Area Forecast Discussion
Issued by NWS Greer, SC
FXUS62 KGSP 290839
Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
339 AM EST Tue Nov 29 2016
Moisture levels will remain high ahead of an approaching frontal
boundary this morning, and remain elevated over the next few days.
This will bring much needed rainfall to the area through Wednesday.
Cool and dry high pressure returns Thursday and lingers through the
.NEAR TERM /THROUGH TONIGHT/...
As of 300 AM Tue: The warm conveyor belt of the deep surface
low over MN/WI is overhead attm; convergence is driving
precip along this axis from the Gulf Coast up into our upper
Piedmont. Exceptional Gulf moisture flux is occurring along
it. Low level flow will reach its maximum in the next hour or two,
if not already. 50 kt SSW winds are seen as low as 5 kft AGL on
KGSP VAD. (Wind Advisory and High Wind Warning will remain as-is
for portions of the mountains, until 12z.) Precip rates are
likely being limited due to robbing by the stronger frontogenetic
bands to our southwest, but so far trends have ben able to keep
fairly steady rainfall up to the NC border. Lighter precip should
fill back in over the mountains over the next few hrs. Hi-res
models depict the current band slowly moving more into the
Piedmont over the morning, but as it does so, winds gradually veer
and weaken, and the models respond by diminishing rain rates.
In response to a shortwave over the Arklatex region, a wave develops
along the same boundary once it settles over the SE portion of the
CWFA. The wave lifts it north again as a warm front this aftn and
tonight. South of the front some guidance depicts a small amount
of surface based instby developing, but prog soundings show too
much inhibition to buy into more than a small chance of thunder
for the forecast. The flow remains strong, as does shear, but
the instability is too much in question to warrant a mentionable
severe threat from either us or SPC. As the front lifts north we
get a slight uptick in low level upglide; this will allow PoPs to
start to ramp back up from the south in the afternoon. Upper jet
streak also begins to move east out of the Mississippi Valley, and
the increasing divergence aloft will also start to enhance chances
again. For tonight, as we had previously done for this morning,
PoPs have been reduced overall and focused more on the terrain,
as depicted by the WRF-based models including 4km NAM. As the
frontal wave moves over TN, 850mb winds again rise above 50 kt for
a time. This jet is not as strong as what is moving overhead now,
and it remains a warm advective flow that is not conducive to much
downward momentum transfer. Mountaintop obs sites this morning are
indicating only occasional gusts above wind-advisory criteria, so
we think it will be unlikely one will be needed for tonight. Hence
no product is being issued with this package.
.SHORT TERM /WEDNESDAY THROUGH THURSDAY NIGHT/...
As of 230 am EST Tuesday: the main challenge for Wednesday will be
the passage of the cold front, which still looks like it will happen
fairly cleanly on Wednesday night. More specifically, what will
transpire ahead of the front. The guidance is in good agreement with
the frontal position to the west of the mtns at sunrise, and in good
agreement with a line of convective precip aligned with the front.
Upslope flow should force light precip close to the Blue Ridge and
over the NC mtns early Wednesday morning, while forcing will be
considerably less farther east over the Piedmont. The guidance
almost uniformly suggests a tighter gradient of precip probability
during the morning hours, with small chances at best over the
eastern edge of the fcst area including Charlotte. With that in
mind, will keep the high precip prob over the mtns, but cut it back
over the eastern zones until the front finally gets closer in the
afternoon. A categorical precip probability still looks like a good
bet, mainly because of the excellent low level forcing. The high
precip chance will be kept over the east for Wednesday evening and
will steadily decrease from the west to the point where precip
chances end by daybreak Thursday.
The environment ahead of the front should be well-endowed with low
level shear given a 50+ kt southwesterly low level jet translating
east across the fcst area during the day. It seems likely that any
convection will help to bring some strong wind gusts down to the
sfc, but fcst soundings show poor lapse rates and a shallow boundary
layer ahead of the front which cuts down on the wind gust potential.
