Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 291832

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
232 PM EDT Sat Apr 29 2017

A Bermuda high will continue the summer-like weather across the
Western Carolinas and Northeast Georgia through the weekend before a
strong cold front approaches from the west Sunday night. The front
will move across our region on Monday. In its wake, weak high
pressure will spread over the southeast for Tuesday and Wednesday.
Another slow moving low pressure system will bring widespread
rainfall to the area later next week.


As of 2pm EDT Saturday: Currently, a warm afternoon with a lot of
cloudiness is unfolding around the region.  BUFKIT soundings show BL
mixing to 5000 ft. this afternoon with fair weather cu at that level
on top of the mixed layer.  Latest satellite shows some cu
enhancement over the terrain in the western third of the CWA.  This
area may see some development of showers or thunderstorms this
afternoon, though no trace of anything breaking the cap has yet been
seen. Latest surface analysis has upwards of 3000 j/kg of CAPE; but
with surface boundaries well north and west of the area, forcing
will be limited to diurnal upslope effects.  CAMS have had some
widely scattered cells of moderate strength, mostly in the
southeastern foothills.  Given very high CAPE levels, some pulse
severe is possible this afternoon with some hail, with activity
rapidly declining with the loss of sunlight this evening.

The synoptic pattern on Sunday will differ marginally from today with
a large  Bermuda high parked off the southeast CONUS, southerly BL
flow over the CWA keeping dewpoints in the mid-60s, and an
approaching frontal system from the west.  Highs on Sunday are
expected to be 5 degrees cooler than today due to increased high
clouds as the central CONUS system begins to affect the area;
consequently, CAPE values Sunday afternoon will be much lower than
today.  However, with southerly flow creating some upslope forcing
in the foothills and mountains, and marginal capping, there will
still be a chance for scattered showers, and possibly thunder around
peak heating on Sunday.

Continued high dewpoints on southerly flow will again create
conditions for fog Sunday morning.  Fog may be reduced if sufficient
cirrus advances by morning to reduce surface cooling.


As of 230 PM EDT Saturday: our main concern was the prospects for
severe thunderstorms with the passage of a strong cold front on
Monday. Before we get to Monday, we still have to deal with Sunday
night. Expect that any diurnally-driven convection will die down
as we go through the evening, while we see a gradually improving
chance of precip forced by low level southerly flow running up
the Blue Ridge Escarpment. Mid/upper forcing remains largely to
the west through most of the overnight hours, so expect the best
chances will quickly pare back to the escarpment and then increase
from there in the early morning hours as the upslope improves. With
that in mind, the precip chances have been brought into a bit
more focus on that feature, with probability lowered over the
Piedmont of the Carolinas, and on the TN border where flow will
be downslope. Temps will remain mild overnight with the increase
in moisture and warm advection ahead of the approaching cold front.

That brings us to the main event. The models are in good agreement
with the overall scenario of a deep upper low moving across the
nrn MS River valley to the upper Gt Lakes and pushing a strong
cold front eastward across our region. Excellent mid/upper support
with a low level focus from a strong sfc boundary and sufficient
moisture all support a likely to categorical precip probability
from west to east through the day. As I look at the new model
data, I am gradually more impressed with our chances of organized
severe storms with this system, mainly because the model timing
has slowed down just enough to keep the front over the wrn part
of the fcst area through peak heating. That suggests the timing
for severe storms might just be ideal for the Piedmont of the
Carolinas, and even better as you go farther east, which is all
to say that Charlotte metro and places east of there might have
the best chance. The operational GFS/NAM indicate sfc-based CAPE
on the order of 1000-1500 J/kg and shear on the order of 40-45 kt
with a curved hodograph. Will be very interested to see some of
the mesoscale model output at this time tomorrow. Right now, the
environment suggests some potential for strong to severe storms,
including supercells and tornadoes. The limiting factor, as in
most cases, will be instability...but I would also temper that
pessimism with the expectation that we will NOT have any cold air
damming to protect us this time. The day 3 outlook shows Marginal
because of the concern over instability, but I would keep an eye
on this one in subsequent outlooks.

