Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 190740

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
340 AM EDT Wed Apr 19 2017

A cool wedge of high pressure will weaken across the region tonight.
That gives way to more seasonable conditions Thursday, in the form
of a warm and moist southerly flow of air, with showers and storms
breaking out in the afternoon. This pattern is expected to linger
into the weekend. A stronger storm system will cross the area on
Sunday, with a more seasonal airmass moving in for the beginning of
the work week.


As of 315 AM Wed: Showers continue to grow in number over the SW
half of the CWFA as moisture flux and WAA pick up above the CAD
wedge. A small amount of MLCAPE is analyzed over NE GA, which
has allowed storms producing intense rainfall and occasional
lightning. PWAT values are not exceptionally high, but weak shear
profiles are allowing these cells to move very slowly. It appears
unlikely these storms would be able to remain sfc-based as they
advect NE, and model profiles above the wedge are not conducive to
deep convection. Therefore the precip we see will be less intense,
though Upstate obs indicate good rainfall rates are occurring.

The peak in moist isentropic lift still looks to be around
daybreak. Models differ on how much precip lasts into the aftn as
the slow CAD breakdown process begins, and notably a lot of the
higher resolution sources really trend down by late morning. As
a rule of thumb we try not to pull down PoPs or reduce sky cover
too fast in the late phase of a wedge, so for now I am holding
onto at least chance PoP thru the day, particularly in light of at
least weak WAA at 925/850 mb. After a lull this morning, however,
an uptick in chances occurs this aftn with diurnal destabilization
near the wedge front. The NAM/GFS generate a few hundred joules
of CAPE over the NC mtns, and in the Piedmont as far NE as the
I-26 corridor, but a look at their profile in Bufkit suggests poor
low-level lapse rates will still exist. Initiation therefore is not
likely and so I have limited thunder chances to slight chc within
the remaining wedge area. Given the weak shear slow-moving heavy
rain producers could cause minor, localized hydrologic issues. Temps
are tricky as most sources reflect wedge erosion, which as noted
is expected to be minimal. A blend of raw guidance biased toward
the particularly cold ECMWF was selected, producing maxes ranging
from the low 60s in the north to mid 70s in the far south.

Tonight the wedge looks to hang on only by a thread, with the
parent high being centered well off the Atlantic coast. Only weak
WAA persists as the wedge becomes very shallow, so PoPs taper off
(or turn to drizzle) and will mainly be associated with upslope
areas. Low clouds should remain trapped under the inversion and
min temps will be held about 10 degrees warmer than normal.


As of 250 AM EDT Wednesday: Damming will be eroded by the beginning
of the short term, with upper ridging building over the area.
Moisture associated with the near-term shortwave will linger on
Thursday, and with temperatures rising back to well above-normal
levels, instability will follow suit with an environment supportive
of pulse thunderstorms Thursday afternoon, especially across the
mountains. A weak front will push into the Ohio Valley late
Thursday, slowly sagging into the Southern Appalachians on Friday,
but the dynamics with this system will be pretty torn up at this
point, with the parent low strongly occluded over southeast Canada
and cut off from the tailing cold front.

However, a strong midlevel wave will dig out of the Rockies into the
Central Plains on Friday, inducing surface low development across
the Texas Panhandle Friday afternoon, with the eastward-extending
warm front developing along the remnants of the weak cold front
lingering across the area. Expect another well above-normal day on
Friday, with general mass fields supportive of even higher
temperatures into the upper 80s across the Upstate, but this may be
tempered by an increase in cloud cover along the front during the
afternoon. Pops will remain pretty high across the mountains
especially, but with even the GFS supporting sbCAPEs over 2000 J/kg
along and east of the Blue Ridge, even with the lack of favorable
mid-level lapse rates, convective instability and increased low-
level lapse rates may support strong to marginally severe pulse
thunderstorms during the afternoon.

By the end of the period, the surface low is progged to be across
northern AR, with the warm front still stretching east along I-40
but beginning the trend of lifting slowly north with increasing low-
level WAA.


As of 315 AM EDT Wednesday: Guidance continues to trend toward the
ECMWF over the weekend, so rather than a damming scenario on
Saturday as earlier progged, we look to be solidly in the warm
sector of the approaching synoptic system, with potential for
another round of well above-normal temperatures (again tempered only
by increasing cloud cover). The GFS does still want to allow for a
high pressure kink in the isobars along the lee of the mountains
Saturday so will continue to watch this. By Saturday night, the
surface low, progged to be somewhere along the Tennessee Valley, has
really wound up with a fairly significant springtime synoptic
system. Bad news for us is that this front and its maximized
instability will be crossing a portion of our area during peak
heating on Sunday, with the surface low crossing the Southern
Appalachians Sunday morning and moving into the NC/SC Piedmont by
the afternoon hours. While bufr soundings are really not all that
impressive with unidirectional winds through the column (and thus no
real mid-level CAA to steepen the lapse rates), the mechanical
forcing along the front itself should be very impressive, plus we`ll
be in the right-entrance region of the upper jet. Result is at least
500-1000 J/kg sbCAPE, with some areas creeping >1000, combined with
40-60kt deep layer (speed) shear. Would like to wait and see what
the SPC Day4-8 outlook says, but would not be a bit surprised if
some low-end severe probabilities are introduced for Day5 and will
word the HWO corresponding to the outlook.

Rainfall could be problematic with the front as well, especially as
totals start piling up across the northern mountains and NC Piedmont
along the warm front Saturday into Sunday, with the cold frontal
convection added to this later on Sunday. For now, official QPF
would not be any reason for concern with 48-hr storm total QPF from
12z Sat through 12z Mon generally 1-2" across the area, but this
might easily change, and depending on how soils are primed with the
near- and short-term convection, at least isolated flash flooding
certainly not out of the question and will continue to monitor.

The front should pass through the area late Sunday with the upper
low diving anomalously south into the Gulf toward the end of the
period, with high pressure in place leading to pleasant spring-like
conditions across the area.


At KCLT and elsewhere: Cold air damming continues over the region,
influencing all terminals. General consensus is that showers will
become more widespread through the early morning as flow atop the
CAD wedge strengthens slightly, and moisture flux increases. Cigs
will lower gradually over the first 6-9 hrs of the TAF as well. Most
guidance has proven too pessimistic so far during this wedge event,
so a blend of guidance weighted toward the best-performing members
does limit IFR to sites nearer the Blue Ridge, where upslope
enhancement will occur. Thus KCLT is fcst to remain low MVFR at
worst. The wedge begins to slowly break down by late morning,
suggesting PoPs will taper off overall, though diurnal instability
near and south of the wedge front allow chances to increase over
the southern sites, incluing KCLT and KAND. Restrictive cigs
persist thru the end of the period at all sites except KCLT.

Outlook: CAD should persist thru Wednesday night with low clouds
and showers expected over the region. The wedge should erode early
Thursday, with a return to more typical diurnal thunderstorm
potential for Thursday and Friday. Another backdoor cold front
may arrive this weekend, and if so restrictions and precip will
become likely.

Confidence Table...

            07-13Z        13-19Z        19-01Z        01-06Z
KCLT       High  94%     Med   79%     Med   66%     High 100%
KGSP       High  81%     High  80%     Med   63%     Low   58%
KAVL       High  81%     Med   75%     Med   69%     Med   77%
KHKY       High 100%     Med   63%     High  86%     High  85%
KGMU       High  82%     High  80%     Med   64%     Low   58%
KAND       High  93%     High  80%     Med   62%     Low   56%

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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