Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 010233

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
1033 PM EDT Sun Apr 30 2017

A cold front will approach the Western Carolinas later tonight and
move across the forecast area on Monday with strong to severe
thunderstorms possible as it does. Dry, weak high pressure builds
across the southeast Tuesday through Wednesday. On Thursday and
Friday, a moist area of low pressure will lift across the region
from the southwest.


As of 1030 PM EDT Sunday:  As expected the prefrontal line
of convection moving into/through the TN Valley has degraded
substantially thanks to encountering more stable air as well
as terrain.  Iso/sct shra, with a few embedded ts highlighted
the weather across the Upstate over the last few hours, however
said convection has now weakened and moved northeast into the
I40 corridor region of the NC Piedmont.  As for the fcst, made
a few tweaks to pops over the next few hours to account for the
aforementioned Piedmont shra, as well as remnant prefrontal shra
moving in from the west.  Otherwise, no sig changes were needed/made
with this update.

Previous Discussion: Area is currently in a well-defined and large
warm sector ahead of major frontal system currently stacked over
the south-central CONUS.  Cold front with line of convection is
currently moving through LA into MS.  Latest timing has front
into the GSP CWA early Monday morning, passing through by late
Monday afternoon.  Significant precipitation/thunder is expected
near and along the front.  Passage of cold front will cool temps
and scour moisture, ending immediate chances for precip. Main
question with this event will be the chances for severe weather.
Instability and wind shear are both in the moderate range with 30
to 40 kts of bulk shear and LI to -5 Monday afternoon.  Shear is
much better further north, but combination of ingredients should be
enough for at least some severe weather on Monday, with wind gusts
in bowing segments of expected QLCS-type system.  An isolated, brief
tornado is not out of the question, but alignment of shear direction
with front orientation favors QLCS and non-super-cellular storms.

850mb flow will also be quite strong with passage of the front,
with southerly to southwesterly winds of 40 to 55 kts from tonight
through Monday.  Surface winds are not expected to reach advisory
levels, except possibly at the highest elevations, and have declined
to issue a product for wind at this time.


As of 120 PM EDT Sunday: a considerably more tranquil weather
scenario is expected for the middle of the week. The back edge
of the precip associated with the cold front might still be over
the eastern-most zones at sunset on Monday, but that should move
away to the east in short order. The upper low lifting NE of the
Gt Lakes on Monday night should allow for the upper trof to the
west to deamplify through Tuesday, resulting in a nearly zonal
upper pattern thru Tuesday night. With that, we welcome weak high
pressure that should bring sunny sky, normal temperatures, and a
dry air mass that will result in a nearly perfect spring day on
Tuesday. The weak high will move to our east on Wednesday as the
upper pattern buckles with the next upper trof digging over the Srn
Plains. That should start a return flow of moisture from the Gulf
ahead of the system late in the day. However, present indication
in the model data is that moisture/precip will not arrive before
the end of the daytime period on Wednesday. That should give us
another mostly dry day with some increasing clouds in the afternoon,
and temps creeping up a bit above normal.


As of 235 PM EDT Sunday: still looking at the development of
another weather system for the late part of the week, but right
now that system remains a moving target. The main issue appears to
be when/where the upper trof cuts off a low.  Will it be Thursday
morning over the Ozark Plateau per the GFS, or the lower OH Valley
Thursday night as in the new ECMWF? Either way, we are looking at
significant moisture transport from the Gulf of Mexico ahead of the
system and lifting northeast across the western Carolinas beginning
Wednesday night. See no problem as the models agree in principle
of providing a wet period centered mainly on Thursday/Thursday
night, so precip probability is ramped up to the likely range
across the whole fcst area. Have resisted the temptation to go
categorical for now, given the run-to-run consistency issues with
the details of this system. The main issue with the guidance creeps
in on Friday and that deals with how quickly the system moves
past. The op GFS remains the slower solution with the center of
the low moving slowly from the Cumberland Plateau to the Central
Appalachians Friday through Saturday morning. Even if the slower
solution comes to pass, most of the area east of the mtns would
develop a downslope flow Friday night that would limit additional
precip, while a more NW flow would enhance or maintain precip
chances near the TN border. The precip probs will be moved in that
direction. Temps should stay below normal Thursday through Saturday
with the extensive cloud cover and threat of precip. Expect the
system to lift out in time Sunday to salvage half the weekend,
with some clearing and temperatures returning to normal.


At KCLT and Elsewhere:  Mostly VFR through the first half of
the taf cycle, with the exception of brief cig/visb reductions at
KGSP/KGMU given sct shra/tsra currently moving through the Upstate.
Otherwise, guidance favors lowering of cigs overnight as moisture
increases ahead of an approaching cold front.  A prefrontal band
of convection working through the TN valley at this time will
likely weaken as it moves east into diurnally cooling/stabilizing
air through the evening into the early morning.  With that,
few residual shra cannot be ruled out at KAVL and the SC sites,
perhaps as far east as KHKY as well through daybreak.  The biggest
impact from this will be lowering cigs, likely into the MVFR/IFR
and perhaps LIFR range.  Beyond that, convection will ramp up yet
again into the late morning hours across the western tier of the
fcst area as the front moves in, with destabilization underway out
ahead across the NC Piedmont and SC Upstate.  Expecting rather
robust tsra into the later part of the taf cycle with some svr
storms possible, thus did prevail tsra at all sites with timing
based upon camguide.  Light/moderate sly/swly winds will prevail
through the period with some gusting likely on Monday, gradually
veering at the westernmost sites post fropa.

Outlook: Behind the cold front exiting by Monday afternoon,
drier weather is expected Tuesday. A return of afternoon/evening
thunderstorms is possible Wednesday, while another cold front may
bring another round of widespread showers and storms by Thursday
into Friday.

Confidence Table...

            02-08Z        08-14Z        14-20Z        20-00Z
KCLT       High  88%     High  88%     High  86%     High  87%
KGSP       High  93%     High  93%     High  94%     High  96%
KAVL       High  85%     Med   71%     High  89%     High 100%
KHKY       High  92%     High  88%     High  98%     High  90%
KGMU       High  93%     High  96%     High  94%     High  98%
KAND       High  96%     High  95%     High  94%     High  98%

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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