Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 181818

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
218 PM EDT Sun Mar 18 2018

Brief drying is expected today before moisture returns tonight and
lingers through midweek, as a strong low pressure system and a
secondary low pressure system bring unsettled weather and colder air
to the area. High pressure towards the end of the week will allow
for dry conditions to return.


As of 145 PM: Current forecast is basically on track. No changes.
Near term discussion will be updated within the next hour.

Otherwise, water vapor imagery shows the departing upper wave moving
off the eastern seaboard, while surface observations have the cold
front slowly slipping southward. Shortwave ridging will build over
the southeast today into tonight, while a strong closed low forms
over the southern plains. At the surface, the passing cold front
will settle south of the region today before returning north as
a warm front tonight.  Fairly good insolation is expected in all
but the northeast parts of the area, where better 850 mb moisture
will linger in the northerly flow. Plenty of 70s temperatures are
expected this afternoon in the southern half.

850 mb flow will gradually return to light southerly late today and
provide some returning upslope moisture and associated cloudiness
along the eastern Blue Ridge by evening. Better low to mid-level
moisture ahead of the plains system will return late tonight along
an activating warm front, and continued weak low-level upglide
should permit light rain to start breaking out atop region during
the early morning hours Monday.


As of 230 AM EDT Sunday: The short term forecast period begins
Monday morning as focus remains on the approaching potent storm
system we`ve been watching for the past few days. As the sfc low
moves eastward out of the central Plains, latest guidance continues
to place it`s extending warm front south of the FA Monday morning,
which is progged to propagate northward throughout the day. While
areas south/southwest of the FA will be more susceptible to severe
weather given the better instability/shear/dynamic support, as the
warm front moves closer to the FA into the afternoon hours, the
potential for thunderstorms (isolated severe) will increase as well.
Before the warm front provides it`s assistance in initiating
afternoon/evening convection, expect rainfall to already be
occurring across the FA Monday morning with the assistance of
isentropic lift.

Guidance suggests the sfc low will quickly move overhead late Monday
night as it`s associated cold front pushes through, allowing for
precip to linger into Tuesday morning. Behind this system, the
suggestion of a Miller B setup continues as another sfc low is
progged to develop across GA/SC/NC Tuesday afternoon ahead of a
secondary cold front just west of the FA. Do expect showers to
continue during the day with a gradual decrease in PoPs across the
Upstate and portions of the NW Piedmont into the evening hours as
the secondary front pushes through. With colder temperatures
advecting in behind the front, especially down the spine of the
Appalachians, anticipate a rain/snow mix along the TN border
beginning Tuesday night, carrying over into Wednesday morning. High
temperatures through the forecast period are expected to remain
below normal.


As of 345 AM EDT Sunday: Unsettled weather will continue well into
the beginning of the medium range forecast period which begins
Wednesday morning, as a sfc low continues to slowly make its way off
the Carolina coast. An upper longwave trough will settle in across
the eastern US for Wednesday and Thursday, as embedded waves of
energy travel within the trough axis, just south of the FA as
generally agreed upon across the models. With available moisture and
supportive dynamics, expect showers to be occurring Wednesday
morning, with a rain/snow mix in place along the TN border. Do
expect precipitation will gradually taper off by Wednesday evening
across the Upstate first, then the NW Piedmont, as the coastal low
slowly moves northeastward up along the eastern seaboard.

As Canadian sfc high begins to slowly build in behind the exiting
system, already colder temperatures in place along the TN border
will allow for a rain/snow mix to persist throughout the day, as a
transition to all snow is expected Wednesday night. Guidance begins
to disagree in regards to how long precipitation will last overnight
Wednesday, but have kept with the general trend of through just
before daybreak on Thursday.

Anticipate conditions to become dry and quiet for Thursday and
Friday as high pressure continues to build in, with an upper ridge
expanding across much of the US. Towards the end of the week,
another system is progged to move out of the central Plains into the
OH Valley, bringing another round of precipitation. However,
disagreement between the models continues both temporally and
spatially with this system. Temperatures throughout the forecast
period will remain below normal.


At KCLT and Elsewhere: VFR, probably thru the end of operations
this evening. Wind this afternoon should generally remain NE,
but a shift to ESE is anticipated in the late afternoon/early
evening. Flight conditions should deteriorate steadily after
roughly midnight. The culprit will be moisture return and the
beginning of upslope/isentropic upglide this evening that will
force the development of low clouds and then light precip near the
Blue Ridge Escarpment at first around 04Z, then expanding quickly
outward from there while also advecting up from the south through
the pre-dawn hours. An MVFR restriction was introduced with a PROB30
group at all TAF sites because of uncertainty regarding the start
of restrictions. Confidence goes up considerably around daybreak as
increasing moist upglide strongly suggests that ceiling will further
drop down into the IFR category, thus we introduce the IFR generally
in the 10Z to 14Z time frame. Note that KCLT was optimistically kept
at the low end of MVFR thru 17Z, but it is almost equally likely
they will also be IFR by 12Z or so. Once the IFR is established,
it will hold thru the end of the period with a light generally
easterly wind. Ceiling may drop into the LIFR range at all sites in
the afternoon, but this detail was omitted for brevity at this time.

Outlook: A complex low pressure system will move thru Monday
night and early Tuesday with continuing flight restrictions and
the possibility of a thunderstorm. As the low passes on Tuesday,
the situation may get more complicated. Colder air may wrap south
into the region behind the system Tuesday night and early Wednesday
and some mixed precipitation cannot be ruled out. Conditions will
finally improve on Wednesday afternoon.

Confidence Table...

            18-24Z        00-06Z        06-12Z        12-18Z
KCLT       High 100%     High 100%     High  92%     High  85%
KGSP       High 100%     High 100%     High  88%     High  92%
KAVL       High 100%     High  97%     High  80%     High  94%
KHKY       High 100%     High 100%     High  96%     High  85%
KGMU       High 100%     High 100%     High  85%     High  89%
KAND       High 100%     High 100%     High  94%     High  82%

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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