Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 200611

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
211 AM EDT Mon Aug 20 2018

An upper level trough will persist over the Eastern CONUS through
Tuesday with abundant gulf moisture persisting over the region. A
fairly robust reinforcing upper trough and its associated cold front
will bring additional widespread showers and thunderstorms late
Tuesday into early Wednesday. In the front`s wake, drier high pressure
will spread over the area for the latter half of the week.


As of 135 AM EDT...Over the past couple of hours, a cluster of
thunderstorms has fired up around the CLT area and are now moving
north and east of there. Besides that, conditions are mostly quiet
across the CWFA. We could see some additional shower/tstorm
development into the early morning, but coverage should be minimal.
Temps are on track to remain nearly steady thru the morning hours.

Otherwise, an upper low dropping down the MO Valley today and will
continue to push east through the near term period as the surface
boundary remains in place. Upstream diffluence aloft will spread
over the southern Appalachians as additional moisture overspreads
the area. Low level moisture advection will continue so expect to
see another round of low clouds in the morning. The stronger upper
support (jet dynamics and dPVA) will hold off into the short term
period, but still with the summertime airmass in place plus the
addition of synoptic lift just from the upper diffluence, should
see more widespread convection Monday afternoon. Near-term guidance
is indicating a much more tropical profile so with the lack of mid
level dry air, concern for wet microbursts is lowered, though low
level lapse rates will remain steep in the shallow boundary layer.
Heavy rain concerns begin to creep up tomorrow as PWs increase to
2" or greater across the area.


As of 215 PM EDT Sunday: Short wave ridging over the area at the
start of the period moves east as an upper low over the Mid MS
Valley opens up and moves east. The trough associated with this
system crosses the area on Tuesday. Broad upper trough then develops
across the eastern CONUS on Wednesday as additional short wave
energy moves in and helps carve out the trough.

At the sfc, a cold front associated with the afore mentioned upper
system moves into the area from the west Tuesday. The combination
of upper and low level forcing, along with increasing moisture and
instability, will lead to high end scattered to numerous coverage of
convection during the day. Looks like there will be enough
instability and shear for some storms to become severe. SPC already
has most of the CWFA in a marginal risk. With PW values near 2
inches, favorable positioning of the upper jet, and storm motions
generally parallel to the slow moving front, heavy rainfall and
isolated flooding are possible. WPC already has portions of the CWFA
in a marginal risk for excessive rainfall.

Despite the broad upper trough, deep moisture moves east on
Wednesday with the departing cold front. Low level flow becomes
northwesterly with some drying taking place. That said, there will
be some lingering moisture and weak instability, so scattered
convective coverage expected over the mountains with isolated
coverage at best elsewhere.

Highs will be near normal both days with lows a couple of degrees
above normal each morning.


As of 145 PM EDT Sunday: Starting Wednesday evening. Taking the
average of the medium range models gives the axis of the 500mb
trough north to south across the Appalachians to the Carolinas for
Wednesday night. The surface front at that time will be near to or
just off the Carolina coast with dry high pressure building over our
region. The center of this high pressure should be over Illinois
early Thursday and over Ohio or PA early Friday. Light wind flow
will adjust as the high moves north of our area. Precipitable water
values 0.5 to 1 inch Thursday and Friday with dry continental
northerly flow then a gradual rise in PWats over the weekend as the
high moves offshore and wind flow from southerly directions begin.
With the departure of the 500mb trough from the East Coast, the
Texas ridge is forecast to build east and amplify to some extent
pushing the jet stream into Canada. Temperatures are forecast to be
3 to 6 degrees below normal through the period but perhaps coolest
on Friday.


At KCLT and elsewhere: A cluster of thunderstorms fired up around
KCLT over the past couple of hours, however the terminal itself was
only briefly impacted. The cluster of storms is currently moving north
and east of the airport and should not impact the terminal. As pockets
of instability linger, we cannot rule out a few more lingering showers
and/or thunderstorms thru the overnight. Otherwise, the latest guidance
continues to show low stratus and some patchy fog developing before
daybreak, mainly over the NC Foothills, Piedmont and mtn valleys. IFR
to MVFR cigs should gradually scatter and lift to VFR by late morning,
with another round of sct to numerous showers and thunderstorms expected
this afternoon/evening. Winds will favor the SW thru the period, with
the exception light and vrb thru the overnight and into the morning.

Outlook: Expect good coverage of diurnal showers and thunderstorms thru
early Wednesday. After which, drier high pressure is expected to build in
behind a passing cold front. Until then, restrictions are likely mainly
during the morning as low clouds form, and then under any afternoon and
evening showers and thunderstorms.

Confidence Table...

            06-12Z        12-18Z        18-24Z        00-06Z
KCLT       High  93%     High  84%     High 100%     High 100%
KGSP       High  88%     High  88%     High 100%     High 100%
KAVL       Med   64%     Med   79%     High 100%     High  88%
KHKY       Low   56%     Med   79%     High 100%     High 100%
KGMU       High  93%     High  89%     High 100%     High 100%
KAND       High  97%     High  84%     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:




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