Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 221518

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
1118 AM EDT Mon May 22 2017

Low pressure will form today along a stalled front over the northern
Gulf coast and track northeast across our region tonight. A deep
upper trough forms over the Mississippi Valley Tuesday night and
Wednesday with the axis crossing the southern Appalachians early
Thursday. This results in cooler than normal temperatures until an
upper ridge builds over our area for the weekend bringing a return
to Spring temperatures.


As of 1055 am Monday: Weak cold front is analyzed extending roughly
from the ATL metro area northeast through the GA/SC/NC Blue Ridge.
Low clouds east of the boundary were in the process of lifting and
scattering from the escarpment through the foothills and Piedmont,
although it appears convective debris will begin wafting over the
area in short order. That being the case, questions about the degree
of afternoon destabilization abound this morning, which is
significant, as the atmosphere will remain weakly forced through the
afternoon, and the magnitude of the PM convective threat will hinge
upon the degree of buoyancy with which terrain lift and boundary
layer convergence have to work. The GFS depiction of 3000+ sbCAPE
across the Upstate is not the least bit plausible (considering the
weak/moist adiabatic mid-level lapse rates, we probably couldn`t
reach that with full sun), and the 1000-2000 values depicted in the
12Z NAM appear much more reasonable. The best combination of low
level lift and buoyancy should exist across the Blue Ridge and the
lower SC/GA Piedmont (essentially expect the current convection in
the ATL area to gradually unzip northeast into our southern zones).
Likely pops will be featured in these areas by early afternoon, with
likely pops overspreading the remainder of the Upstate and much of
the southern NC Piedmont by late afternoon/early evening. A handful
of severe storms (with damaging winds the main, and perhaps the only
threat) cannot be ruled out, but the larger concern will be with
localized flash flooding, especially in locations with wet
antecedent conditions (the foothills and northern Piedmont of SC and
our GA zones, as well as the southern mtns of NC).

Bigger heavy rain concern is with the slug of moisture that lifts
out of the Gulf overnight. All models have some depiction of this,
though the GFS is by far the strongest with this feature, though it
seems to be suffering from some convective feedback issues that can
really be seen in the vorticity fields. Development of 850mb jet
late in the period collocated with the higher PW values also
contributes to concern, but model differences result in slightly
lower confidence. GFS has 50+kt 850mb jet just southeast of the
mountains (surface low progged to track close to the escarpment)
that would add a hefty amount of orographic lift, but NAM and ECMWF
are farther east with this feature. So, confidence in overall QPF is
moderate, but location is low. With this uncertainty, will hold off
on issuing any Flash Flood Watches for now to let the day shift
reevaluate antecedent rainfall, today`s convective trends, and to
see if there is better model consensus in the track of the low
overnight tonight.

Temperatures today will be pretty close to seasonal normals, though
of course that will be dependent on if/when/where convection
develops. Not much change to overnight lows with plentiful low-level
moisture remaining in place.


As of 200 AM Mon: Consensus of the American models is for the
stalled frontal boundary to be on our southeastern fringe Tuesday
morning, extending back southwest toward the central Gulf Coast. The
surface wave of low pressure should exit to the east by midday,
and in its wake some drier air may wrap in from the north. NAM
does show a secondary frontal wave moving thru the I-20 corridor
of AL/GA later in the day; this is not shown on the GFS as it has
the front further south by that time. Hence the latter shows a
more distinct lull in precip activity. The 21/12z EC and 22/00z
CMC wave tracks are similar to the GFS, but slower, suggesting
essentially no lull until Tuesday night.

Wherever the frontal wave sets up Tuesday, the enhanced midlevel
flow preceding it will produce appreciable shear/helicity. On
the warm (southeast) side of the low this is expected to
overlap with sufficient instability to pose a small severe wind
threat. Subsidence and downsloping in the immediate wake of the low
will stabilize the eastern CWFA for the aftn, but meanwhile heights
begin to fall from the west as the upper low cuts off and moves
south into the mid Mississippi Valley. This leads to better lapse
rates and some destabilization at least over the mtns if not the
foothills plus GA and the western Upstate. Though low to midlevel
winds will be weak by the time this occurs, enough shear remains
that a few robust updrafts are possible within multicell clusters.

