-wedged valleys in and around the Smokies look to experience
mid 50s by aftn.

Expect little change in temps and sky cover through most of
tonight, for that matter. The Southeast will be in the warm sector
of the aforementioned low as it moves toward the Great Lakes,
but the ensuing warm advection will only maintain the CAD in our
neck of the woods. 850mb flow will strengthen thru the period,
suggesting not only increasing upslope enhancement to PoPs, but
strong sfc winds on the higher ridgetops. Though gusts do not
look especially strong given WAA and limited mixing, we feel a
Wind Advisory is warranted for elevations above 3500 feet. Precip
rates will increase overnight; as the sfc low continues moving east,
the LLJ and moisture plume will cross the area in the early morning
hours. Locally heavy rain can be expected, especially in the usual
spots along south-facing terrain. With a number of river gages
already above bankfull stage or in minor flood across the area, and
a large swath of our territory north of I-85 having received 3" or
more of rain in the past week--more like 6" along the Escarpment--we
have issued a new Flood Watch effective now, and valid until noon
Sunday. This will give time for the cold front to scour out all
the precipitation, and some time for rivers to crest after it ends.

A small amount of instability will accompany the front as it
moves into the area early Sunday, and the strength of the gradient
allows 0-3km bulk shear to peak at 40-60 kt. Nearly saturated prog
profiles reveal a very "skinny" CAPE profile, and the environment
is characterized by SHERB values less than 0.7 prior to the end
of the period at 12z. Nonetheless we plan to carefully monitor
any convective elements that develop.

&&

.SHORT TERM /SUNDAY THROUGH MONDAY NIGHT/...
As of 230 AM EST Saturday: Weak convection associated with a cold
frontal passage should move quickly east of the mountains Sunday
morning, departing from the Blue Ridge mountains around 12Z and
passing east of I-77 by 15Z. Very little surface-based instability
is expected with the fropa east of the mountains, especially with
the wedge boundary slow to retreat northward, but any sbCAPE
uncovered with the morning fropa would affect the southern piedmont
during that narrow morning window. Otherwise, anticipate rapid post
fropa drying through the day on Sunday, with temperatures in the
upper 60s in many places east of the mountains, touching 70 in a few
spots. Hydro problems are likely to linger along the slower response
main stem rivers and reservoirs through Sunday.

Winds will be the other major concern as cold advection develops
behind the departing front and the surface pressure gradient
tightens ahead of approaching high pressure. Even the more
conservative GFS features a 50+ kt 850 mb jet by Sunday
afternoon/evening across the northern Blue Ridge mountains. Given
the mixing potential in the cold advection, the prudent course
appears to be a northern mountain High Wind Watch for Sunday and
Sunday night, with a Wind Advisory for the central and southern
mountain through late Sunday. Any northwest flow moisture in the
cold air should be shallow and rather short-lived across the western
mountains, so no snow showers will be advertised.

Upper flow should become rather zonal over the east on Monday, with
dry surface high pressure building over to the north of our area.
Maximum temperatures will be a couple of degrees below climo in the
NC mountains, near climo across the rest of western NC, and a degree
or two above climo in SC and northeast GA on Monday afternoon.

&&

.LONG TERM /TUESDAY THROUGH FRIDAY/...
As of 245 AM EST Saturday: There is decent agreement on dry
conditions lasting through at least Tuesday, with some semblance of
a surface high to the north of the area. Forecast confidence takes a
nose dive from mid to late week, however, as the GFS/ECM camps
differ markedly on the strength and permanence of surface high
pressure to the north, and on the redevelopment of shortwave energy
and troughiness in the flow aloft over the Mid and Deep South. The
ECMWF features a relatively dry period of WNW flow over the region
Tuesday through Thursday night, but with modest heights falls over
the plains and mid MS River valley on Friday. Meanwhile, the GFS has
zonal flow early, but with more southern stream shortwave activity
over the deep south by Wednesday through Thursday, and a more
vigorous system on Friday. All told, will stay very close to WPC and
an ensemble blend for the Wednesday through Friday period. This will
mean very low PoPs, possibly peaking on Friday, with mainly liquid
ptypes throughout. The diurnal range on temperatures should get a
bit smaller each day as clouds and PoPs slowly increase.

&&

.AVIATION /09Z SATURDAY THROUGH WEDNESDAY/...
At KCLT and elsewhere: Cold-air damming will continue to drive the
pattern thru tonight, meaning IFR to LIFR cigs will persist. The
only possibility of higher cigs appears to be if SHRA cause
brief erosion of the lowest cloud layer via the downward motion
they will induce. An area of SHRA is expected to blossom near
the NC/SC border and gradually advect northward as warm front
atop the CAD airmass effectively shifts in that direction. Thus,
some TEMPOs feature higher cigs, though as has happened at times
last night, MVFR or IFR vsby can be expected with the precip as
well. Off-and-on SHRA and/or DZ will continue into tonight, with
cigs lowering a bit after sunset as wedge becomes more shallow. A
scouring cold front will enter the area early Sunday morning but
the only effect mentionable in this TAF set is a shift to SW winds
at KCLT just before dawn, and developing LLWS concerns; cigs will
linger there until after 12z Sun.

Outlook: Conditions will become much drier yet gusty on Sunday as
a strong cold front crosses the area. VFR conds will return Sun
and remain through the middle of next week.

Confidence Table...

            08-14Z        14-20Z        20-02Z        02-06Z
KCLT       Med   75%     Med   72%     Med   67%     High  94%
KGSP       Low   58%     Med   78%     Med   72%     High 100%
KAVL       High  89%     High  92%     Med   61%     High  85%
KHKY       Med   70%     Med   76%     Med   61%     High 100%
KGMU       Med   71%     Med   76%     Med   72%     High 100%
KAND       Med   77%     Med   78%     Med   64%     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:

www.weather.gov/gsp/aviation

&&

.GSP WATCHES/WARNINGS/ADVISORIES...
GA...Flood Watch through Sunday morning for GAZ010-017-018-026-028.
NC...Flood Watch through Sunday morning for NCZ033-035>037-048>053-
     056>059-062>065-068>072-501>510.
     High Wind Watch from Sunday morning through Monday morning for
     NCZ033-049-050-501-503-505.
     Wind Advisory from 7 PM this evening to 7 PM EST Sunday for
     NCZ048-051-052-058-059-062>064.
     Wind Advisory from 7 AM to 7 PM EST Sunday for NCZ053-065.
     Wind Advisory from 7 PM this evening to 7 AM EST Sunday for
     NCZ033-049-050.
SC...Flood Watch through Sunday morning for SCZ001>010.

&&

$$
SYNOPSIS...HG
NEAR TERM...Wimberley
SHORT TERM...HG
LONG TERM...HG
AVIATION...Wimberley

NWS GSP Office Area Forecast Discussion