Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 271428
AFDGSP

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
1028 AM EDT SAT AUG 27 2016

.SYNOPSIS...
Surface high pressure will build over the region through the
weekend. Tropical low pressure will slowly strengthen off the
southern tip of Florida early next week and then possibly move into
the Gulf of Mexico. Forecast uncertainty is very high with this
system.

&&

.NEAR TERM /THROUGH TONIGHT/...
1430 UTC Update: Winds were updated with a blend of the lastest NAM
and ADJMAV. Minor changes were made to temperatures, dew points, and
QPF.

1330 UTC Update: Sky cover was reduced early on per satellite trends.

As of 645 AM: The current forecast appears on track this morning. I
will make minor tweaks to sky, vis, and temp to align with obs.

As of 330 AM: The center of 595 DM ridge will lift over the DELMARVA
region as a western Atlantic mid level low drifts slowly toward the
Carolina coast today. Broad high pressure will remain across the
Great Lakes region, ridging south across the forecast area. This
pattern should provide SKC to FEW sky cover through the morning
hours. Winds are forecast to rise to 6 to 8 kts from the ESE during
the daylight hours. The forecast for SHRA and TSRA will be a
challenge this afternoon and evening. It appears that a region of
mid level Q-Vector convergence will develop between the H5 ridge and
western Atlantic low, generally over the I-85 corridor. The llvl ESE
flow should transport an old sea breeze inland across the Midlands
to eastern Piedmont this morning. CAPE values west of the sea breeze should
generally range in the moderate range with low values of CIN. The
combination of the weak mid level forcing and weak llvl boundary
should trigger SCT SHRA/TSRA this afternoon, tracking from NE to SW.
Supported by the HRRR and 4km WRF, I will forecast the greatest
coverage of convection across the I-77 corridor this afternoon. It
is possible that a few damaging wet microbursts will occur within a
field of 1300-1500 J/kg DCAPE, we will highlight in the HWO. Using a
blend of warmer MOS, I will forecast highs near 90 within the mtn
valleys to low to mid 90s across the foothills and Piedmont.

Tonight, convection is expected to dissipate by late evening across
the western Carolinas and NE GA. Debris clouds and convective CU
should thin out by midnight. H85 winds will remain out of the ENE as
the sfc ridge becomes slightly deeper during the overnight hours.
Upslope flow and lingering moisture from wet ground could support cloud
cover along and east of the Blue Ridge overnight. Low temperatures
are forecast to range from the mid to upper 60s across the mtns to
low to mid 70s east.

&&

.SHORT TERM /SUNDAY THROUGH TUESDAY/...
As of 315 AM Saturday: Heights are expected to fall ever so slightly
across the Mid-Atlantic and Southeast during the short term, but
subtropical ridging will nevertheless dominate across much of the
southern 2/3 of the country. Short term guidance remains consistent
in establishing more of a NE low level flow on Sunday, as 1020+
surface high pressure builds into New England, resulting in a
sharpening of the inverted ridge axis down the Eastern Seaboard.
This results in lower theta-E air filtering into the area by Sunday
afternoon, such that there isn`t a whole lot of sbCAPE forecast
across the northeast quarter or so of the forecast area. There should
be enough instability to support widely scattered convection across
the western half of the area, but pops of only 10-20 percent will be
forecast across our eastern zones.

If anything, Monday`s forecast ingredients appear even less
supportive of deep convection, with the NAM and GFS indicating that
a substantial portion of the area will see sbCAPE values < 500 J/kg
Monday afternoon, especially across the heart of the forecast area.
Only slight chance pops will be carried across our far southern and
western areas Mon afternoon into evening. A bit of a potential fly
in the ointment concerns the fate of a weak tropical disturbance,
currently located near Bermuda, that short term guidance tends to
agree will push onto the Carolina coast in the Mon night/early Tue
time frame. The NAM is by far the most aggressive model in advecting
the plume of deep moisture into the western Carolinas, but even it
does not do a whole lot in terms of QPF response in our area. Opted
to just linger a small pop across the southern and eastern zones
Monday night into Tue as a nod to this possibility.

&&

.LONG TERM /TUESDAY NIGHT THROUGH FRIDAY/...
As of 335 AM: The tropics in the vicinity of eastern North America
will be pretty active during the medium range, but primarily in
terms of quantity instead of quality (i.e., intensity). The global
models depict multiple weak disturbances across the Gulf of
Mexico/Caribbean/western Atlantic during this time, so much so that
it`s difficult to determine where one ends and another begins. The
Canadian is the only model that wants to do something of note with
any of these disturbances, developing a significant tropical cyclone
across the eastern gulf by mid-week (it`s been fairly consistent in
doing this), but it`s difficult to put much stock in this
considering the "meh" response among other (more reliable) guidance
sources.

The general consensus is that heights over the eastern conus will
fall sufficiently by mid/late week such that a frontal boundary will
push across the Southeast, sweeping away these various disturbances
from the Conus before they would have a chance of having much of an
impact on sensible weather in our area. It therefore does not appear
the medium range will be particularly active, even in terms of
diurnal convective activity, as low level NE flow acts to keep
relatively low theta-E air over the forecast area through the period.
In fact, the models are not even that enthused with the potential
for convection associated with the passage of the front. Pops will
therefore remain at or a little below climo through the period.
Temps will generally remain a couple of degrees above normal until
the end of the week.

&&

.AVIATION /15Z SATURDAY THROUGH WEDNESDAY/...
At KCLT and elsewhere: The center of 595 DM ridge will lift over the
DELMARVA region as a western Atlantic mid level low drifts slowly
toward the Carolina coast today. Broad high pressure will remain
across the Great Lakes region, ridging south across the forecast
area. This pattern should provide SKC to FEW sky cover through the
morning hours. Winds are forecast to rise to 6 to 8 kts from the ESE
during the daylight hours. The forecast for SHRA and TSRA will be a
challenge this afternoon and evening. It appears that a region of
mid level Q-Vector convergence will develop between the H5 ridge and
low, generally over the I-85 corridor. The llvl ESE flow should
transport a weak sea breeze inland across the Midlands to eastern
Piedmont. CAPE values west of the sea breeze should generally range
in the moderate range with low values of CIN. The combination of the
weak mid level forcing and weak llvl boundary should trigger SCT
SHRA/TSRA this afternoon, tracking from NE to SW. I will highlight
the most likely period of convection with a PROB30 or TEMPO.
Otherwise, sky conditions should favor SCT045-050 CU.

Outlook: A deep ridge of high pressure will persist across the
region through early next week. The environment should support mid
to late afternoon thunderstorms each day, primarily over the mtns
and adjacent foothills. Weak steering flow should favor iso to sct
coverage, with storms dissipating within two hours following sunset.
Patchy fog will be possible any morning following a late afternoon
to early evening shower or thunderstorm.

Confidence Table...

            14-20Z        20-02Z        02-08Z        08-12Z
KCLT       High 100%     High 100%     High 100%     High  83%
KGSP       Med   77%     High 100%     High 100%     Med   68%
KAVL       High 100%     High  88%     Med   78%     Med   72%
KHKY       High 100%     High 100%     High 100%     Med   62%
KGMU       High 100%     High 100%     High 100%     Med   68%
KAND       High 100%     High 100%     High 100%     Low   54%

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...
At 930 AM Satruday: Mixing heights were lowered based on the latest
model soundings, and smoke management paramaters were updated
accordingly.

&&

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

&&

$$
SYNOPSIS...HG
NEAR TERM...JAT/NED
SHORT TERM...JDL
LONG TERM...JDL
AVIATION...NED
FIRE WEATHER...JAT



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