Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 220850

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
450 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 300 AM EDT Monday: Upper low will continue to spin over the
Great Lakes today as the surface low lifts northeast across Ontario
toward James Bay. The attendant cold front will push across the
Appalachians and into the Piedmont during the day, but will stall
across the area. Meanwhile the upper low will split, with the
western portion diving back down into the Upper Midwest, thus
deepening the trough. Divergent flow aloft will increase, and a
shortwave coming out of the Gulf in the southwest flow aloft will
bring another slug of moisture across the Deep South and into the
Southern Appalachians late in the period.

All that said, details remain very tricky. We`re in a bit of a lull
across the area, behind the convection that pushed through yesterday
and still ahead of the advancing front. Some showery activity can
still be expected this morning in the low level WAA regime, but
expect an uptick with frontal forcing increasing today. sbCAPE
values increase to between 1000-1500J/kg especially across southern
zones, and though very little shear is collocated with this, there
remains enough synoptic lift that we could see some marginally
strong storms this afternoon. Lapse rates aren`t particularly
supportive of hail, with profiles looking much more tropical than
severe, but cannot rule out some gusty winds. Hires guidance all
want to develop pulse convection across the area, possibly with some
quasi-linear organization to it, though even then individual members
have wide variation in convective trends today.

Bigger concern remains the heavy rain threat. Rainfall rates could
of course be higher in afternoon convection, but primary issue 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: Winds switched to NW at KCLT after the
last batch of rain moved through, so adjusted prevailing winds
accordingly for the 09z AMD. Also delayed improving conditions based
on current trends and updated guidance. Otherwise, widespread MVFR
to IFR cigs are expected through the morning hours. Could see some
occasional LIFR as well before improvement later this morning to
VFR. Convection chances increase this afternoon and evening and have
extended the previous PROB30s later into the period. Cigs should
drop again to at least MVFR again toward the end of the period.
Winds will be VRB today, but favoring S early in the period for all
but KAVL where WNW winds prevail, veering more SW by daybreak, and
then either backing rapidly SE this evening, swinging around toward
NE toward the end of the period.

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

Confidence Table...

            09-15Z        15-21Z        21-03Z        03-06Z
KCLT       Med   70%     High  91%     High 100%     High  81%
KGSP       Low   59%     High 100%     High 100%     High  84%
KAVL       High  85%     Med   78%     High 100%     Med   75%
KHKY       Med   76%     High  95%     High 100%     High  93%
KGMU       Low   59%     High  88%     High 100%     Med   75%
KAND       Med   76%     High  83%     High 100%     High  82%

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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