Area Forecast Discussion
Issued by NWS Greer, SC

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679
FXUS62 KGSP 300558
AFDGSP

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
158 AM EDT Sun Apr 30 2017

.SYNOPSIS...
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.

&&

.NEAR TERM /THROUGH TONIGHT/...
As of 145 AM EDT: KGSP radar depicts isolated showers continuing to
form in the upslope zone across Transylvania and Henderson counties,
and cannot rule out additional isolated activity all along the Blue
Ridge through daybreak. Thunder chances seem very low, however, as
the instability has waned nocturnally.

Otherwise, deep layer southwesterly flow will increase today between
strong offshore Bermuda high pressure and a deep, closed low
pressure system lifting northeast across the southern plains. Given
the southeast to south flow at low levels, expect scattered showers
to develop once again in the upslope forced areas along the eastern
slopes of the mountains and also near weak piedmont convergence this
afternoon. The main difference from yesterday will be the warm
temperatures aloft appearing in model profiles, which should serve
to keep convection a bit more capped off this afternoon. Still,
isolated to scattered coverage of thunderstorms remains possible
given the terrain forcing, and also in the piedmont where there is a
bit less negative energy in the profiles this afternoon.

Upper forcing will gradually improve tonight as weak vorticity lobes
move toward or into the southern Appalachians east of the deep
plains trough axis. Upper jet divergence should just start arriving
in far southwest parts of the forecast area toward daybreak Monday.
Will thus ramp PoPs up to likely by 10Z Monday along the upslope
forced areas and in the far southwest mountains, with a line of
strong to severe tstms along the approaching front likely knocking
on the door at the end of the period. Gradient winds will also
steadily increase, with elevations above 4000 feet possibly flirting
with Wind Advisory gust criteria. Will hold off on any Advisories
for now, but a high elevation product could be needed if the
southerly low level jet increases any further. Humid conditions will
continue.

&&

.SHORT TERM /MONDAY THROUGH TUESDAY/...
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.

&&

.LONG TERM /TUESDAY NIGHT THROUGH SATURDAY/...
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.

&&

.AVIATION /06Z SUNDAY THROUGH THURSDAY/...
At KCLT and elsewhere: Only the KAVL vicinity appears at any risk of
light rain showers through the first half of the TAF period as
forcing is mainly upslope terrain based. Abundant boundary layer
moisture will be the bigger concern throughout as nearly all
guidance has a period of IFR cigs, and possibly MVFR vsby in fog,
forming before daybreak and lingering into the late morning hours.
Slow recovery to lower end VFR is expected by afternoon, with PROB30
for thunder advertised across the Upstate TAFs and KAVL late this
afternoon. Deeper moisture will then start building from the west
late in the period, with a return to MVFR cigs from 00Z to 06Z.
Anticipate southerly winds around 10 kt or less, but with a few low
end gusts possible late in the day.

Outlook: Widespread showers and thunderstorms are expected
throughout on Monday as a cold front crosses the area. Drier weather
is expected Tuesday. A return of afternoon/evening thunderstorms is
possible Wed, while another cold front may bring another round of
widespread showers and storms by Thursday.

Confidence Table...

            06-12Z        12-18Z        18-24Z        00-06Z
KCLT       High  88%     High  90%     High  95%     High  92%
KGSP       High  80%     High  82%     High  93%     High  94%
KAVL       Med   79%     High  83%     High  98%     High  94%
KHKY       High  85%     Med   79%     High  95%     Med   76%
KGMU       High  80%     Med   79%     High  91%     High  91%
KAND       High  81%     Med   76%     High  92%     High  94%

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:

www.weather.gov/gsp/aviation

&&

.GSP WATCHES/WARNINGS/ADVISORIES...
GA...None.
NC...None.
SC...None.

&&

$$
SYNOPSIS...JPT
NEAR TERM...HG
SHORT TERM...PM
LONG TERM...CDG
AVIATION...HG



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