Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 251106

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
706 AM EDT Thu May 25 2017

An upper level low pressure system will cross the region today, with
showers persisting over parts of the area as a result. Following
the departure of this system, drier air will return tonight and
linger into early Saturday. Moisture and unstable conditions will
then build over the Southeast over the weekend, ahead of a slow
moving cold front.


As of 700 AM EDT Thursday: Have made a round of minor pop
adjustments to account for showers that have defied hires guidance
and managed to push into the Upstate; also seeing some convection
fire in the NW NC Piedmont ahead of the lee trough. Temperatures
look good for now.

Otherwise, much quieter today in the wake of yesterday`s cold front.
The parent upper low will push up the Ohio Valley today, with a
shortwave extension over the Tennessee Valley progged to damp and be
re-absorbed by the primary low. This secondary low is responsible
for the light precip across N AL/GA, and just about all guidance has
this precip dissipating as that southern extension gets eaten up
over the next couple of hours. While it`s not out of the question
that we could see some precip across NE GA/Upstate SC over the next
few hours, pretty much all the hires guidance follow the pattern of
the global models and really diminish the precip into nothingness.
Of greater note will be the main low that rides up the Central
Appalachians toward the eastern Great Lakes today. Lee troughing
induced just east of the Appalachians will keep a firm hold on the
area today, with a brief but notable increase in the pressure
gradient through the day, leading to some breezy winds across the
area. Should see a redevelopment of showers across the northern tier
as the DPVA moves across, and some low-end sbCAPE is progged across
portions of the northwest Piedmont. While deep layer shear isn`t
impressive (30-40kt), that combined with the increased lapse rates
from the CAA aloft plus the surface instability may be enough for
some scattered thunderstorms as well. Gusty winds and small hail
would be the main concerns. SPC`s Day1 outlook includes a sliver of
Davie and Rowan in the Marginal Risk, so cannot rule out a brief
strong to low-end severe storm as well, but chances seem pretty
minimal across western NC (a little better farther east).

Should be another very nice day with the aforementioned CAA, with
high temperatures today a good 10 degrees or so below seasonal
normals. Another cool night tonight, about 5 degrees below normal,
with clearing skies and pops shrinking to only the extreme northern
mountains before ending by the end of the period.


As of 230 AM Thu: A low-amplitude ridge will remain over our region
Friday and Saturday. Initially westerly low-level flow will back
to southwesterly as a warm front lifts north thru the Tenn Valley
and western Carolinas. This will bring back seasonably warm temps
Friday, that trend further upward for Saturday.

Strong subsidence and relatively low dewpoints will preclude
convection on Friday, but by Saturday conditions will be more
favorable. Thus slight chance to chance PoPs will be advertised
northwest of I-85; a subsidence inversion most likely will keep
a lid on things to the southeast. In fact model SBCAPEs are
quite impressive aside from the capping. Furthermore, the height
gradient aloft will be enough to produce some modest upper winds
and deep shear. So Saturday will be a day where we may struggle
to initiate an updraft, but if that does occur the cells quickly
could become troublesome. SREF probs of DCAPE > 1000 J/kg are
in the "chance" range, with the operational GFS generally in the
500-1000 range. Multicell storms producing robust cold pools and
wind damage are not out of the question.

Small PoPs will be retained in the NW half of the CWFA Saturday
night for the possibility that convective debris may linger and/or
a remnant MCV move in. High clouds will increase ahead of the next
frontal system.


As of 300 AM Thu: Another deep upper low will settle into the Great
Lakes during the medium range period. As heights fall to our west and
northwest, low pressure will track through the Ohio Valley.  Our area
will be in the warm sector of this system Sunday before the cold
front drives southeastward Monday. Lapse rates aloft suggest some
convection may develop ahead of the front, though the degree of
low-level capping is in question, given that the front is displacing
a subtropical ridge. As noted for Saturday, relatively robust CAPE,
DCAPE and shear appear likely to be present Sunday, so where the cap
is unable to hinder convection, severe storms easily could develop.
GFS suggests deeper moisture Monday will reduce instability, but it
is too early to say with certainty that the threat will be lower
then. Max temps will remain a few degrees above normal Sunday,
falling back to near normal Monday.

While it looks like the front will have passed by early Tuesday, the
24/12z EC develops a frontal wave that could allow precip to linger
over the area; this is plausible given the upper pattern. A secondary
cold front will push in late Tuesday or Wednesday, reinforcing
slightly below normal temps, but bringing only mediocre forcing for
precip. Dry conditions return following that front.


At KCLT and elsewhere: Area of MVFR/IFR cigs still moving across the
area, but should see improvement with lifting/scattering through the
morning. Upper low moving across will kick off SHRA through the
morning and early afternoon, mainly for the NC TAF sites, and have
added TEMPO SHRA during most probable times. SW winds will pick up
this afternoon 10-15kt with gusts 20-15kt, subsiding somewhat
overnight with VFR conditions.

Outlook: The drying trend that starts this evening will continue
through Saturday. Moisture will return ahead of the next front
starting Sunday.

Confidence Table...

            11-17Z        17-23Z        23-05Z        05-06Z
KCLT       High  89%     High 100%     High 100%     High 100%
KGSP       High  89%     High 100%     High 100%     High 100%
KAVL       High 100%     High 100%     High 100%     High 100%
KHKY       High 100%     High 100%     High 100%     High 100%
KGMU       High  89%     High 100%     High 100%     High 100%
KAND       High 100%     High 100%     High 100%     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:




SHORT TERM...Wimberley
LONG TERM...Wimberley
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