.SHORT TERM /SATURDAY NIGHT THROUGH SUNDAY NIGHT/...
As of 211 PM EDT Friday...
Key Messages
1) A powerful storm system will impact the forecast area from
Saturday night through Sunday morning, producing widespread
thunderstorms, many of which will become severe.
2) Damaging winds and several tornadoes are
likely with this system.
3) Prepare for severe weather - secure outdoor items, prep and
emergency kit, have multiple ways to get critical warnings,
and plan where to seek shelter if your home is in the path of a
tornado or severe thunderstorm.
By Saturday evening, a robust z500
trough axis will be centered
over the southern Great Plains, driving a strong cold
front across
central Louisiana and into the Deep South. Ahead of this
front,
the 12z suite of operational guidance continues to depict a 55-65kt
LLJ, which will funnel a continuous stream of Gulf
moisture into
the prefrontal environment...allowing QLCS activity to proceed
intact all the way across Alabama and Georgia...showing no signs
of deterioration as it approaches our forecast area.
One big question mark still remains regarding the onset of
convection Saturday night. This morning is the first time
we`ve had a full collection of high-res CAM sources to look at,
and there`s still some question over the possibility of some
warm-sector convective initiation over the western Upstate late
Saturday evening - around or after sunset - as the first waves
of strong synoptic forcing begin. Successive runs of the HRRR
and RAP keep hinting at this possibility - and if it pans out,
these storms will develop in a weakly-unstable but highly-sheared
environment with 0-1km on the order of 250+
m2/s2...more than
enough to expect mini-supercells posing a tornadic risk. The 12z
high-res windows and the NAMnest are much less jazzed about this
idea...and with all the ingredients necessary it may wind up being
a waiting game to see whether this activity materializes.
Whatever the case may be Saturday evening, confidence becomes
much higher toward midnight and thereafter that a well-organized
QLCS will advance out of north Georgia and the Tennessee Valley.
By the time it arrives in the western Carolinas, it`ll have
propagated well
downstream of its parent frontal
circulation,
which may still be located as far west as the MS-AL state line.
It`s typical in these strong WAA situations for the boundary
layer to remain well-mixed even at night...and so guidance sources
paint a deceptively stable- looking picture of the atmosphere with
sbCAPE forecasts...but MLCAPE, which in this case may be a much
better metric...surges to 300-400 J/kg over much of the region
as the system arrives. The aforementioned LLJ should still be
intact at this time, and this combined with the advancing upper
trough will provide extensive shear. 0-3km shear vectors will
be oriented a solid 60 degrees relative to the advancing QLCS,
which will support a mixture of continued linear-mode convection
alongside discrete cells ahead of the main line. 0-1km SRH,
meanwhile, could attain values in excess of 200 m2/s2 even as
the LLJ starts to deteriorate. All that to say, any discrete
convection will likely be supercellular in nature, posing a risk
of tornadogenesis; on top of this, we`re likely to see embedded
rotation within the trailing QLCS. The primary/most widespread
threat remains damaging winds, which could occur anywhere along
the QLCS, but tornadoes remain a close second.
Improving clarity, too, on
QPF...with the eastern half of the
forecast area seeing an overall reduction in expected
rainfall
(to less than an inch for the I-77 corridor) but the high terrain
in particular seeing an increase now that the high-res guidance
has become available. The 12z HREF estimates of 3-4 inches in 6
hours, and over 5 in 12 hours, are in line with multiple rounds of
convection affecting favored upslope zones, particularly across
the Smokies and Balsams. Concern remains moderate regarding
flood impacts, with the latest HEFS
ensemble river forecasts still
depicting mainly action stage rises at worst (with the exception
of sites in the Smokies, which simply receive so much
QPF that
action to minor stage flooding isn`t out of the question).
And, with storms looking to move so quickly in a highly-sheared
environment, most zones will steer clear of even localized
flash
flooding...except for
isolated locations where multiple rounds of
heavy
rainfall take place. That`s to say, localized hydro issues
can`t be ruled out, but aren`t expected to pose a major issue.
&&
.LONG TERM /MONDAY THROUGH FRIDAY/...
As of 219 PM EDT Friday...
Key Messages:
1) A second round of
thunder is possible Sunday afternoon.
2) Clear and dry conditions will persist for the first half of
the week.
3) Rain will return Thursday and Friday.
The fun doesn`t end Sunday morning. The general thinking remains
that the main QLCS will exit
GSP`s forecast area to the east by a
few hours after daybreak Sunday...but most of the CAMs have slowed
down the progression of the trailing frontal boundary as it becomes
ever more distant from its parent low. In fact, the 12z HRRR and
NAMnest slow it down so much that it`s still meandering across the
I-77 corridor on Sunday afternoon...late enough that the
atmosphere
manages to destabilize again over the far eastern fringe of the
forecast area, producing yet another round of thunderstorms!
It`s unclear whether this will actually happen - this is wholly
dependent on the evolution of the main system Saturday night,
how much it saps the
atmosphere of its energy, and how the timing
of both the
squall line and the trailing frontal boundary set up.
But if it does, more aggressive models like the RDPS once again
depict some 100-250
J/kg sbCAPE (though with significantly
less-pronounced
shear/
SRH) supportive of at least a few strong -
maybe even some severe - storms in this mess. But, the key here
is that confidence is extremely low.
I'm not sure I have ever seen this in a GSP AFD before. This is serious business. This part is interesting too about the CAPE. They are saying SBCAPE may not matter as much as usual this time. They also are starting to mention the possibility of a 2nd round too.
