Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 172330
AFDGSP

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
730 PM EDT Fri Mar 17 2017

.SYNOPSIS...
A cold front will approach the Western Carolinas later this
evening and move through the area Saturday morning. Dry high
pressure will overspread the region in the front`s wake and
persist into early next week. Another cold front is expected to
approach the forecast area from the north by the middle of next
week.

&&

.NEAR TERM /THROUGH SATURDAY/...
As of 705 pm EDT:  In line with the latest mesoscale model guidance
and radar trends, PoPs have been increased to likely out across the
NC western piedmont for the evening hours. Still anticipate the more
solid band of showers to accompany the main cold frontal passage and
cross the region from the northwest primarily 06Z to 12Z for the
western half, and possibly lingering until late morning in the
southeast sections. The strong forcing along the frontal zone, along
with rather steep mid-level lapse rates around 7 C/km and some weak
MUCAPE, suggest the potential for elevated thunderstorms -
especially across the western and northern zones. A slight to low
chance thunder mention continues in this area. Otherwise, warm
advection, light surface winds, and increasing cloud cover indicate
that tonight will be much warmer than recent mornings, and around 5
degrees above climatology.

The front will sag south of the forecast area throughout the day
Saturday, bringing an end to precip chances in all areas by early
afternoon. While much drier air will return in the wake of the front
tomorrow afternoon, temperature advection will be largely
negligible, and in fact gusty W/NW downslope winds should allow most
areas to warm to about 5 degrees above normal, with a return to the
70s appearing likely across portions of the Piedmont.

&&

.SHORT TERM /SATURDAY NIGHT THROUGH MONDAY/...
As of 230 PM EDT Friday: it looks like the rest of the weekend and
the start of next week are shaping up to be relatively quiet. An
upper low is expected to move down across the nrn Mid-Atlantic
Region Saturday night and early Sunday. This will drive a cold
front, already on the eastern edge of the fcst area at the start of
the period, quickly to the south/east, with strong cold advection
taking hold by Saturday evening. The flow around the upper low
should give the NC mtns a brief shot of NNW flow moisture, which
should provide for a chance of precip Saturday night and early
Sunday near the TN border, mainly in the form of snow showers. No
siginificant accum is expected. Meanwhile, east of the mtns,
we should have low temps down into the frost range early Sunday,
mainly over the NC foothills and NW Piedmont.

The low level moisture should pull out and dry up early Sunday,
leaving us with a fair day as high pressure builds in from the
west. High temps will be just below normal with lots of sun. A
positively-tilted upper ridge will carry this sfc high across
the region Sunday night and Monday. Expect more frostable temps
early Monday over the NC foothills/wrn Piedmont. The high moves
away on Monday and the next system approaches from the west in the
afternoon. Think some of the model guidance is overly-optimistic
that our next system will arrive by the end of the period...00Z
Tuesday...so the fcst was kept dry. Temps will be two categories
warmer on Monday afternoon.

&&

.LONG TERM /MONDAY NIGHT THROUGH FRIDAY/...
As of 245 PM Friday: A broad northwesterly flow aloft will prevail
across the forecast area, at least through the start of this cycle
in the forecast. It appears a pre-frontal trof will be situated in
our forecast area (FA) while the primary cold front will still
be well west of our region - Monday evening.

The cold front will make headway into our region late Monday night,
and then will sag slowly to along the NC/SC state line by Tuesday
evening. This slower response will allow moisture to pool, and H5
heights to fall with steepening lapse rates. We will ramp up the
POPS late Monday night in the NC mountains and over the entire
region Tuesday. SB CAPE values continue to climb with reach model
run therefore have expanded the thunder area Tuesday. We will need
to pay attention to any faster moving convection developing in the
mean NW flow aloft Tuesday. The primary forcing still appears to be
tied into low level surface convergence, and energy diving SE toward
our area in the northwesterly flow aloft.

A bit of a wrinkle in the forecast Tuesday night into Wednesday,
where the GFS is slower to push the front south, with the European
Model a little faster. We took a blend at this point in time, which
then meant hanging onto POPS Tuesday night, then ramping down
Wednesday.

There should be a lull as ridging develops Thursday, then moves off
to the east Thursday night and beyond. This sets up the start of a
potential longer period of isentropic lift as the Gulf of Mexico
opens for business.