Tuesday night, another shortwave moves into the ArkLaMiss region,
reactivating the front and deepening low pressure over the lower
Ohio Valley on Wed. Isentropic upglide will strengthen over our CWFA
allowing PoPs to ramp back up by morning. The warm sector envelops
the area and some degree of buoyancy will develop, though there is
disagreement among guidance how far north this occurs. Wind profiles
will be rather impressive owing to the proximity of the low, posing
a risk of organized severe storms where buoyancy is sufficient
enough. SPC has included a Marginal Risk for the Southeast states
on the new Day 3 outlook, though it includes only a sliver of our
southernmost zones; this certainly could be extended northward if
there were more confidence in the instability progs.

The heavy rain focus will mainly be over the Piedmont Tuesday, near
the front. With that shifting northward, the threat may increase
a bit near the Blue Ridge Wednesday. Most likely the hydrologic
concerns will remain localized to areas hit by multiple rounds of
t-storms thru the early week. For now we will going to advertise
a flood threat in the HWO, with the threat appearing localized
enough to preclude a watch.


As of 145 AM Monday:  The axis of the deep upper trough will pass
over the western Carolinas either Thursday morning or around mid day
Thursday as it moves northeast. Although the ECMWF very little
instability at 18Z Thursday, the GFS has a core of CAPE over I-77
just north of CLT. The 18Z run of the GFS had around 600 CAPE and
the newest run now has around 1000 CAPE from north of CLT to near
the VA border. With the lower freezing level under the upper trough,
this core of higher CAPE may help generate some convection which
would result in hail producers. This higher CAPE will shift to the
RDU and north by 00Z Friday with heights building in from the west
as the upper trough leaves our region.

Weak high pressure dominates our weather from Thursday night through
Saturday night. The shallow upper ridge that forms in the wake of
the departure of the upper trough deamplifies late Saturday and
flattens out for zonal west to east flow on Sunday. A series of
shortwaves pass through west to east next week with the first
bringing convection to the mountains by late Sunday and over all
areas Sunday night. Instability will be increasing from the west
little by little but will moisture arrive in time to result in
storms late Sunday. At this time we are going with slight to low
chance of that happening.

Temperatures start out almost 10 degrees below normal Thursday
rising to above normal over the weekend.


At KCLT and elsewhere: Another complicated forecast this morning,
with low MVFR to IFR cigs in and out across the area this morning,
with occasional dropping to VLIFR and occasional improvement to VFR.
Should see a slow improvement over the next couple of hours, with
VFR everywhere by late morning. Convection should increase again
this afternoon but generally limited to the Piedmont TAFs. Timing
difficult to narrow down but introduced TEMPO TSRA from 19-23z at
KCLT, with similar VCTS timing elsewhere, followed by PROB30.
Another round of widespread rain moves in around midnight, with cigs
dropping again to MVFR and likely IFR by the end of the period.
Winds will be highly dependent on track of the surface low; should
remain NNW at KAVL, but elsewhere should see VRB winds with a
general backing trend from WSW to SW this morning, SE by 00z, and
ENE by the end of the period.

Outlook: Periods of showers and thunderstorms with associated
restrictions will continue off and on through Thursday. Drying is
expected to finally occur through the end of the week.

Confidence Table...

            15-21Z        21-03Z        03-09Z        09-12Z
KCLT       High  93%     High 100%     High  91%     High  88%
KGSP       High  85%     High 100%     High  88%     High  87%
KAVL       Med   76%     High  98%     Med   74%     High  86%
KHKY       High  93%     High 100%     High  82%     Med   68%
KGMU       High  86%     High 100%     High  90%     High  84%
KAND       High  93%     High 100%     High  91%     Med   76%

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:




SHORT TERM...Wimberley
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