The front should move through quickly, so the precip probability
drop off on the back end was also fine tuned to eliminate the
chances quicker Monday evening. The rest of the fcst is fairly
quiet. Cooler air will move in behind the front but not so cool as
to worry about it over the mtns. Tuesday looks close to normal under
zonal flow aloft with weak but dry high pressure at the surface.


As of 200 PM EDT Saturday:  The medium range fcst period kicks
off on Tuesday evening amidst broad longwave troffing with a
series of embedded waves moving through the mean flow aloft,
the most pronounced of which sliding east across Quebec, while a
secondary and eventually more impactful wave begins to take shape
over the Southern Plains.  At the surface, weak lee troffing with
prevail initially across northeast GA and the western Carolinas,
while a broad region of high pressure moves into the OH Valley,
and cyclogenesis initiates beneath the aforementioned Plains wave.

Guidance favored pattern evolution does show some interesting
discontinuities moving into mid/late week.  The GFS remains less
progressive with the eventually track of the closed H5 cyclone,
moving it eastward through weeks end, perhaps getting cutoff
from the southern stream flow altogether by Sunday.  The surface
features during this time frame will include passage of a warm
front on Thursday leading to increasing chances for shra/tsra,
possibly enhanced by a series of shortwave impulses ejecting
through the mean flow aloft.  Given this slow evolution, chances
for svr convection are less likely, with the primary threat being
prolonged rainfall and thus flooding.  On the other hand, the ECMWF
is much more progressive, with the Plains system shifted about 300
miles to the north by midday Thursday, moving atop the OH valley
overnight, all the while pushing a cold front across the deep
south and through the southern Appalachians by Friday morning.
This solution would at least lend itself to relatively higher
chances of deep/svr convection associated with a cold fropa,
and thus less chance for prolonged rainfall.  As for the fcst,
pop trends will favor the ECMWF on the whole, however lending some
credence to the GFS through the weekend.  Thus, pops increase on
Thursday into Friday where likely levels are featured, lowering to
chance levels regionwide through the weekend.  As for temperatures,
expect above normal levels on Wednesday, falling back to around
or just below normal through the remainder of the period given
the abundance of moisture and thus clouds/precip.


At KCLT and elsewhere:  Continued southerly to south-southwesterly
surface winds of 5 to 10 kts with a few gusts to 15 or 20 in the
mountains will maintain the moist low-levels of the atmosphere
through Sunday.  Currently have extensive SCT to BKN clouds with bases
around 2000 to 4000 ft AGL.  Model soundings have mixing with fair
weather cu bases up to 4500 ft this afternoon.  Some convective
showers may develop this afternoon/evening in areas west of I27.
Hi-res models have some scattered convection around peak heating,
but lack of upper support and low wind shear will limit severity.
Went with VCTS for KAVL and KAND, with the chances too low to
mention elsewhere.  Moist low-levels give a good chance for fog
again Sunday morning, and TAFS follow the general pattern of timing
that occurred this morning.  Possibly more high clouds Sunday
morning ahead of next system may reduce fog Sunday morning relative
to what was seen this morning.

Outlook: Patchy fog and low clouds will be possible again Sunday
morning around daybreak. Diurnally-based showers and thunderstorms
will be possible on Sunday afternoon. More organized thunderstorms
are likely on Monday with a passing cold front. Drier weather is
expected Tuesday through Wednesday.

Confidence Table...

            18-24Z        00-06Z        06-12Z        12-18Z
KCLT       High 100%     High 100%     High  85%     High  88%
KGSP       High 100%     High 100%     Med   76%     Med   70%
KAVL       High 100%     High  94%     Med   63%     Med   68%
KHKY       High 100%     High  94%     Med   69%     Med   73%
KGMU       High 100%     High 100%     Med   77%     Med   69%
KAND       High 100%     High 100%     Med   71%     Med   79%

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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