Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 150530

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
1230 AM EST Sun Jan 15 2017

The cool airmass currently in place across the region will erode
on Sunday as the associated surface front lifts north, allowing
well-above normal high temperatures to return. Another cool high
pressure over New England on Monday will force the front south
into Georgia again. The front retreats north again by mid-week as a
cold front approaches from the west. The cold front may stall near
our region late next week allowing rounds of showers to persist
into Saturday.


1230 AM EST Update...made some minor tweaks to hr/ly temps and
td/s. Pops look good for now across the wrn NC mtns...but may adj
downward for late morning/afternoon based on latest hires
guidance and op model differences. Contemplating a DFA across the
srn Upstate and NE GA with a few sites maintaining a quarter mile
or less.

As of 1045 PM EST Saturday:  Well it looks as if the wedge has
nearly eroded across the vast majority of the fcst area, with
nearly all observations favoring southerly flow.  This was likely
aided by westerly H85 downsloping, particularly across the NC
Fthills/Piedmont.  With that, current observations are indicative
of the moist nature of the BL as low stratus and fog have already
begun to develop/spread across portions of Northeast GA and the
Western Upstate, while most Piedmont locales are still only beneath
mid levels ceilings.  Still expecting this to change as guidance
continues to insist on expansion of low stratus/fog northward along
the I85 corridor across GSP metro into CLT by early/mid morning.
Therefore opted not to make any changes to the already pessimistic
visb fcst in place, but wouldn`t be suprised if areas of dense fog
developed warranting even lower visbs and perhaps a localized DFA
at some point overnight.  Lastly, a weak upper impulse riding along
the old stationary frontal axis continues promote shra across the
TN valley, eastward into portions of Northwest NC, therefore kept
pops elevated at likely levels through 2-3AM for these locales.
Otherwise, tweaked t/td trends and left the remainder of the fcst
as it was.

Previous Discussion: The upper ridge now over the Southeast USA
will weaken slightly along the east coast tonight and Sunday,
while slight amplification occurs over the lower MS River Valley.

At the surface, ridging along the Eastern Seaboard will break down
overnight, however cloud cover associated with a stalled surface
front is expected to persist over the area. On Sunday another
ridge begins building down the Eastern Seaboard. Precipitation
chances are only expected to increase slightly, from the the NC
Mountains eastward, as low level winds increase, and resultant
lift increases.  Temperatures will remain above normal, with a
slightly greater diurnal range as the ridge weakens, and cool
northeasterly flow veers to the south.


As of 210 PM EST Saturday: The period begins Sunday night with
excellent model agreement regarding a lifting 500 mb closed low
center over NW TX and a downstream ridge in place over the
southeast. Under the ridge, an 850 mb baroclinic zone will remain
stretched along the northern tier of the forecast area, with
associated weak upglide developing Sunday night into Monday morning.
The upglide will further weaken Monday afternoon as the 850 mb
barocline zone retreats northward, but light southwesterly upslope
forcing may transition the better lingering precipitation chances to
southern/southeastern mountain upslope areas through late day.

Meanwhile, the low pressure system will get gradually absorbed into
a developing longwave trough over the plains Monday. Deeper layer
southwesterly flow will set up over the southern Appalachians Monday
night through Tuesday as the central CONUS trough translates east
toward the lower OH valley. Deeper moisture pooling ahead of the
associated cold front will begin approaching from the west across
western TN/KY through late Tuesday - increasing mountain shower
chances. Anticipate breezy to windy conditions developing over the
high terrain by late Tuesday as southwesterly 850 mb winds increase.

Plenty of clouds are expected throughout the period given the
upglide early, upslope flow late, and continued abundant high clouds
aloft. This will limit insolation, but temperatures will continue
above climo through the period, and surge well above climo in the
solidly warm sector air by Tuesday.


As of 235 PM Saturday, Guidance remains in disagreement through the
medium range keeping confidence low. The GFS remains faster with the
Wednesday cold front and associated upper low. The ECMWF remains
slower. They do agree on a ridge building in on Thursday behind this
system. However, on Friday the ECMWF takes an upper low through the
bottom of the ridge and across our area, while the GFS moves the
ridge axis east and moves some short wave energy over the area in
the SW flow.

Therefore, both models agree on precip over the area Tuesday night
with the GFS drying things out quickly Wednesday and remaining dry
Thursday. The ECMWF has precip through the day Wednesday. It also
stalls the front over the area with a wave of low pressure forming
to the west and moving along the front bringing more precip on
Thursday. Model blend and national guidance suggests moving toward
the wetter solution, so have done that. However, have all PoP but
Tuesday night in the chance range given the uncertainty. Lows
Tuesday night around 20 degrees above normal fall to around 15 above
normal Wednesday night. Highs will be 10 to 15 degrees above normal
both days with Thursday being slightly cooler.

Both models keep precip over the area Friday and Saturday, but
differ in the overall pattern. The GFS brings a warm front into the
area Friday and a cold or occluded front on Saturday. The ECMWF
brings a warm front in, but then brings a wave of low pressure along
the front across the area Saturday. Again, a model blend and
national guidance keeps low end chance over the area each day. Lows
Thursday night will be just under 20 degrees above normal rising to
just above 20 degrees above normal for Friday night. Highs both days
remain nearly steady 10 to 15 degrees above normal.


At KCLT and Elsewhere: Lowering cigs and vsbys at all TAF sites this
morning. KAND and KAVL are already socked in at VLIFR vsby and shud
remain so thru daybreak outside of TEMPO improvements. With moist
bndry stalled across wrn NC...expect KGSP, KGMU, and KCLT to drop
into or remain in VLIFR cigs with vsbys likely dropping into the IFR
range this morning. A passing h5 s/w could instigate -shra
Prob30s are included at KAVL, KHKY late this morning...and this
evening at KCLT. The hires models and a couple op models are
uncertain on the qpf this scenario could change by the
next TAF set or earlier. Otherwise...expect gradual improvement
across all sites...except KAVL and the afternoon then
deteriorating conds return arnd midnight as a moist wedge of high
pressure builds in throughout the overnight period.

Outlook: Cool high pressure ridges down the East Coast Sunday
evening, bringing yet another round of precipitation and
restrictions to start the work week.

Confidence Table...

            05-11Z        11-17Z        17-23Z        23-00Z
KCLT       Med   62%     High 100%     High 100%     Med   66%
KGSP       High 100%     High  83%     Low   56%     Med   78%
KAVL       High  87%     Med   66%     High 100%     High  83%
KHKY       Med   75%     High 100%     High 100%     Med   72%
KGMU       High 100%     High  89%     Med   62%     High  83%
KAND       High 100%     High 100%     High 100%     Med   66%

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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