Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 211752

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

A cold front will approach the forecast area today into Wednesday,
increasing chances for rain and thunderstorms. A few of the
thunderstorms could be strong this evening. Cooler air will return
behind the front to end the work week before another frontal system
approaches the area next weekend.


As of 1200 PM: A nearly stationary front remains across NC and TN
attm; it is difficult to pick out in many spots, 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 are large; generally 1000-1500
J/kg over the NC Piedmont and 1500-2000 J/kg elsewhere, with the
upper Savannah Valley and vicinity seeing some pockets up to 2500
J/kg. Most prog soundings continue to feature at least a small cap
until 20-22z, 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.


As of 240 AM EDT Tuesday: Guidance in good agreement on the overall
pattern, but some significant detail differences show up in the
forecast sensible weather. Some weak short waves move through the NW
flow over the area Wednesday and Wednesday night. Additional weak
short waves move over or near the area Thursday and Thursday night
even as heights rise as a ridge axis moves west toward the area
reaching a line from Lake Michigan to Nashville to Mobile by the end
of the period.

At the surface, high pressure begins to nose into the area from the
NE in a cold air damming pattern on Wednesday behind the departing
cold front. The high strengthens and the ridging remains in place
through the period even as the center of the high slides into the
Atlantic. The GFS has plenty of low level moisture with widespread
cloudiness returning north over the developing cold dome but little
in the way of isentropic lift and resulting precip. The NAM and SREF
have much more in the way isentropic lift and precip. Given the
differences, have gone with a guidance blend which has diminishing
precip Wednesday but brings chance to slight chance PoP back into
the area Wednesday night before diminishing again on Thursday.
Should this precip develop and move into the NC Mountains Wednesday
night temps would be cold enough for a wintry mix. P-type would most
likely be freezing rain given the forecast warm nose, but sleet or
even snow would be possible across the northern mountains. Luckily
QPF would be very light so no significant accumulations would be

Highs will be near normal Wednesday then fall to 10 to 15 degrees
below normal Thursday with the cool high pressure, clouds, and
possibly precip moving in. Lows around 5 degrees below normal
Wednesday night rise a couple of degrees Thursday night putting
frost/freeze concerns back in play.


As of 345 AM EDT Tuesday: Guidance in pretty good agreement through
the medium range. An upper ridge axis moves into the area Friday
then east of the area Saturday and an upper low moves to near the
confluence of the OH and MS rivers. The low then moves to the
eastern Great Lakes or Mid-Atlantic on Sunday as short waves
rotating around the low move over the area. A surface high pressure
ridge remains across the area Friday then weakens Saturday as the
center of the high moves east into the Atlantic. This allows a moist
southerly flow to develop as a cold front moves into the MS valley.
The flow and moisture increase on Sunday as the front continues to
move east. Friday should be dry with precip chances increasing
Saturday and maximizing on Sunday. Will see some thunderstorms
Sunday as instability increases, especially outside of the
mountains. Temps start a little below normal Friday rising above
normal for Saturday and Sunday.

Short wave ridging moves through Monday as a weaker upper low takes
a similar track to the weekend low. The cold front weakens as it
moves into the area Monday. However, enough moisture and instability
remain for a slight chance of showers and storms. Temps rise about 5
degrees from Sunday.


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:




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