Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 201743

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
143 PM EDT Tue Jun 20 2017

A remnant frontal boundary lifts back north this evening to focus
yet another round of showers and storms across the area before drier
high pressure builds in for Wednesday.  Rainfall chances increase
yet again Thursday and Friday as tropical moisture spreads across
the region.  A cold front approaches the area Sunday and moves
through the area early next week with drier weather to follow.


As of 140 PM EDT Tuesday:  Updated discussion for 18z taf issuance.
Tweaked t/td and pop trends over the next few hours for this update.
Otherwise no sig changes were needed/made.  Full fcst discussion
to follow within the hour.

Previous Discussion:  Looks like a problematic fcst over the
next 24 hours with a NW-to-SE gradient of moisture and forcing
persistent across the wrn Carolinas and northeast GA. The plume
of deep moisture should remain strung out across the Piedmont
basically along and S/E of I-85. Drier air will settle in across
the area W of the mtns. Meanwhile, vorticity will advect nwd
today to intersect the deep moisture pooled along the old front,
which will act to help redevelop precip up from the S and into the
Piedmont. The end result should be a fairly high probability of the likely range...along our SE fringe...with perhaps
enough additional rainfall over already wet ground to raise a
flood concern. For the time being, prefer to not entertain a Flood
Watch because of uncertainty as to where the tight gradient near
the front will ultimately settle during the day. We will have to
continue to monitor this potential. Some of this precip may spread
nwd into the drier air filtering over the mtns to result in light
rain/sprinkles across the area N/W of I-85. That area gets a token
chance/slight chance. The clouds/precip over the SE fringe will
keep temps cooler there, with slightly warmer high temps to the
N. The precip chances will only gradually diminish during Tuesday
night, with temps remaining mild.


As of 300 AM EDT Tuesday: A northern stream broad longwave trough
will deamplify and shift east on Wednesday, allowing deep-layer flow
to veer to more due westerly and advect some drier air over the
CWFA. The surface front looks washed out in the guidance as well. So
Wednesday may be relatively dry, although still enough CAPE for
isolated to scattered showers and tstms. Temps will be somewhat
tricky on Wednesday, as well. Assuming we lose the low clouds and
precip remains scattered, temps may bounce back to a couple
categories over Tuesday`s highs. Wednesday night lows will be near
normal under partly cloudy skies.

Thursday thru Thursday night, a tropical system is progged to make
landfall somewhere along the LA or eastern TX coast. As this system
approaches the Gulf Coast, low-level flow is expected to ramp up out
of the south atop the CWFA and PWATs surge to over 2" by Thursday
aftn. The GFS is bullish on near 2.5" PWATs and heavy rain (4-6" in
12 hours) across the SW-facing slopes of the southern Appalachians
Thursday night. The other operational models also show
tropical-enhanced precip in our western zones, but not as heavy as
the GFS. This will certainly bear watching, as the position of the
tropical circuation at that time may put our area within a
deformation zone, allowing for slow-moving or training convection.
Temps will be slightly below normal for highs and above normal for
lows within a humid air mass.


As of 330 AM EDT Tuesday: the 00z operational models agree on the
subtropical ridge reasserting itself across the southeast states
Friday and Saturday. This bring slightly lower-PWAT air into the
area, but results in warmer temps and typical summertime diurnal
convection coverage and trends. The remnants of the expected
landfalling tropical system looks to gradually get absorbed into the
next northern stream wave and frontal system over the Mid-South. The
front and associated tropical moisture will arrive around Saturday
night or Sunday (models diverge on exact timing). So above climo
PoPs will likely return with the front. By Monday, models have
trended toward pushing the front thru the area and allowing a mild
continental air mass to make some inroads. If that does verify
(especially the 00z ECMWF solution), early next week looks dry with
seasonable temps lower-than-usual humidity for this time of year.


At KCLT and the SC Sites:  A mixture of low VFR, MVFR, and
eventually IFR through this period.  A stalled frontal axis just
south of the I85 corridor will continue to be a focus for iso/sct
shra and possibly tsra this afternoon/evening.  With that said,
confidence in tsra at the terminals is fairly low, thus will only
include vcsh at the SC sites while a tempo for shra is favored at
KCLT through the afternoon hours amidst low VFR/MVFR stratocu.
Will continue with a lowering cig trend into the evening and
overnight hours with IFR becoming more likely as shra potential
slips slightly.  IFR cigs amidst MVFR visb will remain in the fcst
through daybreak before both improve by one category to round out
the taf cycle.  All precip timing in tafs was based upon a CAM
guidance consensus.  Winds will remain light sly through the period.

Elsewhere:  Lower chances for restrictions at KAVL/KHKY given
the proximity of the stalled frontal axis being well to the south.
That said, ample BL moisture will allow for at least low end chances
for shra later this evening and overnight amidst development of
MVFR and possible IFR cigs/visb.  Conditions will improve at both
sites into the post dawn hours on Wednesday.  Winds will remain
generally sly through the period.

Outlook: Could have MVFR/IFR/LIFR restrictions across the region
persisting early Wednesday morning due to leftover low moisture.
Drier conditions may finally return during the latter part of
the week if the cold front can push south of the area, however
confidence remains low as possible remnants of a tropical system
may impact the area late in the week.

Confidence Table...

            17-23Z        23-05Z        05-11Z        11-12Z
KCLT       High 100%     Low   53%     Med   72%     Med   78%
KGSP       High 100%     Low   58%     Med   66%     High  85%
KAVL       High 100%     High 100%     Med   72%     High  93%
KHKY       High 100%     High 100%     Med   78%     High  93%
KGMU       High 100%     Low   58%     Med   72%     High  85%
KAND       High 100%     Med   64%     Med   78%     High  93%

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:




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