The GFS continues to be very aggressive, while the European model is
slower and not as amplified late next week into the start of next
weekend. This far out sensible weather would indicate starting small
POPS, and later forecasts will figure whether we need to ramp them
up, ala a GFS scenario, or hold back to see how the European model
responds with later cycles of data.

Temperatures will be starting in a warming mode, ahead of the cold
front. Some areas in upstate SC may approach 80 Tuesday. Once we
have the front south of the FA, temperatures will respond with a
downward trend, as a transitory CAD regime settles in.

&&

.AVIATION /00Z SATURDAY THROUGH WEDNESDAY/...
At KCLT: A lead band of showers has managed to survive the trip
across the mountains and will likely yield VCSH through the evening
hours as the band passes mainly north of the airfield. Still
anticipate the much greater shower coverage to occur with the
frontal passage primarily in the 07Z to 13Z time frame. Steep lapse
rates aloft could support isold thunder but confidence remains too
low to mention at this time. A LLWS group has been added to the TAF
given the potential for flow at 2000 feet to reach 40 to 45 kt from
the SW. Although surface winds remain SW as well, an embedded 200 ft
layer could exist with LLWS conditions met near the surface. The MOS
has trended a bit more pessimistic but will keep cigs at 010 or
higher for now given all the dry near surface air that must be
overcome. SW winds will become westerly and gusty through Saturday
as cigs lift and scatter. Lee troughing could well keep winds south
of west through late day as gusts push into the 20 to 25 kt range.

Elsewhere: Rain shower chances will be best from KAVL to KHKY this
evening with the pre-frontal showers sliding east along and north of
I-40. The better coverage throughout the region will occur with the
main fropa overnight, with SHRA lingering longest at the Upstate
TAFs into Saturday morning before clearing. Isold thunder is
possible throughout but with confidence too low to mention. LLWS is
possible as well as the SW jet ramps up overnight, but SW surface
winds should keep any shear within 200 ft layers sub-LLWS. Will push
all cigs down into the lower end of the MVFR range overnight, but
some brief IFR is starting to look a bit more likely just as the
showers are ending and the boundary layer is most saturated. Winds
will come up rapidly behind the front, toggling quickly NW from KAVL
to KHKY, but with WSW flow possibly holding on at Upstate sites
through late afternoon.

Outlook: A drier airmass will return behind the front by Saturday
night, continuing through at least Monday. Another cold front may
bring scattered showers and perhaps a few thunderstorms next Tuesday.

Confidence Table...

            23-05Z        05-11Z        11-17Z        17-18Z
KCLT       High 100%     High 100%     High  80%     High 100%
KGSP       High 100%     High 100%     High  80%     High 100%
KAVL       High 100%     High 100%     Med   68%     High 100%
KHKY       High 100%     High 100%     High  80%     High 100%
KGMU       High 100%     High 100%     Med   63%     High 100%
KAND       High 100%     High 100%     Low   58%     High 100%

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

&&

.FIRE WEATHER...
Moisture increases more quickly tonight ahead of a cold front which
should bring numerous to widespread showers to the area, especially
late tonight into early Saturday. The Piedmont and foothills are
expected to see rainfall amounts of .10-.25 inch, while the mtns
should see .25-.75 inch, with the highest totals expected near the
TN border.

The moist air will be very short-lived, as more dry air pushes into
the area behind the front. Meanwhile, there`s not really any cool
air behind the front, and in fact temperatures are expected to be
much warmer tomorrow. As a result, it appears likely that many areas
across the mtn valleys, Piedmont, and foothills will see RH dip to
around 25 percent, if not lower tomorrow afternoon. W/NW winds will
also become gusty tomorrow afternoon, but it does not appear that
there will be any overlap of critical RH and critical wind. Even so,
would not rule out the need for a Fire Danger Statement in some
areas tomorrow afternoon, but it seems prudent to wait and see how
the precip situation unfolds tonight before coordinating such
products.

&&

.GSP WATCHES/WARNINGS/ADVISORIES...
GA...None.
NC...None.
SC...None.

&&

$$
SYNOPSIS...JPT
NEAR TERM...HG/JDL
SHORT TERM...PM
LONG TERM...TS
AVIATION...HG/JDL
FIRE WEATHER...JDL/HG



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