Area Forecast Discussion
Issued by NWS Greer, SC

Home | Current Version | Previous Version | Text Only | Print | Product List | Glossary On
Versions: 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50
FXUS62 KGSP 211823

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
223 PM EDT Tue Mar 21 2017

A cold front will move through the area later tonight, increasing
the chances for widespread showers and thunderstorms. A few of the
thunderstorms could be severe. Cooler air will return behind the
front to end the work week before another frontal system approaches
the area next weekend.


As of 215 PM: A nearly stationary front remains across the northern
tier of NC and TN attm; it is difficult to pinpoint the front in
many areas, but it is clear VA and KY are north of the front. At any
rate, an amplified upper ridge will move into the Plains states this
evening, and in response high pressure will shift into the Great
Lakes region, driving the front south through our area tonight.

This afternoon, south of the front, warm and seasonably humid
conditions will be present. Mid-level lapse rates are rather strong,
and accordingly forecast CAPE values range from 750-1000 J/kg over
the NE CWFA to nearly 2000 J/kg over parts of SW NC and NE GA. Most
prog soundings continue to feature at least a small cap until around
21z, and this is reflected in reflectivity/QPF output from the CAMs,
which is minimal until then. 0-6km bulk shear values will be on the
order of 30-35 kt around peak heating, and we continue to expect
storms to organize to some degree. SPC has outlined an Enhanced
Risk centered to our west, but extending into a small portion
of our northeast GA territory; this is surrounded by a narrow
Slight Risk region. At least the Enhanced area will receive the
effects of a compact shortwave during the late afternoon hours,
and models are a bit more aggressive in pushing the front south
thru TN than they are over our area. Also of note, meso model runs
have tended to show a forward-propagating cluster developing in the
Tennessee Valley and tracking east this evening. This may present
a secondary threat to our western areas in addition to that posed
by the greater afternoon instability. Our entire CWFA continues
to be included in a Marginal Risk where the threats appear largely
the same, but the coverage will be lower.

Remnant stratiform precip associated with the decayed MCS is
expected to cross the area overnight, perhaps with a few isolated
rumbles of thunder but otherwise just light rainfall. With any
broad frontal boundary progged to push through in the early morning
hours, I am somewhat cautious about advertising a total "scouring"
of low-level moisture. However, essentially all guidance sources,
including MOS, depict the low clouds pushing out by daybreak, and
cooler and drier conditions lasting thru Wednesday. GFS still tries
to develop a little instability over NE GA, but currently this looks
to be an outlier. All precip mention has been dropped. Sunshine
and gradual downsloping should permit high temps near normal.


As of 2pm EDT Tuesday:  GFS and EC models in good agreement on
synoptic pattern transitioning Wednesday evening to Friday from
northwesterly flow with dry, cool low-level advection to a ridge
centered over the CWA by Friday afternoon.  Dewpoints begin to
improve Friday afternoon as surface high pressure center moves
eastward off the coast and low-level flow veers from easterly to
southeasterly, beginning the advection southeasterly moisture into the

With a dry boundary layer and ridging aloft, there will be little
chance for precipitation Wednesday night through Friday, with only
an outside chance for some orographic-related showers in far western
parts of the CWA.  Main issue will be chances for frost/freeze
Thursday and Friday mornings.  Current lows show freeze possible
with frost likely for western parts of North Carolina, with less
potential for Piedmont areas.  Drier air and few clouds northeast of GSP
leads to lower forecast lows to the northeast with CLT possibly
reaching low to mid 30s Thursday morning and mid to upper 30s Friday


As of 205 PM EDT Tuesday: the medium range fcst picks up at 00z on
Saturday with upper ridging centered over the SE CONUS and a
southern stream closed upper low moving across the southern plains.
Over the next 24 to 48 hrs, the low will approach the fcst area and
as it does, it will lift northward and up over the ridge. As it
does, it`s also expected to deamplify and open up as it rides up the
ridge. As we move into early next week, another H5 shortwave will
approach the CWFA from the west. It also appears to deamplify as it
approaches the CWFA.

At the sfc, broad high pressure will be centered well offshore with
moist SLY flow over the region. By early Sunday, a vigorous low will
lift NE towards the Great Lakes and push a moist warm front over the
CWFA. The cold front associated with the low, looks like it will
largely washout as it lays across our northern fringe early Monday.
As we move into Tuesday, another sfc low follows a similar track
towards the Great Lakes and brings another cold front to our
doorstep by the end of the period. As for the sensible wx, POPs
steadily ramp up from the west on Sat with likely POPs over half of
the CWFA by Sunday morning. Some drying is expected on Monday,
however POPs increase again on Tuesday with the approach of the next
frontal system. Temps start out above normal and remain above normal
thru the period.


At KCLT: A broad frontal zone will push through the area from the
north this evening. Prior to the front, sfc based instability is
expected to permit some TSRA to develop. Best chance thereof is a
bit later than peak heating as it will take time for cap to erode;
also activity may develop further west in the late afternoon and
advect in. TEMPO handles TS threat. While winds are expected to
shift to N behind fropa by 03z, it is possible the shift will
occur a bit earlier. Any TS are more likely than usual to produce
significant wind gusts. The remnant rain shield from earlier
convection may linger for some time overnight, and produce MVFR
cigs/vsby. Guidance is essentially unanimous in showing post-frontal
drying occurring in the early morning; with even MOS depicting this,
it looks unlikely the front will hang up in the area as sometimes
occurs with nocturnal fropas. Northerly and mildly gusty winds
will continue thru the end of the period.

Elsewhere: Main concerns this aftn and evening will be the effects
of TSRA which develop in the unstable atmosphere south of a cold
front. Meso models are consistent in showing isolated initiation of
TSRA near KGSP/KGMU around 21-22z, with isolated activity developing
elsewhere in the Piedmont between 22-00z. Isolated severe gusts and
hail are possible with these TSRA. It is also expected a convective
complex will develop over SE TN or NW GA and move east into SW NC
and the Upstate this evening. Said activity also may bring severe
gusts and may be followed by a period of MVFR cig/vsby. Winds will
shift northerly behind fropa overnight. It does look likely the
front will scour out remaining low level moisture by daybreak,
leaving settled conditions for the remainder of the period.

Outlook: Low level moisture starts to lift back north and deepen by
Thursday, keeping chances of showers and low cigs into the weekend.

Confidence Table...

            18-24Z        00-06Z        06-12Z        12-18Z
KCLT       High 100%     High 100%     High  99%     High 100%
KGSP       High 100%     High 100%     High  99%     High 100%
KAVL       High 100%     High  98%     High  96%     High 100%
KHKY       High 100%     High 100%     High 100%     High 100%
KGMU       High 100%     High 100%     High  99%     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:




NEAR TERM...Wimberley
AVIATION...Wimberley is the U.S. government's official web portal to all federal, state and local government web resources and services.