A wind advisory might ultimately be required for all or part of the
region for Wednesday, but prefer to wait for more guidance before
issuance. The strong shear would support some organization of storms
and would suggest a severe threat, but lapse rates will be poor and
sfc-based CAPE meager at best, and the orientation of the front
roughly parallel to the low level jet does not look especially
troublesome. Cannot disagree with the Day 2 Marginal Risk, but for
now expectations for severe weather are relatively low. The heavier
precip along with the front should translate steadily eastward
through the afternoon and evening, providing a fairly short window
for when rainfall rates would be high enough to cause problems. The
QPF does not raise much alarm at this point, given how dry it has
been leading up to the event.
The front should be long-gone by sunrise on Thursday. The rest of
the fcst will be dry, with temps returning close to normal as high
pressure moves in across the Southeast.
.LONG TERM /FRIDAY THROUGH MONDAY/...
As of 230 AM EST Tuesday: the medium range fcst picks up at 12z on
Friday with heights slowly rising as a broad upper trof lifts NE
of the region. This results in a fairly flat/zonal pattern to begin
the period. As we move into Sunday, the long range models being to
diverge considerably wrt the large scale pattern. The 12z ECMWF develops
a very deep southern stream upper trof and moves it to our doorstep
by early Monday while the newer 00z GFS is not nearly as progressive.
It maintains a mostly zonal patten over the fcst area thru the weekend
and into early next week. The deep trof mentioned above never quite
materializes and the model cuts off an upper low well to our west.
This low eventually gets absorbed by the mean flow early next week.
At the sfc, high pressure will be spread over the SE region to start
the period. Over the weekend, it`s unclear exactly how the sfc pattern
will evolve. The ECMWF develops a robust low over the northern Gulf Coast
and moves it over the fcst area on Sunday. The GFS does not develop a
low until early next week and when it does, the low tracks considerably
farther NW than the one mentioned above. With considerable pattern uncertainty
remaining for most of the period, I did not make any major changes to
the sensible fcst. I still carry the highest POPs on Sunday and into early
Monday with dry conditions on Friday and most of Saturday.
.AVIATION /09Z TUESDAY THROUGH SATURDAY/...
At KCLT and elsewhere: A surface boundary is moving north into
WNC attm. Winds are quickly becoming gusty once the boundary
passes...it has only yet to move thru KAVL and KHKY and will
do so within a couple of hours. A band of rainfall associated
with a midlevel cold front has moved across the mountains and
is expected to weaken as it moves east. Additional heavier rain
bands will move in from the SW later this morning. Generally
MVFR vsby is expected within the rain, with IFR cigs developing
closer to daybreak. Cigs will improve very slowly today, with low
level upglide remaining strong thru the morning and reinforcing
the stratus. That midlevel cold front stalls over the area today
and begins to lift north again as a warm front; this permits some
instability to develop over the Piedmont this afternoon. PROB30
for TS included where warranted. Very strong flow overhead will
continue this morning. LLWS concerns are enough at KAVL and KCLT to
include in TAFs, but criteria may be met in spotty areas thru dawn.
Outlook: Another round of widespread rainfall and associated
restrictions, along with gusty southwesterly winds are expected
on Wednesday, as a slow-moving cold front crosses the region. Dry
high pressure returns Thursday and Friday.
08-14Z 14-20Z 20-02Z 02-06Z
KCLT High 97% High 100% High 87% Med 79%
KGSP High 83% High 83% High 100% Med 78%
KAVL High 92% High 90% High 84% High 93%
KHKY High 90% High 83% High 93% Med 66%
KGMU Med 79% High 87% High 100% Med 60%
KAND High 90% High 98% High 91% High 88%
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:
NC...Wind Advisory until 7 AM EST this morning for NCZ033-049-050-
High Wind Warning until 7 AM EST this morning for NCZ048-051-