Area Forecast Discussion
Issued by NWS Greer, SC
FXUS62 KGSP 291407
Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
907 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/...
1400 UTC Update: Pops were increased over the Foothills and Piedmont
based on radar trends.
As of 645 AM Tue: The warm conveyor belt of the deep surface low
over MN/WI is centered over the area attm; convergence is driving
precip along this axis from the central Gulf Coast up to around
Charlotte. Within the same axis, very strong 60+ kt SSW winds
are seen as low as 5 kft AGL on KGSP VAD. The core of the jet has
moved east of the Appalachians, and we will let the wind headlines
expire on time at 12z. Hi-res models depict another frontogenetic
band of rain moving more into the mountains over the morning as
the other band diminishes, but light showers will persist between.
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. 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.
The flow remains strong, as does shear, but the instability is too
much in question to warrant a severe threat over the vast majority
of the CWFA. However, SPC has extended the marginal risk area
just barely into the western Smokies, for the possibility that
convection will develop to our southwest (in more unstable air)
and move into our area tonight.
.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 /14Z TUESDAY THROUGH SATURDAY/...
At KCLT and elsewhere: A strong southerly low level jet over the
region will diminish this morning, but moisture will continue to
pump northward thru the day, so a day with only slow cig improvement
is expected. LLWS will also be a concern mainly at KCLT before
the jet translates east. Criteria may be met over the mountain
terrain as well. The band of rainfall that extended across Upstate
SC overnight is now approaching KCLT, but is diminishing along with
those winds. A second narrow band will move across the mountains
this morning, but is unlikely to reach the Piedmont sites. Precip
is generally expected to be associated with MVFR to low VFR cigs,
and occasional MVFR vsby. As the boundary stalls and then moves
north again this aftn/evening, and upglide develops over it, VFR
should return to the lower Piedmont (and KCLT). The rest of the
area should remain MVFR, with the upslope areas seeing IFR later
in the night.
Outlook: As the frontal system begins to move out of the region,
another round of widespread rainfall and associated restrictions,
along with gusty southwesterly winds. Dry high pressure returns
behind the front for Thursday thru Saturday.
14-20Z 20-02Z 02-08Z 08-12Z
KCLT High 98% High 93% High 92% Med 71%
KGSP High 87% High 90% High 88% Med 66%
KAVL Med 73% Med 74% Med 70% High 96%
KHKY High 90% High 91% Med 63% Low 55%
KGMU High 95% High 93% Med 79% Med 66%
KAND High 93% High 84% Med 77% Med 66%
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:
Mixing heights were lowered slightly based on the latest model data.
Fire weather parameters were adjusted accordingly.
A strong frontal system is expected to stall just south of our
area today. Another cold front will cross the area Wednesday, with
another round of moderate to heavy rainfall. These systems should
provide a good wetting rain on the ongoing fires. Expect rain totals
of 2 to 4 inches in the mountains and 1 to 2 inches east. Windy
conditions are likely in the mountains again on Wednesday.